Hoʻi hou i ka iwi kuamoʻo.

Return to the backbone.

To return to the homeland or family after being away.

‘Ōlelo No‘eau, compiled by Mary Kawena Pukui, #1024

Introduction

The skeletal system forms the rigid internal framework of the body. The skeletal system includes all of the bones, cartilage, and ligaments that support and give shape to the body and body structures. Cartilage exists in isolated areas of the bony skeleton and provides flexible strength and support for body structures such as the thoracic cage, the external ear, and the trachea and larynx (Chapter 6-Osseous Tissue (link to it). At joints of the body, cartilage can also unite adjacent bones or provide cushioning between them. Ligaments are strong connective tissue bands that connect bone to bone and serve to prevent excessive movements of the joint that would result in injury. Often discussed in conjunction with the skeletal system are muscles because the movement of the skeleton is made possible by muscle contraction. As muscles contract, they pull on the bones to produce movements of the body. The primary functions of the skeleton are to provide mobility, support, storage, and protection for the body in the following way:

• A rigid internal structure that can support the weight of the body against the force of gravity
• A structure upon which muscles can act to produce movements of the body
• Protection of the internal organs, including the brain, spinal cord, heart, lungs, and pelvic organs
• A storage site for important minerals, such as calcium and phosphate, fat, and blood cell-producing tissue of the body

Each bone of the body serves a particular function, and therefore the structure of bones vary in size, shape, and strength based on these functions. For example, the bones of the lower back and lower limb are thick and strong because they need to support the weight of the body. As explained in Chapter 6-Osseous Tissue, the external surfaces of bones have protruding marks where muscles, ligaments, and tendons attach and grooves or holes where a vessel or nerve runs on or through it. The size of a bony landmark that serves as a muscle attachment site on an individual bone is related to the strength of the muscle. Muscles can apply very strong pulling forces to the bones of the skeleton. Bones respond to the forces applied to them by muscles and as a result, bones have enlarged bony landmarks at sites where powerful muscles attach. This means that not only the size of a bone but also its shape is related to its function. For this reason, the identification of bony landmarks is important during your study of the skeletal system and bony landmarks of the skeletal system will be discussed in this chapter.

Bones are also dynamic organs that can modify density and thickness in response to changes in muscle strength or body weight. Muscle attachment sites on bones will thicken in response to a weight-bearing workout program. Similarly, the walls of weight-bearing bones will thicken due to weight gain or in response to weight-bearing exercise such as a new running regimen. In contrast, a reduction in muscle strength or bodyweight will cause bones to become thinner. This may happen during a prolonged hospital stay, following limb immobilization in a cast, or going into the weightlessness of outer space. Even a change in diet, such as eating only soft food due to the loss of teeth, will result in a noticeable decrease in the size and thickness of the jawbones. Refer to Chapter 6-Osseous Tissue (link to it) for an in-depth discussion on the bone remodeling process.

7.1 Divisions of the Skeletal System

The skeletal system consists of two major divisions: the axial skeleton and the appendicular skeleton. The axial skeleton includes the bones of the skull (cranial bones and facial bones), the vertebral column, the hyoid bone, the sternum, and the ribs. The appendicular skeleton consists of the bones of the upper limbs, the lower limbs, the pectoral girdle, and the pelvic girdle. The lower portion of the skeleton is specialized for stability during walking or running. In contrast, the upper skeleton has greater mobility and ranges of motion, features that allow you to lift and carry objects or turn your head and trunk.

Axial and Appendicular Skeleton

Purple – Pectoral Girdle, Green – Upper Limb, Blue – Pelvic Girdle, Brown – Lower Limb

Fig. 7.1: Axial and Appendicular Skeleton 

The axial skeleton of the adult consists of 80 bones, including the skull, the vertebral column, and the thoracic cage. The axial skeleton forms the longitudinal axis of the body, the center or midline of the body around which the limbs rotate, much as the earth spins around its center axis. Additionally, the axial skeleton supports the head, neck, and trunk, while providing rigid protection of the brain, spinal cord, and visceral organs in the thorax region.

Skull: Anterior View

Skull: Posterior View

Skull: Midsagittal View

Fig. 7.2: The Skull

7.2 Skull Geography

The skull is formed by 22 bones and consists of two classes of bones, the cranial bones and the facial bones. It is the shape of the cranial and facial bones of the skull, and how the muscles and skin attached to them, that contribute to the unique individual physical traits of humans. Though all humans have the same bones, they are shaped differently based on ancestral genetics. Indeed, it is the skull bones that provide the framework for the face and attachment for the facial muscles of expression.

The anterior skull consists of the facial bones and provides the bony support for the eyes and structures of the face, and includes the nasal cavity, orbits, and support for the teeth. The cranium can be divided into two distinct regions: a cranial base and the skullcap or cranial vault. The rounded skullcap, or calvaria, is made up of fused cranial bones. These bones surround and protect the brain and house the middle and inner ear structures. The floor of the braincase is referred to as the base of the skull. This is a complex area that varies in depth and has numerous openings for the passage of cranial nerves, blood vessels, and the spinal cord. Inside the skull, the base is subdivided into three large spaces, called the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa (“fossa” = trench or ditch).

Fig. 7.3: Cranial Fossae The bones of the braincase surround and protect the brain, which occupies the cranial cavity. The base of the braincase, which forms the floor of the cranial cavity, is subdivided into the shallow anterior cranial fossa, the middle cranial fossa, and the deep posterior cranial fossa.

From anterior to posterior, the fossae increase in depth. Keeping in mind that structure is related to function in the human body, the shape and depth of each fossa corresponds to the shape and size of the brain region of each house. Most skull bones are flat bones, except for the moveable mandible, which is connected to the rest of the skull via the temporomandibular joint. The flat bones of the skull are united by interlocking stitch-like joints called sutures (see Chapter 9 – Joints). The skull also has several bone markings, each serving a unique function. Most notable are the named openings that provide passageways for the blood vessels, nerves, and the spinal cord.

Cranium

There are eight cranial bones, including the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones.

Parietal Bone

Fig. 7.4 Lateral View of Skull The lateral skull shows the large rounded brain case, zygomatic arch, and the upper and lower jaws. The zygomatic arch is formed jointly by the zygomatic process of the temporal bone and the temporal process of the [pb_glossary id="3547"]zygomatic bone[/pb_glossary]. The shallow space above the zygomatic arch is the temporal fossa. The space inferior to the zygomatic arch and deep to the posterior mandible is the infratemporal fossa.

The paired parietal bone forms most of the superior and lateral sides of the skull (Figure 7.4). The right and left parietal bones join together at the top of the skull, at the sagittal suture. Each parietal bone articulates anteriorly with the frontal bone, meeting at the coronal suture. The parietal bones meet the occipital bone posteriorly at the lambdoid suture and meet the temporal inferiorly at the squamous suture.

Retrieval Practice

The posterior view of the skull presents a modified peace sign.

Temporal Bone

The paired temporal bones form the lower lateral side of the skull (see Figure 7.4 above). The temporal bones articulate with the sphenoid, parietal, and occipital bones. Each temporal bone has a unique shape with four general regions:

• The squamous region — the squamous region is the flat lateral portion of the temporal bone. The zygomatic process of the temporal bone articulates with the temporal process of the zygomatic bone and forms the zygomatic arch, the structure that gives us our “cheekbones”. Inferior to the zygomatic process of the temporal bone is the mandibular fossa, which forms the temporomandibular joint with the mandible bone.
• The tympanic region — the inferior region of the temporal bone is called the tympanic region and it houses the external acoustic meatus. This is the opening to the external auditory canal, which carries sound waves to the middle and inner ear. Prominent in this region is the styloid process, a fragile, narrow process that anchors several muscles associated with the tongue and larynx.
• The mastoid region — this region contains the large mastoid process, which serves as a muscle attachment site. The mastoid process can easily be felt on the side of the head just behind your earlobe. Inside the mastoid process are several tiny sinuses known as mastoid air cells, which are believed to protect the inner structures of the ear and may help regulate air pressure.
• The petrous region — the petrous portion of each temporal bone forms the prominent, diagonally oriented petrous ridge in the floor of the cranial cavity. Located inside each petrous ridge are small cavities that house the structures of the middle and inner ears. There are also several openings in this region, including the foramen lacerum, carotid canal, internal acoustic meatus, and jugular foramen.

Fig. 7.5 Temporal Bone A lateral view of the isolated temporal bone shows the squamous, mastoid, and zygomatic portions of the temporal bone.

Important landmarks of the temporal bone, as shown in Figure 7.5 above, include the following:

• External acoustic meatus (ear canal): This is the large opening on the lateral side of the skull that is associated with the ear.
• Internal acoustic meatus: This opening is located inside the cranial cavity, on the medial side of the petrous ridge. It connects to the middle and inner ear cavities of the temporal bone.
• Mandibular fossa: This is the deep, oval-shaped depression located on the external base of the skull, just in front of the external acoustic meatus. The mandible (lower jaw) joins with the skull at this site as part of the temporomandibular joint, which allows for movements of the mandible during the opening and closing of the mouth.
• Articular tubercle: The smooth ridge located immediately anterior to the mandibular fossa. Both the articular tubercle and mandibular fossa contribute to the temporomandibular joint, the joint that provides for movements between the temporal bone of the skull and the mandible.
• Styloid process: Posterior to the mandibular fossa on the external base of the skull is an elongated, downward bony projection called the styloid process, so named because of its resemblance to a stylus (a pen or writing tool). This structure serves as an attachment site for several small muscles and for a ligament that supports the hyoid bone of the neck. (See also Figure 7.6 below)
• Stylomastoid foramen: This small opening is located between the styloid process and the mastoid process. This is the point of exit for the cranial nerve that supplies the facial muscles.
• Carotid canal: The carotid canal is a zig-zag-shaped tunnel that provides passage through the base of the skull for one of the major arteries that supply the brain. Its entrance is located on the outside base of the skull, anteromedial to the styloid process. The canal then runs anteromedially within the bony base of the skull and then turns upward to its exit in the floor of the middle cranial cavity, above the foramen lacerum.

Also associated with the temporal bones are the three bones in the middle ear, the ear ossicles. [See Chapter 15 – Special Senses]. These three bones, the malleus, incus, and stapes, are the smallest bones in the human body, and function to amplify sound waves traveling from the tympanic membrane to the inner ear.

Fig. 7.6 External and Internal Views of Base of Skull (a) The hard palate is formed anteriorly by the palatine processes of the maxilla bones and posteriorly by the horizontal plate of the palatine bones. (b) The complex floor of the cranial cavity is formed by the frontal, ethmoid, sphenoid, temporal, and occipital bones. The lesser wing of the sphenoid bone separates the anterior and middle cranial fossae. The petrous ridge (the petrous portion of temporal bone) separates the middle and posterior cranial fossae.

 Frontal Bone

The frontal bone is the single bone that forms the forehead, the anterior portion of the cranium. The frontal bone also contributes to the superior portion of the orbit, home to the eyeball, and the anterior cranial fossa. At its anterior midline, between the eyebrows, there is a slight depression called the glabella (see Figure 7.7). The frontal bone also forms the supraorbital margin of the orbit. Near the middle of this margin, is the supraorbital foramina, the opening that provides passage for a sensory nerve to the forehead. The frontal bone is thickened just above each supraorbital margin, forming rounded brow ridges. These are located just behind your eyebrows and vary in size among individuals, although they are generally larger in males. Inside the cranial cavity, the frontal bone extends posteriorly. This flattened region forms both the roof of the orbit below and the floor of the anterior cranial cavity above (see Figure 7.6). Finally, the frontal bone contains the hollow frontal sinuses, which are part of a group of bony cavities that surround the nasal cavity.

Fig. 7.7. Anterior view of the skull — An anterior view of the skull shows the bones that form the forehead, orbits (eye sockets), nasal cavity, nasal septum, and upper and lower jaws.]

Occipital Bone

The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity (Figure 7.8; see also Figure 7.6). On its outside surface, at the posterior midline, is a small protrusion called the external occipital protuberance, which serves as an attachment site for a ligament of the posterior neck. Lateral to either side of this bump is a superior nuchal line (“nuchal” = nape or posterior neck). The nuchal lines represent the most superior point at which muscles of the neck attach to the skull, with only the scalp covering the skull above these lines. On the base of the skull is the most obvious feature of the occipital bone: the large opening of the foramen magnum, which allows for the passage of the spinal cord as it exits the skull. On either side of the foramen magnum is an oval-shaped occipital condyle. These condyles form joints with the first cervical vertebra (C1, the atlas) and thus support the skull on top of the vertebral column.

Fig. 7.8. Posterior View of The Skull This view of the posterior skull shows attachment sites for muscles and joints that support the skull.

Sphenoid Bone

The sphenoid bone is a single, complex bone embedded within the central skull (Figure 7.9). The sphenoid forms much of the base of the central skull (see Figure 7.6) and also extends laterally to contribute to the sides of the skull (see Figure 7.4 and Figure 7.7). To see the entire sphenoid bone, it must be removed from the skull. It serves as a “keystone” bone because it joins with almost every other bone of the skull, including the frontal, parietal, occipital, ethmoid, and temporal bones of the cranium and the palatine, zygomatic, vomer, and maxilla bones of the face.

When viewed on its own, the sphenoid bone looks like a bat or a giant moth with wings extended in flight. The sphenoid bone appears to have a central body and three pairs of processes: the greater wings, the lesser wings, and the pterygoid processes. When comparing the sphenoid bone to a moth, the antennae of the moth are the lesser wings of the sphenoid bone, which are superior and anterior when viewing the sphenoid from a posterior view. The legs of the moth are the pterygoid processes, and the wings, when extended for flight, are the greater wings of the sphenoid bone. The pterygoid processes serve as the attachment site for the pterygoid muscles, used for chewing. The greater wings are large, inferior and posterior.

Between the greater and lesser wings is the superior orbital fissure — a slit through which nerves pass. The sella turcica is a significant bone marking at the midline of the middle cranial fossa, so named because it resembles a Turkish saddle used when riding a horse (a “Turkish saddle” had a high back and tall front). The rounded depression in the floor of the sella turcica is the hypophyseal (pituitary) fossa, which protects the pituitary gland.

Fig. 7.9a-b. Sphenoid Bone Shown in isolation in (a) superior and (b) posterior views, the sphenoid bone is a single midline bone that forms the anterior walls and floor of the middle cranial fossa. It has a pair of lesser wings and a pair of greater wings. The sella turcica surrounds the hypophyseal fossa. Projecting downward are the medial and lateral pterygoid plates. The sphenoid has multiple openings for the passage of nerves and blood vessels, including the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum.

Fig. 7.9c – Giant moth from Hawaiʻi — the Black Witch Moth, Ascalapha odorata

Other important bone markings on the sphenoid bone include:

• Optic canals — passageway for the optic nerves (cranial nerves II)
• Foramen ovale — passageways for branches of cranial nerve V
• Foramen spinosum — passageway for the middle meningeal artery

Ethmoid Bone

The ethmoid bone is a single, unpaired bone in the midline of the skull. Similar to the sphenoid bone, the ethmoid bone is embedded in the skull, located behind the bridge of the nose. It forms the roof and lateral walls of the upper nasal cavity, the upper portion of the nasal septum, and contributes to the medial wall of the orbit (Figure 7.9 and Figure 7.10).  On the interior of the skull, the ethmoid also forms a portion of the floor of the anterior cranial cavity (see Figure 7.6 above).

Fig. 7.10 Sagittal Section of Skull This midline view of the sagittally sectioned skull shows the nasal septum.

In the cranial cavity, on the superior aspect of the ethmoid bone, are several important structures. This portion of the ethmoid bone consists of two significant parts, the crista galli and cribriform plates. The crista galli (“rooster’s comb or crest”) is a small upward bony projection located at the midline. When viewed from the anterior, the crista galli looks like the fin of a shark above the surface of the water. It functions as an anterior attachment point for one of the covering layers of the brain.

Figure 7.11 a Ethmoid Bone The unpaired ethmoid bone is located at the midline within the central skull. It has an upward projection, the crista galli, and a downward projection, the perpendicular plate, which forms the upper nasal septum. The cribriform plates form both the roof of the nasal cavity and a portion of the anterior cranial fossa floor. The lateral sides of the ethmoid bone form the lateral walls of the upper nasal cavity, part of the medial orbit wall, and give rise to the superior and middle nasal conchae. The ethmoid bone also contains the ethmoid air cells.

Fig. 7.11 b Shark

To either side of the crista galli is the cribriform plate (“cribrum” = sieve), a small, flattened area with numerous small openings termed olfactory foramina. These foramina allow small nerve branches from the olfactory areas of the nasal cavity to pass through and enter the brain.

The lateral portions of the ethmoid bone are located between the orbit and upper nasal cavity, and thus form the lateral nasal cavity wall and a portion of the medial orbit wall. Located inside this portion of the ethmoid bone are several small, air-filled spaces that are part of the paranasal sinus system of the skull.

Fig. 7.12 Detailed image of the cribriform plate. Also see animated model.

Fontanels

The skull of a human fetus is noticeably different from that of an adult human. The cranial bones of a fetus remain separated from each other by large areas of dense connective tissue, each of which is called a fontanelle (Figure 7.12).

Fig. 7.13 Newborn Skull The bones of the newborn skull are not fully ossified and are separated by large areas called fontanelles, which are filled with fibrous connective tissue. The fontanelles allow for the continued growth of the skull after birth. At the time of birth, the facial bones are small and underdeveloped, and the mastoid process has not yet formed.

The fontanelles are the “soft spots” on an infant’s head. They are important during birth because these areas allow the skull to change shape as it squeezes through the birth canal. After birth, the fontanelles allow for continued growth and expansion of the skull as the brain enlarges. The largest fontanelle is located on the anterior head, at the junction of the frontal and parietal bones.

The fontanelles decrease in size and eventually disappear around age 2. However, the skull bones remain separated from each other at the sutures. The connective tissue of the sutures allows for the continued growth of the skull bones as the brain enlarges during childhood growth.

As discussed in Chapter 9 – Joints, in adults, sutures are immobile joints between adjacent bones of the skull. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The long sutures located between the bones of the braincase are not straight, but instead follow irregular, tightly twisting paths. These twisting lines serve to tightly interlock the adjacent bones, thus adding strength to the skull for brain protection.

Facial Bones

The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The facial bones include 14 bones, with six paired bones and two unpaired bones. The paired bones are the maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones. The unpaired bones are the vomer and mandible bones. Although classified with the cranial bones, the ethmoid bone also contributes to the nasal septum and the walls of the nasal cavity and orbit.

Maxillary Bone

The maxillary bone often referred to simply as the maxilla (plural = maxillae), is one of a pair that together form the upper jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose (see Figure 7.7). The curved, inferior margin of the maxillary bone that forms the upper jaw and contains the upper teeth is the alveolar process of the maxilla (Figure 7.13). Each tooth is anchored into a deep socket called an alveolus. On the anterior maxilla, just below the orbit, is the infraorbital foramen. This is the point of exit for a sensory nerve that supplies the nose, upper lip, and anterior cheek. On the inferior skull, the palatine process from each maxillary bone can be seen joining together at the midline to form the anterior three-quarters of the hard palate (see Figure 7.6). The hard palate is the bony plate that forms the roof of the mouth and floor of the nasal cavity, separating the oral and nasal cavities.

Fig. 7.14 Maxillary Bone The maxillary bone forms the upper jaw and supports the upper teeth. Each maxilla also forms the lateral floor of each orbit and the majority of the hard palate.

Palatine Bone

The palatine bone is one of a pair of irregularly shaped bones that contribute small areas to the lateral walls of the nasal cavity and the medial wall of each orbit. The largest region of each of the palatine bones is the horizontal plate. The horizontal plate articulates with the maxillary bones and forms the posterior portion of the hard palate (see Figure 7.6). Thus, the palatine bones are best seen in an inferior view of the skull and hard palate.

Fig. 7.15 Child with Cleft Lip and Palate 

A more severe developmental defect is the cleft palate, which affects the hard palate. The hard palate is the bony structure that separates the nasal cavity from the oral cavity. It is formed during embryonic development by the midline fusion of the horizontal plates from the right and left palatine bones and the palatine processes of the maxilla bones. Cleft palate affects approximately 1:2500 births and is more common in females. It results from a failure of the two halves of the hard palate to completely come together and fuse at the midline, thus leaving a gap between them. This gap permits communication between the nasal and oral cavities, and can negatively affect chewing and swallowing. These orofacial clefts are one of the most common craniofacial malformations of the newborn and are prevalent in Hawaiʻi.

Zygomatic Bone

The zygomatic bone articulates with the zygomatic processes of the temporal bones posteriorly, which creates what is commonly known as the cheekbone. Each of the paired zygomatic bones forms much of the lateral wall of the orbit and the lateral-inferior margins of the anterior orbital opening. [See Figure 7.4 and Figure 7.7].

Nasal Bone

The nasal bone is one of two small bones that articulate (join) with each other to form the bony base (bridge) of the nose. They also support the cartilages that form the lateral walls of the nose (see Figure 7.9 above). These are the bones that are damaged when the nose is broken.

Lacrimal Bone

Each lacrimal bone is a small, rectangular bone that forms the anterior, medial wall of the orbit (see Figure 7.7). The anterior portion of the lacrimal bone forms a shallow depression called the lacrimal fossa, and extending inferiorly from this is the nasolacrimal canal. The lacrimal fluid (tears of the eye), which serves to maintain the moist surface of the eye, drains at the medial corner of the eye into the nasolacrimal canal. This duct then extends downward to open into the nasal cavity, behind the inferior nasal concha. In the nasal cavity, the lacrimal fluid normally drains posteriorly, but with an increased flow of tears due to crying or eye irritation, some fluid will also drain anteriorly, causing a runny nose.

Fig. 7.16 Frontal view of the lacrimal apparatus showing the flow of tears from the lacrimal glands to the nasolacrimal duct and nasal cavity. Note that the lacrimal canaliculi are shown as lacrimal ducts in the figure. (By CNX OpenStax — https://cnx.org/contents/5CvTdmJL@4.4, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=53728100) – [this image of the eye is a direct copy from the special senses chapter]

Inferior Nasal Conchae

The right and left inferior nasal conchae form a curved bony plate that projects into the nasal cavity space from the lower lateral wall (see Figure 7.16). The inferior concha is the largest of the nasal conchae and can easily be seen when looking into the anterior opening of the nasal cavity.

Fig. 7.17 Lateral Wall of Nasal Cavity The three nasal conchae are curved bones that project from the lateral walls of the nasal cavity. The superior nasal concha and middle nasal concha are parts of the ethmoid bone. The inferior nasal concha is an independent bone of the skull.

Vomer Bone

The unpaired vomer bone, often referred to simply as the vomer, is triangular-shaped and forms the posterior-inferior part of the nasal septum (see Figure 7.16). The vomer is best seen when looking from behind into the posterior openings of the nasal cavity (see Figure 7.6). In this view, the vomer is seen to form the entire height of the nasal septum. A much smaller portion of the vomer can also be seen when looking into the anterior opening of the nasal cavity.

Mandible

The mandible, considered the strongest bone in the face, forms the lower jaw and is the only moveable bone of the skull. At the time of birth, the mandible consists of paired right and left bones, but these fuse together during the first year to form the single U-shaped mandible of the adult skull. Each side of the mandible consists of a horizontal body and posteriorly, a vertically oriented ramus of the mandible (“ramus” = branch). The outside margin of the mandible, where the body and ramus come together, is called the angle of the mandible (Figure 7.16).

The ramus on each side of the mandible has two upward-pointing bony projections. The more anterior projection is the flattened coronoid process of the mandible, which provides attachment muscles of mastication. The posterior projection is the condylar process of the mandible, which is topped by the oval-shaped condyle. The condyle of the mandible articulates with the mandibular fossa and articular tubercle of the temporal bone. Together these articulations form the temporomandibular joint, which allows for the opening and closing of the mouth (see Figure 7.4). The broad U-shaped curve located between the coronoid and condylar processes is the mandibular notch.

Fig. 7.18 Isolated Mandible The mandible is the only moveable bone of the skull.

Important landmarks for the mandible include the following:

• Alveolar process of the mandible — This is the upper border of the mandibular body and serves to anchor the lower teeth
• Mental protuberance — The forward projection from the inferior margin of the anterior mandible that forms the chin (“mental” = chin)
• Mental foramen — The opening located on each side of the anterior-lateral mandible, which is the exit site for a sensory nerve that supplies the chin
• Mylohyoid line — This bony ridge extends along the inner aspect of the mandibular body. The muscle that forms the floor of the oral cavity attaches to the mylohyoid lines on both sides of the mandible.
• Mandibular foramen — This opening is located on the medial side of the ramus of the mandible. The opening leads into a tunnel that runs down the length of the mandibular body. The sensory nerve and blood vessels that supply the lower teeth enter the mandibular foramen and then follow this tunnel. Thus, to numb the lower teeth prior to dental work, the dentist must inject anesthesia into the lateral wall of the oral cavity at a point before where this sensory nerve enters the mandibular foramen.
• Lingula — This small flap of bone is named for its shape (“lingula” = little tongue). It is located immediately next to the mandibular foramen, on the medial side of the ramus. A ligament that anchors the mandible during the opening and closing of the mouth extends down from the base of the skull and attaches to the lingula.

The Orbits

The orbit is the bony socket that houses the eyeball and muscles that move the eyeball or open the upper eyelid. The upper margin of the anterior orbit is the supraorbital margin. Located near the midpoint of the supraorbital margin is a small opening called the supraorbital foramen. This provides for the passage of a sensory nerve to the skin of the forehead. Below the orbit is the infraorbital foramen, which is the point of emergence for a sensory nerve that supplies the anterior face below the orbit.

The walls of each orbit include contributions from seven skull bones (Figure 7.18). The frontal bone forms the roof and the zygomatic bone forms the lateral wall and lateral floor. The medial floor is primarily formed by the maxilla, with a small contribution from the palatine bone. The ethmoid bone and lacrimal bone make up much of the medial wall and the sphenoid bone forms the posterior orbit.

At the posterior apex of the orbit is the opening of the optic canal, which allows for the passage of the optic nerve from the retina to the brain. Lateral to this is the elongated and irregularly shaped superior orbital fissure, which provides passage for the artery that supplies the eyeball, sensory nerves, and the nerves that supply the muscles involved in eye movements.

Fig. 7.19 Bones of the Orbit Seven skull bones contribute to the walls of the orbit. Opening into the posterior orbit from the cranial cavity are the optic canal and superior orbital fissure.

The Nasal Septum and Nasal Conchae

The nasal septum consists of both bone and cartilage components (Figure 7.19).  The upper portion of the septum is formed by the perpendicular plate of the ethmoid bone. The lower and posterior parts of the septum are formed by the triangular-shaped vomer bone. The anterior nasal septum is formed by the septal cartilage, a flexible plate that fills in the gap between the perpendicular plate of the ethmoid and vomer bones. This cartilage also extends outward into the nose where it separates the right and left nostrils. The septal cartilage is not found in the dry skull, thus the wide opening in the facial area can be seen, in place of the nose, when viewing the skull anteriorly.

Labeled MRI image from an axial slice through human head. The image shows the nose, maxillary sinuses and nasal septum.	Cross section through the human skull and represents the kieselbachi’s plexus. The nasal septum (lat. Nasal septum) is marked with blue color.

Fig. 7.20 Nasal Septum

Attached to the lateral wall on each side of the nasal cavity are the superior, middle, and inferior nasal conchae (singular = concha), which are named for their positions (see Figure 7.16). These are bony plates that curve downward as they project into the space of the nasal cavity. They serve to swirl the incoming air, which helps to warm and moisturize it before the air moves into the delicate air sacs of the lungs. This also allows mucus, secreted by the tissue lining the nasal cavity, to trap incoming dust, pollen, bacteria, and viruses. The largest of the conchae is the inferior nasal concha, which is an independent bone of the skull. The middle concha and the superior conchae, which is the smallest, are both formed by the ethmoid bone.

Paranasal Sinuses

The paranasal sinuses are hollow, air-filled spaces located within certain bones of the skull [see Figure 7.20 below paranasal sinus figure/image]. All of the sinuses communicate with the nasal cavity (“paranasal” = next to the nasal cavity) and are lined with nasal mucosa. The sinuses act to reduce bone mass and thus lighten the skull, and are associated with resonance of the voice. This second feature is most obvious when sinus congestion is present. Inflammation can result in swelling of the mucosa and excess mucus production, which can obstruct the narrow passageways between the sinuses and the nasal cavity, causing the voice to sound different. This blockage can also allow the sinuses to fill with fluid, with the resulting pressure producing pain and discomfort.

The paranasal sinuses are named for the skull bone that each occupies. The frontal sinus is located just above the eyebrows, within the frontal bone. This irregular space may be divided at the midline into bilateral spaces, or these may be fused into a single sinus space. The frontal sinus is the most anterior of the paranasal sinuses. The largest sinus is the maxillary sinus. These are paired and located within the right and left maxillary bones, where they occupy the area just below the orbits. The maxillary sinuses are most commonly involved during sinus infections. Because their connection to the nasal cavity is located high on their medial wall, they are difficult to drain. The sphenoid sinus is a single, midline sinus. It is located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, thus making it the most posterior of the paranasal sinuses. The lateral aspects of the ethmoid bone contain multiple small spaces separated by very thin bony walls. Each of these spaces is called an ethmoid air cell. These are located on both sides of the ethmoid bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae.

Paranasal Sinuses

Nose and Nasal Cavities

Fig. 7.21 Paranasal Sinuses

Hyoid Bone

The hyoid bone is an independent bone that does not contact any other bone and thus is not part of the skull. The hyoid is held in position by a series of small muscles that attach to it either from above or below. But because it is near the skull, it is often discussed at the same time as the bones of the skull. The hyoid is a small U-shaped bone located superior to the larynx near the level of the inferior mandible, which serves as an attachment site for the muscles of the tongue, larynx, and pharynx. It consists of a body and two pairs of horns, a greater horn and a lesser horn. Its position and attachments aid in swallowing and speaking.

Fig. 7.22 Hyoid Bone The hyoid bone is located in the upper neck and does not join with any other bone. It provides attachments for muscles that act on the tongue, larynx, and pharynx.

Bone Markings: Markings of the cranium viewing the skull from the inferior view.	Bone Markings: Markings of the cranium viewing the skull from the superior view of the cranial cavity.

Markings of the cranium with the following views: (A) anterior view, (B) lateral view of the left side of the skull, (C) posterior view, and (D) lateral view of the right side of the skull.	Bone Markings: Markings of the cranium Also see: Moving skull illustration

Figure 7.23 Bone Markings of the Skull

Sex Differences in the Skull

There is little difference between the skull of the female and that of the male. Differences become more apparent at puberty and into adulthood. In general, the skull of an adult female is lighter and smaller than that of an adult male, with thinner walls that are less marked with ridges. When compared to the oblique forehead and obvious glabella seen in the male skull, female skulls have a straighter frontal bone and less pronounced glabella. Skeletal differences are also obvious in the more pronounced zygomatic and mandible bones of males, in which angles and processes are more robust.

7.3 The Vertebral Column

The vertebral column is also known as the spinal column or spine. It consists of a sequence of vertebrae (singular = vertebra), each of which is separated and united by an intervertebral disc. Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae. People jump off the side of the Waikiki wall for fun. When they dive into shallow waters and hit the ocean floor, it could result in damage to the vertebral column and lead to a permanent spinal injury. Next time, when you are tempted to jump off, think about the consequences!

Fig. 7.24 Vertebral Column 

General Characteristics

In the fetus and infant, the vertebral column originally develops as a series of 33 vertebrae, but this number is reduced to 24, as nine of the individual bones fuse to form two composite bones: the sacrum and the coccyx. In adults, the vertebral column consists of 24 bones, each called a vertebra, plus the sacrum and coccyx. The vertebral column is subdivided into five regions, with the vertebrae in each area named for that region and numbered in descending order. In the neck, there are seven cervical vertebrae, each designated with the letter “C” followed by its number. Below these are the 12 thoracic vertebrae, designated T1–T12. The lower back contains the L1–L5 lumbar vertebrae. The single sacrum, which is also part of the pelvis, is formed by the fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results from the fusion of four small coccygeal vertebrae. However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age.

Curvatures of the Vertebral Column

The adult vertebral column does not form a straight line but instead has four curvatures along its length. These curves increase the vertebral column’s strength, flexibility, and ability to absorb shock. When the load on the spine is increased, by carrying a heavy backpack, for example, the curvatures increase in depth (become more curved) to accommodate the extra weight. The curvature returns to normal shape when the weight is removed. The four adult curvatures are classified as either primary or secondary curvatures. Primary curves are retained from the original fetal curvature, while secondary curvatures develop after birth.

During fetal development, the body is flexed anteriorly into the fetal position, giving the entire vertebral column a single curvature that is concave anteriorly. In the adult, this fetal curvature is retained in two regions of the vertebral column as the thoracic curve, which involves the thoracic vertebrae, and the sacrococcygeal curve, formed by the sacrum and coccyx. Each of these is thus called a primary curve because they are retained from the original fetal curvature of the vertebral column.

Figure 7.25a (Left) image of baby and highlight primary curvature, Figure 7.25b (right) images of curvatures in adult vertebral column (By CNX OpenStax — http://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=49935127) ()

A secondary curve develops gradually after birth as the child learns to sit upright, stand, and walk and the bone develops in response to the forces put upon it. Secondary curves are concave posteriorly, opposite in direction to the original fetal curvature. The cervical curve of the neck region develops as the infant begins to hold their head upright when sitting. Later, as the child begins to stand and then to walk, the lumbar curve of the lower back develops. In adults, the lumbar curve is generally deeper in females.

Disorders associated with the curvature of the spine include kyphosis (excessive posterior curvature of the thoracic region), lordosis (excessive anterior curvature of the lumbar region), and scoliosis (an abnormal, lateral curvature, accompanied by twisting of the vertebral column).

Developmental anomalies, pathological changes, or obesity can enhance the normal vertebral column curves, resulting in the development of abnormal or excessive curvatures. Kyphosis, also referred to as humpback or hunchback, is an excessive posterior curvature of the thoracic region. This can develop when osteoporosis causes weakening and erosion of the anterior portions of the upper thoracic vertebrae, resulting in their gradual collapse. Lordosis, or swayback, is an excessive anterior curvature of the lumbar region and is most commonly associated with obesity or late pregnancy. The accumulation of body weight in the abdominal region results in an anterior shift in the line of gravity that carries the weight of the body. This causes an anterior tilt of the pelvis and a pronounced enhancement of the lumbar curve.

Scoliosis is an abnormal, lateral curvature, accompanied by twisting of the vertebral column. Compensatory curves may also develop in other areas of the vertebral column to help maintain the head positioned over the feet. Scoliosis is the most common vertebral abnormality among girls. The cause is usually unknown, but it may result from weakness of the back muscles, defects such as differential growth rates in the right and left sides of the vertebral column, or differences in the length of the lower limbs. When present, scoliosis tends to get worse during adolescent growth spurts. Although most individuals do not require treatment, a back brace may be recommended for growing children. In extreme cases, surgery may be required.

Excessive vertebral curves can be identified while an individual stands in the anatomical position when compared to a plumb line. The vertebral profile can be observed from the side and then from behind to check for kyphosis or lordosis. Scoliosis is best detected when flexion at the waist reveals that the right and left sides of the back are not level with each other in the bent position.

Figure 7.26 Abnormal curvatures of the vertebral column. (By OpenStax College — Anatomy and Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC By 3.0, https://commons.wikimedia.org/w/index.php?curid=30131448)

Intervertebral Discs of the Vertebral Column

The bodies of adjacent vertebrae are strongly anchored to each other by an intervertebral disc. An intervertebral disc is a fibrocartilaginous pad that fills the gap between adjacent vertebral bodies. This structure provides padding between the vertebrae during weight-bearing. Because it can change shape, this allows for slight movement between the vertebrae. Although the total amount of movement available between any two adjacent vertebrae is small, when these movements are summed together along the entire length of the vertebral column, large body movements can be produced.

Figure 7.28 Intervertebral Disc The bodies of adjacent vertebrae are separated and united by an intervertebral disc, which provides padding and allows for movements between adjacent vertebrae. The disc consists of a fibrous outer layer called the annulus fibrosus and a gel-like center called the nucleus pulposus. The intervertebral foramen is the opening formed between adjacent vertebrae for the exit of a spinal nerve. (Note: there is a typo in the figure: Annulus is correct spelling).

Each disc is anchored to the bodies of its adjacent vertebrae, thus strongly uniting these. The intervertebral discs are thin in the cervical region and thickest in the lumbar region, which carries the most bodyweight. In total, intervertebral discs account for approximately 25 percent of body height between the top of the pelvis and the base of the skull.

Each intervertebral disc consists of two parts. The annulus fibrosus is the tough, fibrous outer layer of the disc. It forms a circle (“annulus” = ring) and is firmly anchored to the outer margins of the adjacent vertebral bodies. Inside is the nucleus pulposus, consisting of a softer, more gel-like material. It has a high water content that serves to resist compression and thus is important for weight-bearing. With increasing age, the water content of the nucleus pulposus gradually declines. This causes the disc to become thinner, decreasing total body height slightly over time, and reduces the flexibility and range of motion of the disc, making movement more difficult.

The gel-like nature of the nucleus pulposus also allows the intervertebral disc to change shape as one vertebra rocks side to side or forward and back in relation to its neighbors during movements of the vertebral column. Thus, bending forward causes compression of the anterior portion of the disc but expansion of the posterior disc. If the posterior annulus fibrosus is weakened due to injury or increasing age, the pressure exerted on the disc when bending forward and lifting a heavy object can cause the nucleus pulposus to protrude posteriorly through the annulus fibrosus, resulting in a herniated disc (ruptured or slipped disc) (Figure 7.28). The posterior bulging of the nucleus pulposus can cause compression of a spinal nerve at the point where it exits through the intervertebral foramen, with resulting pain and/or muscle weakness in those body regions supplied by that nerve. The most common sites for disc herniation are the L4/L5 or L5/S1 intervertebral discs, which can cause sciatica, a widespread pain that radiates from the lower back down the thigh and into the leg. Similar injuries of the C5/C6 or C6/C7 intervertebral discs, following forcible hyperflexion of the neck from a collision accident or football injury, can produce pain in the neck, shoulder, and upper limb.

Fig. 7.29 Herniated Intervertebral Disc Weakening of the annulus fibrosus can result in herniation (protrusion) of the nucleus pulposus and compression of a spinal nerve, resulting in pain and/or muscle weakness in the body regions supplied by that nerve. Note that there is a typo in the figure: Annulus is correct spelling.

Ligaments of the Vertebral Column

Ligaments that extend along the length of the vertebral column also contribute to its overall support and stability. Adjacent vertebrae are united by ligaments that run the length of the vertebral column along both its posterior and anterior aspects. These serve to resist excess forward or backward bending movements of the vertebral column, respectively.

The anterior longitudinal ligament runs down the anterior side of the entire vertebral column, uniting the vertebral bodies. It serves to resist excess backward bending (extension) of the vertebral column. Protection against this movement is particularly important in the neck, where extreme posterior bending of the head and neck can stretch or tear this ligament, resulting in a painful whiplash injury. Before the mandatory installation of seat headrests, whiplash injuries were common for passengers involved in a rear-end automobile collision.

The supraspinous ligament is located on the posterior side of the vertebral column, where it interconnects the spinous processes of the thoracic and lumbar vertebrae. This strong ligament supports the vertebral column during forward bending motions. In the posterior neck, where the cervical spinous processes are short, the supraspinous ligament expands to become the nuchal ligament (nuchae = “nape” or “back of the neck”). The nuchal ligament is attached to the cervical spinous processes and extends upward and posteriorly to attach to the midline base of the skull, out to the external occipital protuberance. It supports the skull and prevents it from falling forward.

Additional ligaments are located inside the vertebral canal, next to the spinal cord, along the length of the vertebral column. The posterior longitudinal ligament is found anterior to the spinal cord, where it is attached to the posterior sides of the vertebral bodies. Posterior to the spinal cord is the ligamentum flavum (“yellow ligament”). This consists of a series of short, paired ligaments, each of which interconnects the lamina regions of adjacent vertebrae. The ligamentum flavum has large numbers of elastic fibers, which have a yellowish color, allowing it to stretch and then pull back. Both of these ligaments provide important support for the vertebral column when bending forward.

Fig. 7.30 Ligaments of Vertebral Column The anterior longitudinal ligament runs the length of the vertebral column, uniting the anterior sides of the vertebral bodies. The supraspinous ligament connects the spinous processes of the thoracic and lumbar vertebrae. In the posterior neck, the supraspinous ligament enlarges to form the nuchal ligament, which attaches to the cervical spinous processes and the base of the skull.

General Structure of the Vertebrae

Within the different regions of the vertebral column, vertebrae vary in size and shape, but they all follow a similar structural pattern. A typical vertebra will consist of a body, a vertebral arch, and seven processes.

The vertebral body is the anterior portion of each vertebra and this is the part that supports the body weight. Because of this, the vertebral bodies progressively increase in size and thickness going down the vertebral column. One of the most common bone fractures associated with the loss of bone density is the vertebral compression fracture when too much pressure is placed on a weakened vertebral body. Local foods like salmon and tofu can increase your intake of calcium and contribute to better bone health. The vertebral arch forms the posterior portion of each vertebra. It consists of four parts, the right and left pedicles and the right and left laminae. Each pedicle forms one of the lateral sides of the vertebral arch. The pedicles are anchored to the posterior side of the vertebral body. Each lamina forms part of the posterior roof of the vertebral arch. The large opening between the vertebral arch and body is the vertebral foramen, which contains the spinal cord. In the intact vertebral column, the vertebral foramina of all of the vertebrae align to form the vertebral (spinal) canal, which serves as the bony protection and passageway for the spinal cord down the back. When the vertebrae are aligned together in the vertebral column, notches in the margins of the pedicles of adjacent vertebrae together form an intervertebral foramen, the opening through which a spinal nerve exits from the vertebral column.

Seven processes arise from the vertebral arch (Figure 7.30). The single spinous process (vertebral spine) projects posteriorly at the midline of the back. The vertebral spines can easily be felt like a series of bumps just under the skin down the middle of the back. Each paired transverse process projects laterally and arises from the junction point between the pedicle and lamina. The transverse and spinous processes serve as important muscle attachment sites. A superior articular process extends or faces upward, and an inferior articular process faces or projects downward on each side of a vertebra. The paired superior articular processes of one vertebra join with the corresponding paired inferior articular processes from the next higher vertebra. These junctions form slightly moveable joints between the adjacent vertebrae. The shape and orientation of the articular processes vary in different regions of the vertebral column and play a major role in determining the type and range of motion available in each region.

Fig. 7.31 Parts of a Typical Vertebra A typical vertebra consists of a body and a vertebral arch. The arch is formed by the paired pedicles and paired laminae. Arising from the vertebral arch are the transverse, spinous, superior articular, and inferior articular processes. The vertebral foramen provides for passage of the spinal cord. Each spinal nerve exits through an intervertebral foramen, located between adjacent vertebrae. Intervertebral discs unite the bodies of adjacent vertebrae.

Regional Vertebral Characteristics

In addition to the general characteristics of a typical vertebra described above, vertebrae also display characteristic size and structural features that vary between the different vertebral column regions. Cervical vertebrae are smaller than lumbar vertebrae due to differences in the proportion of body weight that each supports. For example, the cervical vertebrae must only support the weight of the skull and associated muscles, whereas the lumbar vertebrae support the weight of the entire upper body. Thoracic vertebrae have sites for rib attachment, and the vertebrae that give rise to the sacrum and coccyx have fused into single bones.

Cervical Vertebrae

The first cervical vertebra and the second cervical vertebra are unique among all vertebrae, each having a distinctive appearance (Figure 7.31). The first cervical (C1) vertebra is called the atlas because this is a vertebra that supports the skull on top of the vertebral column. In Greek mythology, Atlas was the god who supported the heavens on his shoulders. The C1 vertebra does not have a body or spinous process. Instead, it is ring-shaped, consisting of an anterior arch and a posterior arch. The transverse processes of the atlas are longer and extend more laterally than do the transverse processes of any other cervical vertebrae. The superior articular processes or facets face upward and are deeply curved for articulation with the occipital condyles on the base of the skull. These joints allow you to move your head when you nod to signify “yes”. The inferior articular processes or facets are flat and face downward to join with the superior articular processes of the C2 vertebra.

The second cervical (C2) vertebra is called the axis, because it serves as the axis for rotation when turning the head toward the right or left (the atlas and axis articulate to form a pivot joint; when shaking the head “no”, the atlas is rotating on the axis). The axis resembles typical cervical vertebrae in most respects but is easily distinguished by the dens (odontoid process), a bony projection that extends upward from the vertebral body. The dens join with the inner aspect of the anterior arch of the atlas, where it is held in place by a transverse ligament.

Fig. 7.32 Cervical Vertebrae A typical cervical vertebra has a small body, a bifid spinous process, transverse processes that have a transverse foramen and are curved for spinal nerve passage. The atlas (C1 vertebra) does not have a body or spinous process. It consists of an anterior and a posterior arch and elongated transverse processes. The axis (C2 vertebra) has the upward projecting dens, which articulates with the anterior arch of the atlas.

The remaining cervical vertebrae have several characteristic features that differentiate them from thoracic or lumbar vertebrae (Figure 7.31). Cervical vertebrae have a small body, reflecting the fact that they carry the least amount of body weight. Cervical vertebrae usually have a bifid (Y-shaped) spinous process (sometimes described as looking like a snake’s tongue). The spinous processes of the C3–C6 vertebrae are short, but the spine of C7 is much longer. The prominent C7 spine located at the base of the neck can be palpated during cervical flexion, when the chin is touching the chest, by running a finger over the vertebral column and feeling the bulge at the posterior, inferior aspect of the neck. The transverse processes of the cervical vertebrae are sharply curved (U-shaped) to allow for the passage of the cervical spinal nerves. Each transverse process also has an opening called the transverse foramen. The transverse foramina are present only in the cervical vertebrae, specifically. An important artery that supplies the brain ascends the neck by passing through these openings. The superior and inferior articular processes of the cervical vertebrae are flattened and largely face upward or downward, respectively.

Thoracic Vertebrae

The bodies of the thoracic vertebrae are larger than those of cervical vertebrae (Figure 7.32). The characteristic feature for a typical mid-thoracic vertebra is the spinous process, which is long and has a pronounced downward angle that causes it to overlap the next inferior vertebra. The superior articular processes of thoracic vertebrae face anteriorly and the inferior processes face posteriorly. These orientations are important determinants for the type and range of movements available to the thoracic region of the vertebral column.

Thoracic vertebrae have several additional articulation sites, each of which is called a facet, where a rib is attached. Most thoracic vertebrae have two facets located on the lateral sides of the body, each of which is called a costal facet (“costal” = rib). These are for articulation with the head (end) of a rib. An additional facet is located on the transverse process for articulation with the tubercle of a rib.

In the posterolateral view, the thoracic vertebra can resemble the head of a giraffe. In this analogy, the giraffe’s snout is the spinous process, the giraffe’s ears are the transverse processes, the horns are the superior articulating processes, and the giraffe’s wide lower jaw is the inferior articulating processes.

Fig. 7.33 Thoracic Vertebrae A typical thoracic vertebra is distinguished by the spinous process, which is long and projects downward to overlap the next inferior vertebra. It also has articulation sites (facets) on the vertebral body and a transverse process for rib attachment. Rib Articulation in Thoracic Vertebrae Thoracic vertebrae have superior and inferior articular facets on the vertebral body for articulation with the head of a rib, and a transverse process facet for articulation with the rib tubercle.

Lumbar Vertebrae

Lumbar vertebrae carry the greatest amount of body weight and are thus characterized by the large size and thickness of the vertebral body (Figure 7.33). Lumbar vertebrae have short transverse processes and a short, blunt spinous process that projects posteriorly. The articular processes are large, with the superior process facing backward and the inferior facing forward.

When observing a lateral view of the lumbar vertebrae, the shape can be compared to that of a moose’s head. The moose’s nose represents the blunt lumbar spinous process, the moose’s horns are the superior articular processes and the moose’s furry chin (bell) represents the inferior articular process.

Fig. 7.34 Lumbar Vertebrae Lumbar vertebrae are characterized by having a large, thick body and a short, rounded spinous process.

Use this link to compare unique characteristics of each vertebral column region and view the bones, intervertebral discs, and ligaments of the vertebral column.

Cervical vertebra, lateral view: The lateral view of a typical cervical vertebra.	Thoracic vertebra: Image of a typical thoracic vertebra.
Lumbar vertebra: Image of typical lumbar vertebra.	Coccyx: Lateral view of coccyx shown beneath the sacrum.

Fig. 7.35 Vertebrae Comparison Chart

Sacrum and Coccyx

The sacrum is a triangular-shaped bone that is thick and wide across its superior base where it is weight-bearing and then tapers down to an inferior, non-weight-bearing apex (Figure 7.35). It is formed by the fusion of five sacral vertebrae, a process that does not begin until after the age of 20. On the anterior surface of the older adult sacrum, the lines of vertebral fusion can be seen as four transverse ridges. On the posterior surface, running down the midline, is the median sacral crest, a bumpy ridge that is the remnant of the fused spinous processes (median = “midline”; while medial = “toward, but not necessarily at, the midline”). Similarly, the fused transverse processes of the sacral vertebrae form the lateral sacral crest.

The sacral promontory is the anterior lip of the superior base of the sacrum. Lateral to this is the roughened auricular surface, which joins with the ilium portion of the hipbone to form the immobile sacroiliac joints of the pelvis. Passing inferiorly through the sacrum is a bony tunnel called the sacral canal, which terminates at the sacral hiatus near the inferior tip of the sacrum. The anterior and posterior surfaces of the sacrum have a series of paired openings called sacral foramina (singular = foramen) that connect to the sacral canal. Each of these openings is called a posterior (dorsal) sacral foramen or anterior (ventral) sacral foramen. These openings allow for the anterior and posterior branches of the sacral spinal nerves to exit the sacrum. The superior articular process of the sacrum, one of which is found on either side of the superior opening of the sacral canal, articulates with the inferior articular processes from the L5 vertebra.

The coccyx, or tailbone, is derived from the fusion of four very small coccygeal vertebrae (Figure 7.35). It articulates with the inferior tip of the sacrum. It is not weight-bearing in the standing position but may receive some bodyweight when sitting.

Fig. 7.36 Sacrum and Coccyx The sacrum is formed from the fusion of five sacral vertebrae, whose lines of fusion are indicated by the transverse ridges. The fused spinous processes form the median sacral crest, while the lateral sacral crest arises from the fused transverse processes. The coccyx is formed by the fusion of four small coccygeal vertebrae.

Thoracic Cage

The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of 12 pairs of ribs with their costal cartilages and the sternum (Figure 7.36). The ribs are anchored posteriorly to the 12 thoracic vertebrae, T1 to T12. One primary function of the thoracic cage is to protect the heart and lungs.

Fig. 7.37 Thoracic Cage The thoracic cage is formed by the (a) sternum and (b) 12 pairs of ribs with their costal cartilages. The ribs are anchored posteriorly to the 12 thoracic vertebrae. The sternum consists of the manubrium, body, and xiphoid process. The ribs are classified as true ribs (1–7) and false ribs (8–12). The last two pairs of false ribs are also known as floating ribs (11–12).

Sternum

The sternum is the flat, long bone in the center of the anterior thoracic cage (Figure 7.36). It is divided into three parts: the manubrium, body, and xiphoid process. The manubrium is the wide, superior portion of the sternum that articulates with the clavicles. This articulation is at the top of the manubrium and is marked by a shallow, U-shaped border called the jugular (suprasternal) notch. You can easily find this bony landmark at the anterior base of the neck, between the medial ends of the clavicles. The clavicular notch is the shallow depression located on either side at the superior-lateral margins of the manubrium. This is the site of the sternoclavicular joint, between the sternum and clavicle.

The elongated, central portion of the sternum is the body. The body is the largest part of the sternum and the sides of the body have notches where it articulates with the costal cartilages of the ribs. The manubrium and body join together at the sternal angle, so-called because the junction between these two components is not flat, but forms a slight bend. The sternal angle is commonly used as a landmark when palpating, as it is at the level of the second pair of ribs. The first ribs also attach to the manubrium and it is hidden behind the clavicle. Therefore, the second rib is the highest rib that can be identified by palpation. Thus, the sternal angle and second rib are important landmarks for the identification and counting of the lower ribs during a clinical examination. Ribs 2 to 7 are attached to the sternal body through their costal cartilages.

The inferior tip of the sternum is the xiphoid process. This small structure is cartilaginous early in life but gradually becomes ossified, starting during middle age.

Clinical Application

When administering chest compressions during cardiopulmonary resuscitation (CPR), hand placement is important as any pressure on the xiphoid process should be avoided because it can cause the xiphoid process to break off, resulting in punctures or lacerations to the soft tissue below it.

Cardiopulmonary Resuscitation (CPR) – Administering Chest Compressions	Xiphoid Process

 Fig. 7.38 Cardiopulmonary Resuscitation (CPR) Technique

Ribs

We have 12 pairs of ribs. Each rib is a curved and flat bone that contributes to the wall of the thorax. The ribs articulate posteriorly with the T1 to T12 thoracic vertebrae, and most attach anteriorly via their costal cartilages to the sternum. The ribs are numbered from 1 to 12 per the thoracic vertebrae. With an exception to Ribs 11 and 12, each rib ends in a costal cartilage. These cartilages are made of hyaline cartilage and serve as an attachment site to the sternum.

The 12 pairs of ribs are commonly divided into groups based on how they articulate with the sternum: true ribs and false ribs (Figure 7.36 above). Ribs 1 to 7 are classified as true ribs (vertebrosternal ribs) where their costal cartilages attach directly to the sternum. Ribs 8 to 12 are called false ribs (vertebrochondral ribs) where their costal cartilages do not attach directly to the sternum. False ribs are further divided into groups. For ribs 8 to 10, the costal cartilages are attached to the cartilage of the next higher rib. The last two false ribs (11 and 12) are also called floating ribs (vertebral ribs) with no attachment to the sternum at all. Instead, their small costal cartilages terminate within the musculature of the lateral abdominal wall.

Parts of a Typical Rib

Each rib is classified as a flat bone (Figure 7.37). The longest part of the rib is called the body, or shaft, and it connects the two ends of the rib. The two ends of the rib look different from each other. The posterior end of a typical rib is called the head of the rib. The head has two flat surfaces that articulate primarily with the costal facet located on the body of the same numbered thoracic vertebra. Lateral to the head is the narrowed neck of the rib. A small bump on the posterior rib surface is the tubercle of the rib, which articulates with the facet located on the transverse process of the same numbered thoracic vertebra. Just lateral to the tubercle is the angle of the rib, the point at which the rib has its greatest degree of curvature. The angles of the ribs form the most posterior extent of the thoracic cage. In the anatomical position, the angles align with the medial border of the scapula. A shallow costal groove for the passage of blood vessels and a nerve is found along the inferior margin of each rib.

Fig. 7.39 Bony Landmarks of Rib

Chapter Summary

Key Terms

alveolar process of the mandible

upper border of mandibular body that contains the lower teeth

alveolar process of the maxilla

curved, inferior margin of the maxilla that supports and anchors the upper teeth

angle of the mandible

rounded corner located at outside margin of the body and ramus junction

angle of the rib

portion of rib with greatest curvature; together, the rib angles form the most posterior extent of the thoracic cage

anterior (ventral) sacral foramen

one of the series of paired openings located on the anterior (ventral) side of the sacrum

anterior arch

anterior portion of the ring-like C1 (atlas) vertebra

anterior cranial fossa

shallowest and most anterior cranial fossa of the cranial base that extends from the frontal bone to the lesser wing of the sphenoid bone

anterior longitudinal ligament

ligament that runs the length of the vertebral column, uniting the anterior aspects of the vertebral bodies

appendicular skeleton

all bones of the upper and lower limbs, plus the girdle bones that attach each limb to the axial skeleton

articular tubercle

smooth ridge located on the inferior skull, immediately anterior to the mandibular fossa

atlas

first cervical (C1) vertebra

axial skeleton

central, vertical axis of the body, including the skull, vertebral column, and thoracic cage

axis

second cervical (C2) vertebra

brain case

portion of the skull that contains and protects the brain, consisting of the eight bones that form the cranial base and rounded upper skull

calvaria

(also, skullcap) rounded top of the skull

carotid canal

zig-zag tunnel providing passage through the base of the skull for the internal carotid artery to the brain; begins anteromedial to the styloid process and terminates in the middle cranial cavity, near the posterior-lateral base of the sella turcica

cervical curve

posteriorly concave curvature of the cervical vertebral column region; a secondary curve of the vertebral column

cervical vertebrae

seven vertebrae numbered as C1–C7 that are located in the neck region of the vertebral column

clavicular notch

paired notches located on the superior-lateral sides of the sternal manubrium, for articulation with the clavicle

coccyx

small bone located at inferior end of the adult vertebral column that is formed by the fusion of four coccygeal vertebrae; also referred to as the “tailbone”

condylar process of the mandible

thickened upward projection from posterior margin of mandibular ramus

condyle

oval-shaped process located at the top of the condylar process of the mandible

coronal suture

joint that unites the frontal bone to the right and left parietal bones across the top of the skull

coronoid process of the mandible

flattened upward projection from the anterior margin of the mandibular ramus

costal cartilage

hyaline cartilage structure attached to the anterior end of each rib that provides for either direct or indirect attachment of most ribs to the sternum

costal facet

site on the lateral sides of a thoracic vertebra for articulation with the head of a rib

cranial cavity

interior space of the skull that houses the brain

cranium

skull

cribriform plate

small, flattened areas with numerous small openings, located to either side of the midline in the floor of the anterior cranial fossa; formed by the ethmoid bone

crista galli

small upward projection located at the midline in the floor of the anterior cranial fossa; formed by the ethmoid bone

dens

bony projection (odontoid process) that extends upward from the body of the C2 (axis) vertebra

ear ossicles

three small bones located in the middle ear cavity that serve to transmit sound vibrations to the inner ear

ethmoid air cell

one of several small, air-filled spaces located within the lateral sides of the ethmoid bone, between the orbit and upper nasal cavity

ethmoid bone

unpaired bone that forms the roof and upper, lateral walls of the nasal cavity, portions of the floor of the anterior cranial fossa and medial wall of orbit, and the upper portion of the nasal septum

external acoustic meatus

ear canal opening located on the lateral side of the skull

external occipital protuberance

small bump located at the midline on the posterior skull

facet

small, flattened area on a bone for an articulation (joint) with another bone, or for muscle attachment

facial bones

fourteen bones that support the facial structures and form the upper and lower jaws and the hard palate

false ribs

vertebrochondral ribs 8–12 whose costal cartilage either attaches indirectly to the sternum via the costal cartilage of the next higher rib or does not attach to the sternum at all

floating ribs

vertebral ribs 11–12 that do not attach to the sternum or to the costal cartilage of another rib

fontanelle

expanded area of fibrous connective tissue that separates the brain case bones of the skull prior to birth and during the first year after birth

foramen lacerum

irregular opening in the base of the skull, located inferior to the exit of carotid canal

foramen magnum

large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium

foramen rotundum

round opening in the floor of the middle cranial fossa, located between the superior orbital fissure and foramen ovale

foramen spinosum

small opening in the floor of the middle cranial fossa, located lateral to the foramen ovale

frontal bone

unpaired bone that forms forehead, roof of orbit, and floor of anterior cranial fossa

frontal sinus

air-filled space within the frontal bone; most anterior of the paranasal sinuses

glabella

slight depression of frontal bone, located at the midline between the eyebrows

hard palate

bony structure that forms the roof of the mouth and floor of the nasal cavity, formed by the palatine process of the maxillary bones and the horizontal plate of the palatine bones

head of the rib

posterior end of a rib that articulates with the bodies of thoracic vertebrae

horizontal plate

medial extension from the palatine bone that forms the posterior quarter of the hard palate

hyoid bone

small, U-shaped bone located in upper neck that does not contact any other bone

hypoglossal canal

paired openings that pass anteriorly from the anterior-lateral margins of the foramen magnum deep to the occipital condyles

hypophyseal (pituitary) fossa

shallow depression on top of the sella turcica that houses the pituitary (hypophyseal) gland

inferior articular process

bony process that extends downward from the vertebral arch of a vertebra that articulates with the superior articular process of the next lower vertebra

inferior nasal concha

one of the paired bones that project from the lateral walls of the nasal cavity to form the largest and most inferior of the nasal conchae

infraorbital foramen

opening located on anterior skull, below the orbit

internal acoustic meatus

opening into petrous ridge, located on the lateral wall of the posterior cranial fossa

intervertebral disc

structure located between the bodies of adjacent vertebrae that strongly joins the vertebrae; provides padding, weight bearing ability, and enables vertebral column movements

intervertebral foramen

opening located between adjacent vertebrae for exit of a spinal nerve

jugular (suprasternal) notch

shallow notch located on superior surface of sternal manubrium

jugular foramen

irregularly shaped opening located in the lateral floor of the posterior cranial cavity

kyphosis

(also, humpback or hunchback) excessive posterior curvature of the thoracic vertebral column region

lacrimal bone

paired bones that contribute to the anterior-medial wall of each orbit

lacrimal fossa

shallow depression in the anterior-medial wall of the orbit, formed by the lacrimal bone that gives rise to the nasolacrimal canal

lambdoid suture

inverted V-shaped joint that unites the occipital bone to the right and left parietal bones on the posterior skull

lamina

portion of the vertebral arch on each vertebra that extends between the transverse and spinous process

lateral sacral crest

paired irregular ridges running down the lateral sides of the posterior sacrum that was formed by the fusion of the transverse processes from the five sacral vertebrae

lesser wings of the sphenoid bone

lateral extensions of the sphenoid bone that form the bony lip separating the anterior and middle cranial fossae

ligamentum flavum

series of short ligaments that unite the lamina of adjacent vertebrae

lingula

small flap of bone located on the inner (medial) surface of mandibular ramus, next to the mandibular foramen

lordosis

(also, swayback) excessive anterior curvature of the lumbar vertebral column region

lumbar curve

posteriorly concave curvature of the lumbar vertebral column region; a secondary curve of the vertebral column

lumbar vertebrae

five vertebrae numbered as L1–L5 that are located in lumbar region (lower back) of the vertebral column

mandible

unpaired bone that forms the lower jaw bone; the only moveable bone of the skull

mandibular foramen

opening located on the inner (medial) surface of the mandibular ramus

mandibular fossa

oval depression located on the inferior surface of the skull

mandibular notch

large U-shaped notch located between the condylar process and coronoid process of the mandible

manubrium

expanded, superior portion of the sternum

mastoid process

large bony prominence on the inferior, lateral skull, just behind the earlobe

maxillary bone

(also, maxilla) paired bones that form the upper jaw and anterior portion of the hard palate

maxillary sinus

air-filled space located with each maxillary bone; largest of the paranasal sinuses

median sacral crest

irregular ridge running down the midline of the posterior sacrum that was formed from the fusion of the spinous processes of the five sacral vertebrae

mental foramen

opening located on the anterior-lateral side of the mandibular body

mental protuberance

inferior margin of anterior mandible that forms the chin

middle cranial fossa

centrally located cranial fossa that extends from the lesser wings of the sphenoid bone to the petrous ridge

middle nasal concha

nasal concha formed by the ethmoid bone that is located between the superior and inferior conchae

mylohyoid line

bony ridge located along the inner (medial) surface of the mandibular body

nasal bone

paired bones that form the base of the nose

nasal cavity

opening through skull for passage of air

nasal conchae

curved bony plates that project from the lateral walls of the nasal cavity; include the superior and middle nasal conchae, which are parts of the ethmoid bone, and the independent inferior nasal conchae bone

nasal septum

flat, midline structure that divides the nasal cavity into halves, formed by the perpendicular plate of the ethmoid bone, vomer bone, and septal cartilage

nasolacrimal canal

passage for drainage of tears that extends downward from the medial-anterior orbit to the nasal cavity, terminating behind the inferior nasal conchae

neck of the rib

narrowed region of a rib, next to the rib head

nuchal ligament

expanded portion of the supraspinous ligament within the posterior neck; interconnects the spinous processes of the cervical vertebrae and attaches to the base of the skull

nucleus pulposus

gel-like central region of an intervertebral disc; provides for padding, weight-bearing, and movement between adjacent vertebrae

occipital bone

unpaired bone that forms the posterior portions of the brain case and base of the skull

occipital condyle

paired, oval-shaped bony knobs located on the inferior skull, to either side of the foramen magnum

optic canal

opening spanning between middle cranial fossa and posterior orbit

orbit

bony socket that contains the eyeball and associated muscles

palatine bone

paired bones that form the posterior quarter of the hard palate and a small area in floor of the orbit

palatine process

medial projection from the maxilla bone that forms the anterior three quarters of the hard palate

paranasal sinuses

cavities within the skull that are connected to the conchae that serve to warm and humidify incoming air, produce mucus, and lighten the weight of the skull; consist of frontal, maxillary, sphenoidal, and ethmoidal sinuses

parietal bone

paired bones that form the upper, lateral sides of the skull

pedicle

portion of the vertebral arch that extends from the vertebral body to the transverse process

perpendicular plate of the ethmoid bone

downward, midline extension of the ethmoid bone that forms the superior portion of the nasal septum

petrous ridge

petrous portion of the temporal bone that forms a large, triangular ridge in the floor of the cranial cavity, separating the middle and posterior cranial fossae; houses the middle and inner ear structures

posterior (dorsal) sacral foramen

one of the series of paired openings located on the posterior (dorsal) side of the sacrum

posterior arch

posterior portion of the ring-like C1 (atlas) vertebra

posterior cranial fossa

deepest and most posterior cranial fossa; extends from the petrous ridge to the occipital bone

posterior longitudinal ligament

ligament that runs the length of the vertebral column, uniting the posterior sides of the vertebral bodies

primary curve

anteriorly concave curvatures of the thoracic and sacrococcygeal regions that are retained from the original fetal curvature of the vertebral column

ramus of the mandible

vertical portion of the mandible

ribs

thin, curved bones of the chest wall

sacral canal

bony tunnel that runs through the sacrum

sacral foramina

series of paired openings for nerve exit located on both the anterior (ventral) and posterior (dorsal) aspects of the sacrum

sacral hiatus

inferior opening and termination of the sacral canal

sacral promontory

anterior lip of the base (superior end) of the sacrum

sacrococcygeal curve

anteriorly concave curvature formed by the sacrum and coccyx; a primary curve of the vertebral column

sacrum

single bone located near the inferior end of the adult vertebral column that is formed by the fusion of five sacral vertebrae; forms the posterior portion of the pelvis

sagittal suture

joint that unites the right and left parietal bones at the midline along the top of the skull

scoliosis

abnormal lateral curvature of the vertebral column

secondary curve

posteriorly concave curvatures of the cervical and lumbar regions of the vertebral column that develop after the time of birth

sella turcica

elevated area of sphenoid bone located at midline of the middle cranial fossa

septal cartilage

flat cartilage structure that forms the anterior portion of the nasal septum

skeleton

bones of the body

skull

bony structure that forms the head, face, and jaws, and protects the brain; consists of 22 bones

sphenoid bone

unpaired bone that forms the central base of skull

sphenoid sinus

air-filled space located within the sphenoid bone; most posterior of the paranasal sinuses

squamous suture

joint that unites the parietal bone to the squamous portion of the temporal bone on the lateral side of the skull

sternal angle

junction line between manubrium and body of the sternum and the site for attachment of the second rib to the sternum

sternum

flattened bone located at the center of the anterior chest

styloid process

downward projecting, elongated bony process located on the inferior aspect of the skull

stylomastoid foramen

opening located on inferior skull, between the styloid process and mastoid process

superior articular process

bony process that extends upward from the vertebral arch of a vertebra that articulates with the inferior articular process of the next higher vertebra

superior articular process of the sacrum

paired processes that extend upward from the sacrum to articulate (join) with the inferior articular processes from the L5 vertebra

superior nasal concha

smallest and most superiorly located of the nasal conchae; formed by the ethmoid bone

superior nuchal line

paired bony lines on the posterior skull that extend laterally from the external occipital protuberance

superior orbital fissure

irregularly shaped opening between the middle cranial fossa and the posterior orbit

supraorbital foramen

opening located on anterior skull, at the superior margin of the orbit

supraorbital margin

superior margin of the orbit

supraspinous ligament

ligament that interconnects the spinous processes of the thoracic and lumbar vertebrae

suture

junction line at which adjacent bones of the skull are united by fibrous connective tissue

temporal bone

paired bones that form the lateral, inferior portions of the skull, with squamous, mastoid, and petrous portions

temporal fossa

shallow space on the lateral side of the skull, above the level of the zygomatic arch

temporal process of the zygomatic bone

short extension from the zygomatic bone that forms the anterior portion of the zygomatic arch

thoracic cage

consists of 12 pairs of ribs and sternum

thoracic curve

anteriorly concave curvature of the thoracic vertebral column region; a primary curve of the vertebral column

thoracic vertebrae

twelve vertebrae numbered as T1–T12 that are located in the thoracic region (upper back) of the vertebral column

transverse foramen

opening found only in the transverse processes of cervical vertebrae

transverse process

paired bony processes that extends laterally from the vertebral arch of a vertebra

true ribs

vertebrosternal ribs 1–7 that attach via their costal cartilage directly to the sternum

tubercle of the rib

small bump on the posterior side of a rib for articulation with the transverse process of a thoracic vertebra

vertebra

individual bone in the neck and back regions of the vertebral column

vertebral (spinal) canal

bony passageway within the vertebral column for the spinal cord that is formed by the series of individual vertebral foramina

vertebral arch

bony arch formed by the posterior portion of each vertebra that surrounds and protects the spinal cord

vertebral column

entire sequence of bones that extend from the skull to the tailbone

vertebral foramen

opening associated with each vertebra defined by the vertebral arch that provides passage for the spinal cord

vomer bone

unpaired bone that forms the inferior and posterior portions of the nasal septum

xiphoid process

small process that forms the inferior tip of the sternum

zygomatic arch

elongated, free-standing arch on the lateral skull, formed anteriorly by the temporal process of the zygomatic bone and posteriorly by the zygomatic process of the temporal bone

zygomatic bone

cheekbone; paired bones that contribute to the lateral orbit and anterior zygomatic arch

zygomatic process of the temporal bone

extension from the temporal bone that forms the posterior portion of the zygomatic arch

Sources

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(http://www.ulukau.org/elib/cgi-bin/library?e=d-0qlcc1-000Sec–01en-50-20-frameset-book–1-010escapewin&a=d&d=D0.3.183&toc=0)