Axial Skeleton

Axial Skeleton

Skeletal Organization

The skeleton is divided into two major portions: the axial skeleton (FIGURE 7-16 ) and the appendicular skeleton. Including those of the middle ear, there are 206 bones in the human body.

Axial Skeleton

The axial skeleton supports and protects the head, neck, and trunk. It includes the skull, hyoid bone (a single bone in the neck that supports the tongue and its muscles), vertebral column, and thoracic cage.

Skull

The human skull is made up of 22 firmly interlocked bones. These are divided into the cranium and the facial bones. The cranium is made up of eight bones and the face is made up of 14. The lines where the bones of the skull lock together are called sutures. The only movable bone in the skull is the mandible (lower jaw), which is attached to the cranium by ­ligaments. The cranium houses and protects the brain. ­Air-filled spaces inside the cranial bones called paranasal sinuses help the voice to resonate and also reduce the weight of the skull. The cranial bones enclose the chamber that supports the brain, which is known as the cranial cavity. FIGURES 7-17A and 7-17B show various views of the human skull and its bones.

Cranium

The cranium consists of eight bones

■■ Frontal bone: This bone forms the anterior skull above the eyes, with each eye orbit (the eye socket) aving a supraorbital foramen (notch). Blood vessels and nerves pass through this structure to the forehead tissues. The frontal bone contains two frontal sinuses above the central part of the eyes. The frontal squama is also known as the forehead, forming the anterior, superior portion of the cranium. It provides a surface area where the facial muscles attach. The lacrimal fossa is a shallow depression marking the location of the lacrimal (tear) gland.

■■ Parietal bones: Located on each side of the skull behind the frontal bone, these two bones form the sides and roof of the cranium and are fused in the middle along the sagittal suture. They meet the frontal bone along the coronal suture.

■■ Occipital bone: Joining the parietal bones along the lambdoid suture, the occipital bone forms the back of the skull and base of the cranium. A large opening at the lower portion of this bone (the foramen­ magnum) allows nerve fibers to pass through from the brain into the spinal cord. The jugular foramen is between the occipital and temporal bones and allows the internal jugular vein to pass through. Rounded occipital condyles on each side of the foramen magnum articulate with the first vertebra of the spine. The hypoglossal canals begin at each occipital condyle’s lateral base, ending on the inner surface of the occipital bone near the foramen magnum. The hypoglossal nerves pass through them. The ­external occipital protuberance is a small bump on the inferior surface, at the midline, of the occipital bone. The external occipital crest begins here, marking the attachment of a ligament that helps stabilize the neck vertebrae.

■■ Temporal bones: These two bones join the parietal bone on each side of the skull along the ­squamous suture, and form parts of the sides and base of the cranium. An opening called the external acoustic meatus leads through each temporal bone to the inner ear. The ­mandibular fossae are depressions that articulate with the mandible. Two projections below each external acoustic meatus (the mastoid process and the styloid process) provide points of attachment. The mastoid process attaches to certain neck muscles, and the styloid process attaches to muscles of the tongue and pharynx. The zygomatic process projects from the temporal bone to join the zygomatic bone, helping to form the cheek at the zygomatic arch. The ­squamous part of the temporal bone is convex and irregular, bordering the squamous suture. The petrous part of the temporal bone encloses the structures of the inner ear. The auditory ossicles are located inside the tympanic cavity (middle ear). They transfer sound vibrations from the eardrum to the inner ear. The carotid canal provides a passageway for the internal carotid artery of the brain. The foramen lacerum is thin, extending between the sphenoid and temporal bones, containing hyaline cartilage and small arteries supplying the inner surface of the cranium. The stylomastoid foramen is posterior to the base of the styloid process, allowing the facial nerve to pass through. The internal acoustic meatus is a canal that carries blood vessels and nerves to the inner ear as well as the facial nerve to the stylomastoid foramen.

■■ Sphenoid bone: This complex, bat-shaped bone forms part of the base of the cranium, sides of the skull, and floors and sides of the eye orbits (­FIGURE 7-18). The eye orbits are actually formed by seven bones in total, known as the orbital complexes. Each orbital complex consists of portions of the sphenoid, frontal, maxilla, lacrimal, ethmoid, palatine, and zygomatic bones. The superior­ portion of the sphenoid bone has an indentation that forms the sella turcica (Turk’s saddle), which contains the pituitary gland in its “seat” known as the hypophyseal fossa. It is considered the cranium’s keystone because it articulates with all other cranial bones. The sphenoid bone is described as having a central body and three pairs of processes known as the greater wings, lesser wings, and pterygoid processes. The body of the sphenoid bone houses two ­sphenoidal sinuses . The greater wings project laterally from the body of the sphenoid bone. They form parts of the middle cranial fossa, the posterior orbit walls, and the external skull wall. At this final point, they are flag-shaped, areas medial to the zygomatic arch. The sphenoid bone’s lesser wings resemble horns and form part of the anterior cranial fossa’s floor as well as part of the orbits’ medial walls. The pterygoid processes are narrow depressions, projecting inferiorly from where the body and greater wings join. They anchor the chewing-­ related pterygoid muscles. The sphenoid bone has many openings. The optic canals, anterior to the sella turcica, allow the optic nerves to reach the eyes. A crescent- shaped row of four open-ings lies on each side of the sphenoid body. The most anterior opening is the superior orbital fissure, which appears as a long slit between the greater and lesser wings. This fissure allows cra-nial nerves III, IV, and VI to enter the orbit. It can be easily seen in an anterior skull view. Two other openings, the foramen rotundum and foramen ovale , create passageways for cranial nerve V to reach the face. The foramen rotundum­ is usually oval (not round) in shape, regardless of its name. The large foramen ovale lies posterior to the foramen rotundum and is also seen in an inferior view of the skull. The small foramen spi-nosum lies posterolateral to the foramen ovale and allows the middle meningeal artery to pass through, serving certain cranial bones. The sphe-noid bone serves as a bridge uniting the cranial and facial bones.

■■ Ethmoid bone: Located in front of the sphenoid bone, the ethmoid bone forms a mass on each side of the nasal cavity that is joined by thin cribriform plates that partially form the roof of the nasal cav-ity. Between the cribriform plates, a triangular pro-cess (the crista galli) attaches to membranes that enclose the brain. Parts of the ethmoid bone form pieces of the cranial floor, walls of the eye orbits, and walls of the nasal ­cavity. A ­perpendicular plate forms most of the nasal ­septum. The supe-rior nasal conchae and middle nasal conchae project inward toward the perpendicular plate, with the lateral ethmoid bone ­containing many ethmoidal sinuses (FIGURE 7-19). The lateral masses contain the ethmoidal labyrinth, which has interconnected ethmoidal air cells opening into the nasal cavity on either side. The olfactory foramina inside the crib-riform plate allow passage of the olfactory nerves.

Fontanels

In infants, the cranial bones are connected by fibrous membranes through fontanels (soft spots) that allow the cranium to slightly change shape. When the infant is born, the cranium compresses somewhat in order to pass through the birth canal. The fontanels­ eventually close as the cranium ossifies and the bones grow together. The skull of an infant fractures­ less easily than that of an adult. The two fontanels are termed anterior and posterior . The anterior fon-tanel closes at 18 months of age and the posterior­ ­fontanel closes at two months of age.

Facial Bones

The facial skeleton consists of the following 14 bones (Figures 7-17A and 7-17B):

■■ Maxillae: These two bones form the upper jaw, anterior roof of the mouth (hard palate), floors of the eye orbits, and the nasal cavity sides and floor. The maxillae contain the upper teeth sockets as well as the maxillary sinuses, which are the largest sinuses in the skull. The orbital rim protects the eye and other structures in the eye orbit. As the human body grows, palatine processes of the maxillae grow together and fuse to form the ­anterior hard palate. Along with the alveolar process, the alveolar arch (dental arch) is formed, where the teeth are bound via dense connective tissue. The nasolacrimal canal is formed by a maxilla and lacrimal bone, protecting the lacrimal sac and nasolacrimal duct, through which tears flow from the orbit to the nasal cavity. The infraorbital­ foramen allows passage of a major sensory nerve reaching the brain through the foramen rotundum of the sphenoid bone. Between the maxillae and sphenoid, the inferior orbital fissure allows passage of blood vessels and cranial nerves.

■■ Zygomatic bones: These two bones form the cheek prominences below the eyes as well as the ­lateral walls and floors of the eye orbits. A ­temporal process extends from the zygomatic bones to form a zygomatic arch. The zygomaticofacial foramen on each zygomatic bone’s anterior surface allows passage of a sensory nerve that innervates the cheek.

■■ Nasal bones: These two long, thin bones lie side by side, fusing at the midline to form the bridge of the nose. Flexible cartilages support the distal nose and extend along with soft tissues to the superior bor-der of the external nares, which are the entrances to the nasal cavity. The nasal complex consists of bones that enclose the nasal cavities as well as the paranasal sinuses. These bones include the fron-tal, sphenoid, ethmoid, maxilla, lacrimal, ethmoid, palatine, and inferior nasal conchae.

■■ Vomer bone: This thin, flat bone is found along the midline of the nasal cavity, joining the ethmoid bone to form the nasal septum.

■■ Inferior nasal conchae: These two bones are scroll-shaped, attached to the lateral nasal cavity walls, and support the mucous membranes of the cavity.

■■ Lacrimal bones: These two thin structures are located in the medial wall of each eye orbit between the maxillae and ethmoid bone. A groove along the anterior lateral surface, known as the lacrimal sulcus, marks the location of the lacrimal sac.

■■ Palatine bones: Located behind the maxillae, the two L-shaped palatine bones form the posterior hard palate and nasal cavity floor as well as the nasal cavity lateral walls. The horizontal plate actually forms the posterior hard palate, whereas the perpendicular plate extends from the horizontal plate to the orbital process, forming part of the floor of the orbit.

■■ Mandible: This horseshoe-shaped bone projects upward at each end with the mandibular condyle and coronoid process. These processes are separated by the mandibular notch. The mandible articulates with the temporal bone and provides attachments for the muscles needed for chewing. The curved alveolar arch contains the hollow sockets for the lower teeth. The mandible is the only movable bone of the facial skeleton. The body of the mandible is its horizontal portion, whereas the ramus is the ascending portion beginning at the mandibular angle on each side. The mental foramina open to allow nerves to pass through that carry sensory information from the lips and chin to the brain. The mandibular foramen is where the mandibular canal begins, which allows blood vessels and nerves to pass that service the lower teeth. The mandible articulates with the temporal bone at the mandibular fossa.

Hyoid Bone

The hyoid bone is unique in that it is not actually part of the skull and lies just below the mandible in the anterior neck. It actually resembles the man-dible in shape but is much smaller (FIGURE 7-20). It is the only bone in the body that does not artic-ulate directly with any other bone, but is instead anchored by thin stylohyoid ligaments to the sty-loid processes of the temporal bones. Its somewhat “horseshoe” shape consists of a body and two pairs of cornua (horns). The greater horns (cornua) help to support the larynx and attach to the tongue muscles. The lesser cornua are attached to the stylohyoid ligaments. The hyoid one is a movable base for the tongue, serving to provide attachment points for neck muscles that control laryngeal movements during speech and swallowing.

Spine

The vertical axis of the human skeleton is formed by the vertebral column (backbone), which extends from the skull to the pelvis. It is made up of 26 bony vertebrae, separated by intervertebral discs made of cushioning cartilage, connected by ligaments (­FIGURE 7-21). Each vertebra has a drum- shaped body, making up the thick anterior portion of the bone.

The head and trunk are supported by the vertebral column, which also protects the spinal cord. The spinal cord passes through a vertebral canal created by open-ings in the vertebrae. At the bottom of the backbone,some vertebrae are fused to form the sacrum (a part of the pelvis) and the coccyx (tailbone), which is attached to the end of the sacrum.

Vertebrae Structure

The vertebral body (centrum) is the area of a verte-bra that transfers weight along the vertebral column’s axis. Two short stalks (pedicles) project from each drum-shaped vertebra, with two plates called lam-inae that fuse to become a spinous process. These ­structures collectively form a bony vertebral arch around the vertebral­ foramen, where the spinal cord passes through. A transverse process projects pos-teriorly, attached to ligaments and muscles. Superior and inferior articular processes project upward and downward with cartilage coverings, joined to the ver-tebra above and below. Each articular process has a smooth concave surface known as an articular facet. Notches align with adjacent vertebrae forming open-ings (­intervertebral foramina) through which the spinal nerves pass.

Beginning with those located at the top of the spine with the others listed sequentially (moving down the spine), the vertebrae that make up the spine are:

■■ Cervical vertebrae: These seven structures com-prise the neck, with distinctive transverse pro-cesses and round transverse foramina, which allow the arteries leading to the brain to pass through. The forked processes of the second through to the fifth cervical vertebrae provide attachments for muscles. The atlas (first vertebra supports the head with two kidney-shaped facets articulating with the occipital condyles. It is dif-ferent from the other vertebrae, because it lacks a body and spinous process and has a large, round vertebral foramen that is bounded by anterior and posterior arches. The axis (second vertebra) has a process (the dens) that projects upward into the ring of the atlas. When the head turns side to side, the atlas pivots around the dens. A notched spinous process, such as those on the C₂ to C₆ vertebrae, is referred to as bifid. The transverse processes are fused laterally to the costal processes, originating near the ventrolateral portion of the vertebral body. A partial or complete dislocation of the cervical vertebrae may result from sudden acceleration or deceleration, such as in a car crash, causing an injury to the muscles, ligaments, and spinal cord that is referred to as whiplash. The last cervical vertebra (C₇) resembles the first thoracic vertebra (T₁). This rule is generally true where each different section of the vertebrae joins the next. C₇, the vertebra prominens, has a long, thin spinous process ending in a broad tubercle that can be felt through the skin at the base of the neck. The ligamentum nuchae is a thick elastic­ ligament that begins at C₇ and extends to insert along the skull’s occipital crest.

■■ Thoracic vertebrae: These 12 structures are larger than the cervical vertebrae and have long processes­ that slope downward to articulate with the ribs. The thoracic vertebrae increase in size down the spine to bear increasing loads of body weight. Each thoracic vertebra articulates with ribs along the body’s dorsolateral surfaces. The costal facets on the vertebral bodies articulate with the heads of the various ribs. The transverse processes of vertebrae, T₁ to T₁₀, are relatively thick. They contain transverse costal facts for rib articulation.

■■ Lumbar vertebrae: These five structures in the lower back are even larger than the thoracic ver-tebrae, supporting more body weight. The lum-bar vertebrae do not have costal facts, but their slender transverse processes project dorsolater-ally. They have a triangular vertebral foramen, with short spinous processes projecting dorsally. The superior articular processes face medially, whereas the inferior articular processes face laterally.

■■ Sacrum: This triangular structure forms the posterior­ wall of the pelvis. It contains five fused ­vertebrae, S₁ to S₅, and forms the vertebral column’s­ base ( FIGURE 7-22). The sacrum ­articulates superiorly via the superior articular processes with L₅ . It also articulates inferiorly with the coccyx. Its auricular surfaces are lat-erally articulated with the hip bones, forming the sacroiliac­ joints of the pelvis. The sacral promontory is the anterosuperior­ margin of the first sacral vertebra. It bulges anteriorly into the pelvic cavity. Its body’s center of gravity lies approximately 1 cm posterior to the sacral prom-ontory. Four transverse ridges cross the con-cave anterior aspect and mark lines of fusion of the sacral vertebrae. The anterior sacral foram-ina, at the lateral ends of the transverse ridges, transmit blood vessels and anterior rami of the sacral spinal nerves. Lateral to these foramina are expanded superior regions with a wing-like appearance. They are called alae . The poste-rior midline sacral surface is made rough by the fused spinous processes of the sacral vertebrae, which form the median sacral crest. Flank-ing this crest ­laterally are the posterior sacral foramina. These openings­ transmit the pos-terior rami of the sacral spinal nerves and the lateral sacral crests, which are remnants of the transverse processes of S₁ to S₃. The sacral canal continues through the sacrum to an exter-nal opening called the sacral hiatus, where four pairs of anterior sacral ­foramina allow nerves and blood vessels to pass. The sacral hiatus forms because the laminae of the fifth and sometimes fourth sacral vertebrae do not fuse medially. The sacral hiatus is located at the inferior end of the sacral canal.

■■ Coccyx: Also known as the tailbone, the coccyx is the lowest part of the vertebral column and is composed of four fused vertebrae. It is attached to the sacral hiatus by ligaments. The prominent laminae of the first coccygeal vertebrae are called the coccygeal cornua.

Spinal Curvature

The vertebral column consists of four spinal curves: the cervical, thoracic, lumbar, and sacral curves. The thoracic and sacral curves are called primary curves. They are also called accommoda-tion curves because they accommodate the thoracic and abdominopelvic­ viscera. The cervical and ­lumbar curves are called secondary curves (­FIGURE 7-23). They are also called compensation curves because they help shift body weight to allow an upright posture.

Thorax

The thorax is composed of the thoracic cage, which includes 12 pairs of ribs connected posteriorly to the thoracic vertebrae (FIGURE 7-24), the sternum, and the costal cartilages, which attach the ribs to the sternum anteriorly. The thoracic cage supports the pectoral gir-dle and upper limbs and protects the ­visceral organs inside the thoracic and upper abdominal cavities.

Sternum

The sternum, also known as the breastbone, is located in the middle anterior thoracic cage. It is composed of an upper manubrium, a middle body or gladiolus­, and a lower xiphoid process (FIGURE 7-25). The manubrium attaches to the clavicles via clavicular notches. It also articulates with the first two pairs of ribs. The jugular notch is a shallow indentation between the clavicular articulations, on the manubrium’s superior surface. It can be easily felt through the skin of the upper chest, and is usually in line with the disc between the second and third thoracic vertebrae as well as the point where the left common carotid artery emerges from the aorta. The sternum is about six inches in length. Its gladiolus is the largest portion, with notches present at the points with which it articulates with the costal cartilages of the second to the seventh ribs. The xiphoid process may vary in shape, and during early life is formed from hya-line cartilage. Past the age of 40, it usually ossifies. The xiphoid process only articulates with the gladiolus and is an attachment point for certain abdominal muscles. In certain people, the xiphoid process projects ­posteriorly, and chest trauma may plunge it into the heart or liver, resulting in serious hemorrhaging.

Aside from the jugular notch, the other two important anatomical landmarks of the sternum are the sternal angle and the xiphisternal joint. The sternal angle is a horizontal ridge across the front of the sternum where the manubrium meets the ster-nal body. It is cartilaginous, acting as a hinge to allow the gladiolus to move anteriorly during inhala-tion. It is lined with the disc between the fourth and fifth thoracic vertebrae, at the level of the second rib pair. It aids in finding the second rib during physical examinations, used in counting the ribs and a point to listen to sounds made by certain heart valves. The ­xiphisternal joint is located where the gladiolus is fused with the xiphoid process. It is located at the level of the ninth thoracic vertebra. The heart is located on the diaphragm, just deep to the xiphisternal joint.

Ribs

The ribs are attached, in pairs, to each of the 12 thoracic vertebrae, totaling 24 ribs in all. The first seven pairs are true ribs (vertebrosternal ribs), attached to the sternum via costal cartilages. The last five pairs are false ribs (meaning their cartilages do not reach the sternum directly). The cartilages of the upper three false rib pairs join the cartilages of the seventh true ribs. The final two false rib pairs are called floating ribs (vertebral ribs) because they do not attach to the sternum via cartilage. Ribs are curved with enlarged ends (heads), allowing them to attach to the sternum via facets (surfaces where bones meet). The transverse process of the vertebrae articulates with a tubercle (projection) close to the rib’s head. The angle of a rib is where the tubular shaft (body) begins to curve toward the sternum. FIGURE 7-26 illus-trates the structure of the sixth rib in its anterior view.

1. Compare the axial skeleton with the appendicular skeleton.

2. Explain the bones of the cranium.

3. List the facial bones.

4. Compare the mandible with the hyoid bone.

1. List the numbers of cervical, thoracic, and lumbar vertebrae.

2. Explain the structure of the sternum.

3. Distinguish between true, false, and floating ribs.