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Notes - 5th Edition

Dr. Steven Schwartz                                      AP1                                         IRSC/PBCC

Chapter 8

The Skeletal System

I. Overview of the Skeletal System (pp. 242–244)

A. The axial skeleton forms the central supporting axis of the body and includes the skull, auditory ossicles, hyoid bone, vertebral column, and thoracic cage (ribs and sternum). (p. 242) (Fig. 8.1)

B. The appendicular skeleton includes bones of the upper limb and pectoral girdle, and bones of the lower limb and pelvic girdle. (p. 242) (Fig. 8.1)

C. The bones of the skeletal system typically number 206 in an adult, but at birth there are 270 and even more form during childhood. (p. 242–244)

1. The number of bones decreases as separate bones fuse during aging.

2. This fusion is completed by late adolescence to the mid-20s to bring about the average number of 206; however this number varies among adults. (Table 8.1)

a. Sesamoid bones are bones that form within some tendons in response to stress, such as the patella (kneecap).

b. Some people have extra bones in the skull called sutural or wormian bones (Fig. 8.6)

D. Bones exhibit a variety of anatomical features including ridges, spines, bumps, depressions, canals, slits, cavities, and articular (joint) surfaces; many of these bone markings can be felt on your own body. (p. 244) (Fig. 8.2) (Table 8.2)

II. The Skull (pp. 244–258)

A. The skull is the most complex part of the skeleton. (pp. 244–249) (Figs. 8.3, 8.4, 8.5, 8.6)

1. The skull is composed of 22 bones, and sometimes more; most of these are connected by immovable joints called sutures. (Fig. 8.4)

2. The skull contains several prominent cavities. (Fig. 8.7)

a. The largest is the cranial cavity, which encloses the brain.

b. Other cavities include the orbits (eye sockets), nasal cavity, buccal cavity (mouth), middle- and inner-ear cavities, and paranasal sinuses.

i. The paranasal sinuses are named for the bones in which they occur: frontal, sphenoid, ethmoid, and maxillary sinuses. (Fig. 8.8)

ii. These cavities are connected with the nasal cavity, lined by a mucous membrane, and filled with air.

c. Bones of the skull have conspicuous foramina (sing. foramen), holes that allow passage of nerves and blood vessels. (Table 8.3)

B. The cranium forms the cranial cavity that protects the brain and associated sensory organs; it is composed of eight bones: 1 frontal, 2 parietal, 2 temporal, 1 occipital, 1 sphenoid, and 1 ethmoid. (pp. 249–255)

1. The cranium is rigid with an opening, the foramen magnum, where the spinal cord enters.

a. Brain swelling inside the cranium is an important consideration in head injuries; it can increase damage and may be fatal.

b. The brain is separated from the cranial bones by three membranes called the meninges, the thickest and toughest of which is the dura mater.

2. The cranium consists of the calvaria and the base.

a. The calvaria (skullcap) forms the roof and walls of the cranium. (Fig. 8.6)

b. The base of the cranium consists of three basins, or fossae. (Fig. 8.9)

i. The anterior cranial fossa is crescent shaped and accommodates the frontal lobes of the brain.

ii. The middle cranial fossa is abruptly deeper and bilateral, and accommodates the temporal lobes.

iii. The posterior cranial fossa is the deepest and houses the cerebellum.

3. The frontal bone of the skull extends from the forehead back to a prominent coronal suture, which crosses the crown of the head, joining the frontal bone to the parietal bones. (Figs. 8.3, 8.4)

a. The frontal bone forms the anterior wall and about one-third of the roof of the cranial cavity and turn inward to form nearly all of the anterior cranial fossa and the roof of the orbit.

b. Interior to the eyebrows is a ridge called the supraorbital margin, the center of which on each side is perforated by a supraorbital foramen; this foramen sometimes breaks through the margin to form a notch. (Figs. 8.3, 8.14)

c. The smooth area of the frontal bone just above the root of the nose is called the glabella.

d. The frontal bone also contains the frontal sinuses, although these may be absent in some people or sections.

e. Along the cut edge of the calvaria, the diploe is visible (the layer of spongy bone in the middle of flat bones). (Fig. 8.5b)

4. The right and left parietal bones form most of the cranial roof and part of its walls. (Figs. 8.4, 8.6)

a. Each parietal bone is bordered by four sutures.

i. The sagittal suture lies between the two parietal bones.

ii. The coronal suture is at the anterior margin.

iii. The lambdoid suture is at the posterior margin.

iv. The squamous suture is at the lateral margin.

b. Small sutural (wormian) bones are often seen along the sagittal and lambdoid sutures.

c. Markings on the inside of the parietal and frontal bones that resemble river channels are places where the bone has been molded around blood vessels of the meninges.

d. Externally, the parietal bones have few features

i. A parietal foramen sometimes occurs near the corner of the lambdoid and sagittal sutures (Fig. 8.6).

ii. Additionally, a pair of slight thickenings called the superior and inferior temporal lines mark the attachment of the temporalis muscle that moves the mandible. (Fig. 8.4a)

5. The right and left temporal bones form much of the lower wall and part of the floor of the cranial cavity; you can feel this bone just above and anterior to each ear. (Fig. .8.10)

a. The name of the bone refers to time—people often get their first gray hairs in this area as they age.

b. The complex shape of the temporal bone can be broken down into four parts: squamous, tympanic, mastoid, and petrous.

c. The squamous part is flat and vertical and is encircle by the squamous suture; it has two prominent features.

i. The zygomatic process forms part of the zygomatic arch (cheekbone).

ii. The mandibular fossa is the site at which the mandible articulates with the cranium.

d. The tympanic part is a ring of bone that borders the external acoustic meatus (ear canal opening).

i. On its inferior surface is the styloid process, which resembles a stylus (writing implement).

ii. The styloid is an attachment for muscles of tongue, pharynx, and hyoid bone.

e. The mastoid part is posterior to the tympanic part.

i. The mastoid part includes the mastoid process, which is filled with small air sinuses that can be subject to infection (mastoiditis).

ii. A groove called the mastoid notch lies medial to the mastoid process and is the origin of the digastric muscle that opens the mouth. (Fig. 8.5a)

iii. The notch is perforated by the stylomastoid foramen at its interior end and the mastoid foramen at its posterior end.

e. The petrous part is located in the cranial floor, where it resembles a little mountain range separating the middle cranial fossa from the posterior fossa. (Fig. 8.10b)

i. The petrous part houses the middle- and inner-ear cavities.

ii. The inter acoustic meatus on its posteromedial surface allows passage of the vestibulocochlear nerve that carries sensations of hearing and balance to the brain.

iii. On the inferior surface are two prominent foramina named for the blood vessels that pass through them: the carotid canal and the jugular foramen; three cranial nerves also pass through the jugular foramen. (Table 8.3)

6. The occipital bone forms the rear of the skull, or occiput, and much of its base. (Fig. 8.5)

a. The most conspicuous feature of the occipital bone is the foramen magnum through which the spinal cord passes and to which the dura mater attaches.

b. The occipital bone continues anterior to the foramen magnum as a plate called the basilar part.

i. On either side of the foramen magnum is a smooth knob called the occipital condyle; the skull rests on the vertebral column on these condyles.

ii. At the anterolateral edge of each condyle is the hypoglossal canal, named for the nerve that passes through it to innervate the tongue.

ii. In some people, a condylar canal is posterior to each condyle.

c. Internally, the occipital bone has impressions or grooves left by large venous sinuses that drain blood from the brain; they terminate at the jugular foramina. (Fig. 8.b)

d. Some external features of the occipital bone can be palpated.

i. The external occipital protuberance is a medial bump to which the nuchal ligament attaches, binding the skull to the vertebral column

ii. A ridge, the superior nuchal line, can be traces horizontally from the external occipital protuberance toward the mastoid process; it provides attachment for several neck and back muscles. (Fig. 8.5a)

iii. The inferior nuchal line provides attachment for some of the deep neck muscles; it cannot be palpated on a living person but is visible on an isolated skull.

7. The sphenoid bone has a complex shape, with a thick medial body and outstretched greater and lesser wings. (Fig. 8.11)

a. The sphenoid is best seen from the superior perspective. (Fig. 8.11)

i. The lesser wings form the posterior margin of the anterior cranial fossa.

ii. The lesser wings end at a sharp bony crest, where the sphenoid drops to the greater wings.

b. The greater wing forms about half of the middle cranial fossa and are perforated by several foramina; it also forms part of the lateral surface of the cranium anterior to the temporal bone. (Fig. 8.4a)

c. The lesser wing forms the posterior wall of the orbit.

i. The lesser wing contains the optic foramen through which the optic nerve and ophthalmic artery pass. (Fig. 8.14)

ii. A pair of bony spines, the anterior clinoid processes, are found near the optic foramina.

iii. The superior orbital fissure angles upward from the posterior wall of the orbit on each side of the optic foramen; it serves as a passage for nerves supplying the eye muscles.

d. The sella turcica is a saddle-like prominence in the body of the sphenoid; it consists of three regions.

i. The hypophyseal fossa is a deep pit in which the pituitary gland (hypophysis) is housed.

ii. The tuberculum sellae is a raised anterior margin.

iii. The dorsum sellae is the rear margin.

iv. In life, a fibrous membrane covers the sella turcica, and a stalk penetrates this membrane to connect the pituitary to the brain.

e. Two foramina, the foramen rotundum and foramen ovale, are found on each side of the sella turcica; branches of the trigeminal nerve pass through these openings.

f. The foramen spinosum, near the foramen ovale, allows passage of an artery of the meninges.

g. The irregularly shaped foramen lacerum occurs at the junction of the sphenoid, temporal, and occipital bones; it is filled with cartilage in life and does not serve as a passage. (Fig. 8.5a)

h. Viewed inferiorly, the sphenoid forms part of the posterior nasal apertures, or choanae.

i. Lateral to each aperture are two plates, the medial pterygoid plate and lateral pterygoid plate, each of which has a narrow extension, the pterygoid process; these serve as muscle attachments for some jaw muscles. (Fig. 8.5a)

j. The sphenoid sinus occurs within the body of the sphenoid bone.

8. The ethmoid bone is an anterior bone located between the eyes that contributes to the medial wall of the orbit, the roof and walls of the nasal cavity, and the nasal septum; it has three major portions. (Figs. 8.7, 8.12)

a. The perpendicular plate is a thin, median vertical plate of bone the forms the superior two-thirds of the nasal septum, dividing the nasal cavity into the right and left nasal fossae.

b. The cribriform plate is a horizontal plate that forms the roof of the nasal cavity.

i. The crista galli is a median crest on this plate that forms an attachment point for the meninges.

ii. A depression is found on each side of the crista; the olfactory bulbs rest in these depressions.

iii. The cribriform foramina in the depressed area allow passage for olfactory nerves from the nasal cavity to the bulbs.

c. The labyrinth is a large mass on each side of the perpendicular plate that contains a maze of air spaces called the ethmoidal cells.

i. The ethmoidal cells collectively constitute the ethmoid sinus.

ii. The lateral surface of each labyrinth is the slightly concave orbital plate (Fig. 8.14)

iii. The medial surface of gives rise to two plates, the superior nasal conchae and middle nasal conchae.

iv. Along with the inferior nasal conchae, a facial bone, the superior and inferior nasal conchae occupy most of the space in the nasal cavity, creating turbulence in air flow.

v. The sensory cells of smell are borne on the superior conchae and adjacent part of the nasal septum.

d. The ethmoid is difficult to see in an intact skull. (Figs. 8.3, 8.5b, 8.14)

Insight 8.1 Injury to the Ethmoid Bone

C. The facial bones are those having no direct contact with the brain or meninges; there are 14 facial bones: 2 maxillae, 2 palatine bones, 2 zygomatic bones, 2 lacrimal bones, 2 nasal bones, 2 inferior nasal conchae, 1 vomer, and 1 mandible. (p. 255–257)

1. The maxillae are the largest facial bones; they form the upper jaw and meet at the median intermaxillary suture. (Figs. 8.3, 8.4a, 8.5a)

a. Alveolar processes are small points of maxillary bone that grow into the spaces between the bases of the teeth.

i. The root of each tooth is inserted into a deep socket, or alveolus; if a tooth is lost, the alveolar processes are resorbed and the alveolus fills in with new bone.

ii. Although present in the skull, teeth are not bones.

b. Each maxilla extends from the teeth to the inferomedial wall of the orbit.

c. Just below the orbit it exhibits an infraorbital foramen through which a blood vessel and nerve pass.

d. The maxilla forms part of the floor of the orbit, where it exhibits a gash called the inferior orbital fissure. (Fig. 8.14)

i. The inferior and superior orbital fissures form a sideways V with apex near the optic foramen.

ii. The inferior orbital fissure is a passage for blood vessels and sensory nerves of the face.

e. The palate forms the roof of the mouth and floor of the nasal cavity; it consists of a bony hard palate and a fleshy soft palate.

i. Most of the hard palate is formed by horizontal extensions of the maxilla called palatine processes. (Fig. 8.5a)

ii. An incisive foramen lies near the anterior margin of each palatine process, just behind the incisors.

iii. The palatine processes normally meet at the intermaxillary suture at about 12 weeks of gestation; failure to join results in cleft palate.

Insight 8.2 Evolutionary Significance of the Palate

2. The palatine bones for the rest of the hard palate, part of the wall of the nasal cavity, and part of the floor of the orbit. (Figs. 8.5a, 8.13)

a. Two greater palatine foramina are found at the posterolateral corners of the hard palate.

3. The zygomatic bones form the angles of the cheeks at the inferolateral margins of the orbits and part of the lateral wall of each orbit; they extend halfway to the ear. (Figs. 8.4a, 8.5a)

a. Each zygomatic bone has an inverted T shape and a small zygomaticofacial foramen near the stem and crossbar of the T.

b. The zygomatic arch that flares from each side of the skull is formed by the union of the zygomatic process of the temporal bone and the temporal process of the zygomatic bone. (Fig. 8.4a)

c. Waste materials pass in the other direction and diffuse into the blood vessels.

4. The lacrimal bones form part of the medial wall of each orbit and are the smallest bones of the skull—about the size of the little fingernail.

a. A depression called the lacrimal fossa houses a membranous lacrimal sac where tears from the eye collect and drain into the nasal cavity.

5. Two small rectangular nasal bones form the bridge of the nose and support cartilages that shape the nose’s lower portion. (Fig. 8.3)

6. The inferior nasal conchae, the largest of the three nasal conchae, is a separate bone; the other conchae are parts of the ethmoid.

7. The vomer forms the inferior half of the nasal septum and resembles the blade of a plow.

a. The vomer and perpendicular plate of the ethmoid support a wall of septal cartilage that forms the anterior nasal septum.

8. The mandible is the strongest bone of the skull and the only one that can move. (Fig. 8.15)

a. The mandible supports the lower teeth and provides attachment for mastication (chewing) muscles and facial expression.

b. It develops as separate right and left bones joined at the median by the mental symphysis at the point of the chin.

i. The mental symphysis ossifies in early childhood, uniting the halves into a single bone.

ii. The point of the chin is called the mental protuberance.

c. The mandible has two major parts on each side: the horizontal body and the ramus; these meet at a corner called the angle.

i. The body of the mandible exhibits pointed alveolar processes between the teeth like the maxilla.

ii. Slightly lateral to the mental symphysis are mental foramina through which nerves and blood vessels of the chin pass.

iii. Ridges and depressions on the inner surface accommodate muscles and salivary glands.

iv. Mental spines on the inner surface near the mental protuberance are for attachment of certain chin muscles.

d. The angle of the mandible has a rough lateral surface for insertion of the masseter, a muscle of mastication.

e. The ramus is somewhat Y shaped.

i. The posterior branch, the condylar process, bears the mandibular condyle that articulates with the mandibular fossa of the temporal bone to form the temporomandibular joint (TMJ).

ii. The anterior branch, called the coronoid process, is the point of insertion for the temporalis muscle that closes the teeth to bite.

iii. The U-shaped arch between the two processes is called the mandibular notch.

iv. Below the notch on the medial surface of the ramen is the mandibular foramen, through which blood vessels and nerves supplying the lower teeth pass.

D. Seven bones are closely associated with the skull but not considered part of it: the three auditory ossicles in each of the two ears, and the hyoid bone beneath the chin. (p. 257)

1. The auditory ossicles are the malleus (hammer), incus (anvil) and stapes (stirrup).

2. The hyoid bone is a slender bone between the chin and larynx. (Fig. 8.16)

a. It does not articulate with any other bone, and is suspended from the styloid processes of the skull.

b. The media body of the hyoid is flanked on either side by the greater and lesser horns (cornua).

c. The hydoid bone serves as attachment for several muscles that control the mandible, tongue, and larynx.

d. A fractures hydoid is considered evidence of strangulation.

E. The skull bones of an infant are not fused, which allows it to pass through the pelvic outlet of the mother during birth. (p. 257–258)

1. Spaces between the unfused cranial bones are called fontanels, meaning little fountain, because the pulsation of blood can be felt there.

a. The bones at these points are joined only by fibrous membranes; intramembranous ossification takes place later.

b. Four prominent fontanels are the anterior, posterior, sphenoid (anterolateral), and mastoid (posterolateral) fontanels. (Fig. 8.17)

c. Most fontanels ossify by one year of age, but the largest one, the anterior fontanel, is still evident 18 to 24 months after birth.

2. The frontal bone and mandible are separate right and left bones at birth, but fuse medially in early childhood; the frontal bones around age 5 or 6.

a. In some cases a metopic suture persists between the frontal bones and may be evident in the adult skull.

3. The face of a newborn is flat and the cranium relatively large.

a. The skull grows more rapidly than the rest of the skeleton during childhood, to accommodate the growing brain; it is nearly full size by 8 or 9 years.

b. The heads of babies and children are larger in proportion to the trunk than the heads of adults.

c. In humans and other animals the large, rounded heads of the young are thought to promote survival by stimulating parental caregiving instincts.

Insight 8.3 Cranial Assessment of the Newborn

III. The Vertebral Column and Thoracic Cage (pp. 258–267)

A. The vertebral column (spine) physically supports the skull and trunk, allows for their movement, protects the spinal cord, absorbs stresses of walking, running, and lifting; and provides attachment for the limbs, thoracic cage, and postural muscles. (pp. 258–259)

1. The vertebral column consists of a chain of 33 vertebrae with intervertebral discs of fibrocartilage between most of them.

2. The vertebral column averages about 71 cm (28 in.) long, with the discs accounting for about 1/4 of the length.

3. The vertebrae are divided into five groups: 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the chest, 5 lumbar vertebrae in the lower bac, 5 sacral vertebrae at the base of the spine, and 4 tiny coccygeal vertebrae at the very end. (Fig. 8.18)

a. Variations in the number of vertebrae occur in about 1 person in 20, with lumbar and sacral vertebrae showing most variation.

4. Beyond the age of 3 years, the vertebral column is slightly S-shaped, with four bends called the cervical, thoracic, lumbar, and pelvic curvatures. (Fig. 8.19)

5. Newborns have one continuous C-shaped curve, as do monkeys, apes, and other four-legged animals. (Fig. 8.20

6. The S-curve, which develops as the infant learns to crawl and walk, make sustained bipedal walking possible.

a. The thoracic and pelvic curvatures are called primary curvatures because they are remnants of the original infantile curvature.

b. The cervical and lumbar curvatures are called secondary curvatures because they develop later.

Insight 8.4 Abnormal Spinal Curvatures (Fig. 8.21)

B. The most obvious feature of a vertebra is the body (centrum), a mass of spongy bone and red bone marrow covered with a thin shell of compact bone; this is the weight bearing portion of the vertebra. (p. 259–260) (Fig. 8.22)

1. Posterior to the body is a triangular canal, the vertebral foramen.

a. The vertebral foramina collectively form the vertebral canal through which the spinal cord passes.

2. The foramen is bordered by a vertebral arch composed of two parts on each side: a pillarlike pedicle and platelike lamina.

a. Extending from the apex of the arch is a projection called the spinous process, which is directed posteriorly and downward.

b. A transverse process extends laterally from the point where the pedicle and lamina meet.

c. These processes provide attachment points for ligaments and muscles.

3. A pair of superior articular processes project upward from one vertebra and meet a similar pair of inferior articular processes projecting downward from the vertebra just above. (Fig. 8.23)

a. Each process has a flat articular surface (facet) facing that of the adjacent vertebra.

b. These processes restrict twisting of the spine, which could damage the spinal cord.

3. When two vertebrae are joined, they exhibit an opening between their pedicles called the intervertebral foramen.

a. This foramen allows passage of spinal nerves that connect with the spinal cord at regular intervals.

b. Each foramen is formed by an inferior vertebral notch in the superior vertebra, and a superior vertebral notch in the inferior vertebra. (Fig. 8.23b)

C. An intervertebral disc is a pad consisting of a gelatinous nucleus pulposus surrounded by a ring of fibrocartilage, the anulus fibrosus. (p. 261) (Fig. 8.22)

1. The discs help bind adjacent vertebrae, support the weight of the body, and absorb shock.

2. Excessive stress can crack the anulus and cause the nucleus to ooze out, a condition termed a herniated disc (ruptured or slipped disc).

3. To relieve the pressure, a laminectomy may be necessary—each lamina is cut and the spinous process is removed.

D. Vertebrae differ from one another depending on the region of the spine in which they are located; these difference reflects their functions. (pp. 261–264)

1. The cervical vertebrae (C1–C7) are the smallest and lightest, other than the coccygeals. (Fig. 8.24)

a. Vertebra C1 is called the atlas because it supports the head reminiscent of the Titan in Greek mythology who held up the world.

i. It has no body and is little more than a ring surrounding a vertebral foramen.

ii. On each side is a lateral mass with a deeply concave superior articular facet that articulates with the occipital condyle of the skull; this joint allows the nodding motion of the head.

iii. The inferior articular facets articulate with C2.

iv. The lateral masses are connected by an anterior arch and posterior arch, which bear protuberances called the anterior and posterior tubercles.

b. Vertebra C2 is called the axis, and it allows the head to swivel from side to side.

i. It has a prominent knob, the dens or odontoid process, on its anterosuperior side, which projects into the vertebral foramen of the atlas; it is nestled in a facet and held in place by a transverse ligament. (Fig. 8.24c)

ii. A heavy blow to the top of the head can cause a fatal injury by driving the dens through the foramen magnum into the brainstem.

iii. The articulation between the atlas and the cranium is the atlanto-occipital joint; the articulation between the atlas and axis is the atlantoaxial joint.

vi. The axis is the first vertebrae to exhibit a spinous process.

c. In vertebrae C2 to C6, the spinous process is forked, or bifid, providing attachment for the nuchal ligament of the back of the neck. (Fig. 8.25a)

d. All seven cervical vertebrae have a prominent transverse foramen in each transverse process.

i. These foramina provide passage and protection for the vertebral arteries that supply blood to the brain and the vertebral veins that return blood to the heart.

ii. No other vertebrae have these foramina, so their presence identifies a cervical vertebrae.

e. Cervical vertebrae C3–C6 are similar to each other and to the general vertebrae described earlier, except for the bifid spinous process and the transverse foramina.

f. Cervical vertebra C7 does not have a bifid spinous process, and the process is especially long, forming a bump at the base of the neck (vertebral prominence).

2. The twelve thoracic vertebrae (T1–T12), which correspond to the 12 pairs of ribs attached to them, lack transverse foramina and bifid processes

a. The thoracic vertebrae have four distinguishing features. (Fig. 8.25b)

i. The spinous processes are relatively pointed and angle sharply downward.

ii. The body is somewhat heart shaped and more massive than that of the cervical vertebrae but less than the lumbar vertebrae.

iii. The body has small concave costal facets for attachment of the ribs.

iv. Vertebrae T1–T10 have a cuplike transverse costal facet at the end of each transverse process that are a second point of articulation for ribs 1–10; ribs 11 and 12 attach only to the bodies of vertebrae 11 and 12.

b. No other vertebrae have ribs articulating with them.

i. A rib inserts between two vertebrae, so each vertebra contributes half of the articular surface.

ii. A rib joins the inferior costal facet of the upper vertebra and the superior costal facet of the lower vertebra.

iii. Vertebrae T1 and T10–T12 have complete costal facets on the vertebral body; the corresponding ribs articulate on the bodies rather than between vertebrae.

c. Each thoracic vertebra has a pair of superior articular facets that face posteriorly and a pair of inferior articular facets that face anteriorly (except for T12); these articulate with adjacent vertebrae.

d. In T12, the inferior articular facets face somewhat laterally; this is a transition between the thoracic pattern and lumbar pattern.

3. The five lumbar vertebrae (L1–L5) are distinguished by a thick, stout body and blunt, squarish spinous processes, with articular processes oriented differently from other vertebrae. (Fig. 8.25c)

a. The superior processes face medially and the inferior processes face laterally.

b. This arrangement resists twisting of the spine, protecting the spinal cord.

4. The sacrum is a bony plate that forms the posterior wall of the pelvic cavity. (Fig. 8.26)

a. In children, there are five separate sacral vertebrae (S1–S5); these begin to fuse around age 16 and are fully fused by age 26.

b. The anterior surface of the sacrum is smooth and concave and has four transverse lines indicating where the vertebrae have fused.

i. Four large anterior sacral (pelvic) foramina allow passage of nerves and arteries to the pelvic organs.

c. The posterior surface is very rough, with crests and foramina.

i. The spinous processes fuse into a ridge called the median sacral crest.

ii. The transverse processes fuse into the less prominent lateral sacral crest on each side of the median crest.

iii. Four pairs of posterior sacral foramina correspond to the anterior foramina.

d. The sacral canal, which contains spinal nerve roots, runs through the sacrum and ends in the posterior sacral hiatus.

e. On each side of the sacrum is an ear-shaped region called the auricular surface, which articulates with the os coxae and forms the nearly immovable sacroiliac joint. (8.36b)

f. The body of vertebra S1 juts anteriorly to form a sacral promontory that supports vertebra L5.

i. A pair of superior articular processes lateral to the sacral median crest articulate with L5.

ii. Lateral to the superior articular processes are a pair of large winglike extensions called the alae.

5. The coccyx usually consists of four (sometimes five) small vertebrae, Co1–Co4, which fuse by the age of 20 or 30 into a single triangular bone. (Fig. 8.26)

a. Vertebra Co1 has a pair of horns (cornua) that serve as attachments for ligaments that bind the coccyx to the sacrum.

b. The coccyx can be fractured by a difficult childbirth or a hard fall to the buttocks.

c. It is the vestige of a tail, but nevertheless provides attachment for muscles of the pelvic floor.

E. The thoracic cage consists of the thoracic vertebrae, sternum, and ribs. (p. 264–267) (Fig. 8.27)

1. The thoracic cage forms a more or less conical enclosure for the lungs and heart and provides attachment for the muscles of the pectoral girdle and upper limb; it is expanded by respiratory muscles to draw air into the lungs.

2. The inferior border is formed by a downward arc called the costal margin.

3. The thoracic cage protects the spleen, liver, and most of the kidneys as well as lungs and heart.

4. The sternum (breastbone) is a bony plate anterior to the heart; it is subdivided into three regions.

a. The manubrium is the broad superior portion.

i. It has a median suprasternal notch (jugular notch) which can be felt between the clavicles.

ii. Left and right clavicular notches form articulations with the clavicles.

b. The body, or gladiolus, is the longest part of the sternum, joining the manubrium at the sternal angle, which forms a ridge at the point where the sternum projects farthest forward.

i. The second rib attaches at the sternal angle, making it a useful landmark.

ii. The manubrium and body have scalloped lateral margins where cartilages of the ribs are attached.

c. At the inferior end of the sternum is a small, pointed xyphoid process that provides attachment for some abdominal muscles.

d. Improperly performed chest compressions in cardiopulmonary resuscitation can drive the xiphoid process into the liver and cause a fatal hemorrhage.

5. There are 12 pairs of ribs, with no difference between the sexes. (Table 8.4)

a. Each rib is attached at its posterior (proximal) end to the vertebral column.

b. Most are also attached at the anterior (distal) end to the sternum; the anterior attachment is through a long strip of hyaline cartilage called the costal cartilage.

c. The ribs generally increase in length from 1 through 7 and then decrease through 12; they are increasingly oblique (slanted) from 1 through 9 and then less so from 10 through 12.

d. Rib 1 is atypical; it attaches just below the base of the neck, and much of this rib lies just above the level of the clavicle. (Fig. 8.28)

i. Rib 1 is a short, flat, C-shaped plate of bone. (Fig. 8.28a)

ii. The vertebral end has a knobby head that articulates with the body of T1; distal to the head, the rib narrows to a neck and then widens to form a rough area called the tubercle, where it attaches to the transverse costal facet of T1.

iii. Beyond the tubercle, the rib flattens and widens into a sloping bladelike shaft.

iv. The shaft ends distally in a squared-off, rough area where the costal cartilage begins and spans the distance to the upper sternum.

e. Ribs 2 through 7 are more typical in appearance. (Fig. 8.28b)

i. Each rib has a head, neck, and tubercle at the proximal end with the head inserting between two vertebrae.

ii. The wedge-shaped head has a superior articular facet and an inferior articular facet, which join with the inferior and superior costal facets, respectively.

iii. The tubercle articulates with the transverse costal facet of each same-numbered vertebrae. (Fig. 8.29)

iv. Each rib make a sharp curve around the side of the torso, called the angle of the rib, and then progresses anteriorly to the sternum. (Fig. 8.27)

v. The inferior margin of the shaft has a costal groove, the path of intercostals blood vessels and nerves.

f. Because they each have a costal cartilage connecting with the sternum, ribs 1 through 7 are termed true ribs.

g. Ribs 8 through 12 are called false ribs because they lack independent cartilaginous connections to the sternum.

i. In 8 through 10, the costal cartilages sweep upward and end on the costal cartilage of rib 7. (Fig. 8.27)

ii. Rib 10 attaches to a single vertebra (T10) rather than between vertebrae.

iii. Ribs 11 and 12 articulate with the bodies of T11 and T12, but they do not have tubercles and do not attach to the transverse processes of the vertebrae; they are called floating ribs because there is no cartilaginous connection to the higher costal cartilages.

IV. The Pectoral Girdle and Upper Limb (pp. 267–272)

A. The pectoral girdle (shoulder girdle) supports the arm; it consists of two bones, the clavicle (collarbone) and the scapula (shoulder blade). (pp. 267–268)

1. The medial end of the clavicle articulates with the sternum and the sternoclavicular joint, and its lateral end with the scapula and the acromioclavicular joint. (Fig. 8.27)

2. The scapula also articulates with the humerus at the glenohumeral joint.

3. The human shoulder is far more flexible than that of most other mammals.

4. The clavicle is a slightly S-shaped, somewhat flattened bone of the upper thorax. (Fig. 8.30)

a. The medial sternal end has a rounded, hammerlike head.

b. The lateral acromial end is markedly flattened, and near this end is a tuberosity called the conoid tubercle, a ligament attachment.

c. The clavicle braces the shoulder; without the clavicles the chest muscles would pull the shoulders forward and medially.

d. Fracture of the clavicle is common because it is close to the surface and because people often reach out with their arms to break a fall.

5. The scapula is named for its resemblance to a spade or shovel; it is a triangular plate that posteriorly overlies ribs 2 to 7. (Fig. 8.31)

a. The three sides of the triangle are called the superior, medial (vertebral), and lateral (axillary) borders, and the three angles are the superior, inferior, and lateral angles.

b. The suprascapular notch in the superior border provides passage for a nerve.

c. The broad anterior surface, or subscapular fossa, is slightly concave a relatively featureless.

d. The posterior surface has a transverse ridge called the spine, and deep indentation superior to the spine called the supraspinous fossa, and a broad surface inferior called the infraspinous fossa.

e. The most complex region of the scapular is its lateral angle, with three main features.

i. The acromion is a platelike extension that forms the apex of the shoulder and articulates with the scapula; this is the sole point of attachment of the scapula and upper limb to the rest of the skeleton.

ii. The coracoid process is shaped like a bent finger but named for a resemblance to a crow’s beak; it provides attachments for tendons of muscle of the arm.

iii. The glenoid cavity is a shallow socket that articulates with the head of the humerus to form the glenohumeral joint.

B. The upper limb is divided into four regions and contains 30 bones per limb. (pp. 269–272)

1. The four regions are the brachium, the antebrachium, the carpus, and the manus.

a. The brachium, or arm proper, extends from shoulder to elbow and contains only the humerus.

b. The antebrachium, or forearm, extends from elbow to wrist and contains two bones, the radius and ulna; in anatomical position, the radius is lateral to the ulna.

c. The carpus, or wrist, contains eight small bones arranged in two rows.

d. The manus, or hand, contains 19 bones in two groups: 5 metacarpals in the palsm and 14 phalanges in the fingers.

2. The humerus has a hemispherical head that articulates with the glenoid cavity of the scapula. (Fig. 8.32)

a. The head is bordered by a groove called the anatomical neck.

b. Greater and lesser tubercles at the proximal end are muscle attachments, and an intertubercular sulcus between them accommodates a tendon of the biceps.

c. The surgical neck is a narrowing of the bone distal to the tubercles and is a common fracture site.

d. The deltoid tuberosity is a rough area on the shaft that is the insertion for the deltoid muscle.

3. The distal end of the humerus has two smooth condyles.

a. The lateral condyle, the capitulum, is shaped somewhat like a wide tire and articulates with the radius.

b. The medial condyle, called the trochlea, is pulley-like and articulates with the ulna.

c. Immediately proximal to these condyles, the humerus flares out to form two bony processes, the lateral and medial epicondyles.

i. The medial epicondyle protects the ulnar nerve, which passes close to the surface; this epicondyle is the “funny bone.”

d. Just above these epicondyles are the lateral and medial supracondylar ridges, which are attachments for some forearm muscles.

4. The distal end of the humerus also shows three deep pits: one posterior and two anterior.

a. The posterior pit, the olecranon fossa, accommodates the olecranon of the ulna when the arm is extended.

b. The anterior medial pit, the coronoid fossa, accommodates the coronoid process of the ulna when the arm is flexed.

c. The anterior lateral pit, the radial fossa, is named for the nearby head of the radius.

5. The radius has a distinctive disc-shaped head at its proximal end. (Fig. 8.33)

a. When the forearm is rotated, the circular superior surface of this disc spins on the capitulum of the humerus, and the edge of the disk spins on the radial notch of the ulna.

b. Immediately distal to the head, the radius has a narrow neck and then widens to the radial tuberosity on its medial surface; the distal biceps tendon terminates here.

6. The distal end of the radius has three features, from lateral to medial.

a. The styloid process is a bony point proximal to the thumb.

b. Two articular facets articulate with the scaphoid and lunate bones of the wrist.

c. The ulnar notch articulates with the end of the ulna.

7. The ulna has a deep, C-shaped trochlear notch at its proximal end; this notch wraps around the trochlea of the humerus.

a. The posterior side of the notch is formed by the olecranon, the bony point of the elbow.

b. The anterior side is formed by the less prominent coronid process.

c. Laterally, the head of the ulna has a radial notch that accommodates the head of the radius.

8. The distal end of the ulna has a medial styloid process.

9. The radius and ulna are attached along their shafts by a ligament, the interosseous membrane (IM), attached to an angular ridge on each bone called the interosseous margin.

a. Most fibers of the IM are oriented obliquely, slanting upward from ulna to radius.

b. The IM enables the two elbow joints (humerus–radius and humerus–ulna) to share a load more evenly, and also serves as an attachment for certain forearm muscles.

10. The carpal bones, which form the wrist, are arranged in two rows of four bones each. (Fig. 8.34)

a. The proximal row, starting at the lateral (thumb) side, are the scaphoid, lunate, triquetrum, and pisiform.

i. The pisiform is a sesamoid bone that is not present at birth but develops around age 9 to 12 in the tendon of the flexor carpi ulnaris muscle.

b. The distal row, also starting on the lateral side, are the trapezium, trapezoid, capitate, and hamate.

i. The hamate can be recognized by a prominent hook called the hamulus on the palmar side. (Fig. 8.34b)

11. The metacarpals are the bones of the palm.

a. Metacarpal I is proximal to the base of the thumb, and metacarpal V is proximal to the base of the little finger.

b. The proximal end of a metacarpal bone is called the base, the shaft is called the body, and the distal end is called the head; the heads form knuckles when you clench your fist.

12. Phalanges are the bones of the fingers; the singular is phalanx.

a. Two phalanges are found in the pollex (thumb) and three in each of the other digits.

b. Phalanges are identified by Roman numerals I–V preceded by proximal, middle, and distal; proximal phanlanx I is the basal segment of the thumb, and distal phalanx V is the tip of the little finger.

c. Each phalanx contains a base, body, and head.

V. The Pelvic Girdle and Lower Limb (pp. 273–282)

A. The pelvic girdle is composed of two hip (coxal) bones; these are also sometimes called the ossa coxae or innominate bones. (pp. 273–275)

1. The pelvic girdle and sacrum together constitute the pelvis, which supports the trunk on the lower limbs and provides a protective enclosure of organs of the pelvic cavity. (Fig. 8.35)

2. The pelvis has a bowl-like shape with the greater (false) pelvis between the flar of the hips and the narrower less (true) pelvis below.

a. The two pelves are separated by a somewhat round margin called the pelvic brim.

b. The opening circumscribed by the brim is called the pelvic inlet; the lower margin of the lesser pelvis is called the pelvic outlet.

3. The hip bones have three distinctive features.

a. The iliac crest is the superior crest of the hip.

b. The acetabulum is the hip socket.

c. The obturator foramen is a large round-to-triangular hole below the acetabulum, closed by a ligament called the obturator membrane.

4. The adult hip bone is formed by the fusion of three childhood bones: the ilium, the ischium, and the pubis. (Fig. 8.36)

a. The ilium, the largest bone, extends from the iliac crest to the center of the acetabulum.

i. The iliac crest extends from an anterior point or angle, the anterior superior spine, to a sharp posterior angle, the posterior superior spine.

ii. The anterior superior spines form visible protrusions in a lean person; the posterior superior spines are sometimes marked by dimples above the buttocks.

iii. Below the superior spines are the anterior and posterior inferior spines.

iv. Below the posterior inferior spine on each side is the greater sciatic notch, named for the sciatic nerve that passes through it.

v. The posterolateral surface of the ilium is rough textured and serves as attachment for several muscles of the buttocks and thighs.

vi. The anteromedial surface is the smooth iliac fossa, covered in life by the iliacus muscle.

vii. Each ilium has an auricular surface that joins with the sacrum to form the sacroiliac joint.

b. The ischium is the inferoposterior portion of the hip bone.

i. The body of the ischium is marked with a prominent ischial spine.

ii. Inferior to the spine is the lesser sciatic notch and then the thick ischial tuberosity, which supports the body when sitting.

iii. The ramus of the ischium joins the inferior ramus of the pubis anteriorly.

c. The pubis (pubic bone) is the most anterior portion of the hip bone and has a superior and inferior ramus and triangular body.

i. The body of one pubis meets the body of the other at the pubic symphysis; the pubis and ischium encircle the obturator foramen.

ii. The pubis is often fractured when the pelvis is compressed front to back, as in seatbelt injuries.

5. The pelvis is sexually dimorphic, and the sex of skeletal remains can be identified largely from the anatomy of the pelvis. (Fig. 8.37) (Table 8.5)

a. The male pelvis is heavier and thicker than that of the female.

b. The female pelvis is wider and shallower, with a larger pelvic inlet and outlet to accommodate the needs of pregnancy and childbirth.

B. The lower limb, like the upper limb, is divided into four regions and contains 30 bones per limb; these bones have adaptations for weight-bearing and locomotion. (pp. 275–280)

1. The four regions are the femoral region, the crural region, the tarsal region, and the pedal region.

a. The femoral region, or thigh, extend from hip to knee and contains the femur.

i. The patella (kneecap) is a sesamoid bone at the junction of the femoral and crural regions.

b. The crural region, or leg proper, extends from knee to ankle and contains two bones, the medial tibia and lateral fibula.

c. The tarsal region (tarsus), or ankle, is the union of the crural region with the foot; the tarsal bones are treated as part of the foot.

d. The pedal region (pes), or foot, is composed of the 7 tarsal bones, 5 metatarsals, and 14 phalanges in the toes.

2. The femur is the longest and strongest bone of the body, with a hemispherical head that articulates with the acetabulum in a ball-and-socket joint. (Fig. 8.38)

a. A ligament extends from the acetabulum to a pit, the fovea capitis, in the head of the femur.

b. Distal to the head is a constricted neck and then two massive processes called the greater and lesser trochanters on which powerful muscles of the hip insert.

i. The trochanters are connected posteriorly by the intertrochanteric crest and anteriorly by the intertrochanteric line.

c. The primary feature of the shaft of the femur is a posterior ridge called the linea aspera at its midpoint.

i. At the upper end, the linea aspear forks into the medial spiral (pectineal) line and the lateral gluteal tuberosity.

ii. The gluteal tuberosity serve as attachment of the powerful gluteus maximus muscle of the buttock.

iii. At the lower end, the linea aspera forks into medial and lateral supracondylar lines, which continue down to the respective epicondyles.

d. The medial and lateral epicondyles are the widest points of the femur at the knee; these and the supracondylar lines are attachments for certain thigh and leg muscles and knee ligaments.

3. At the distal end of the femur are the two surfaces of the knee joint, the medial and lateral condyles, separated by a groove called the intercondylar fossa.

a. On the anterior side of the femur, a smooth medial depression, the patellar surface, articulates with the patella.

b. On the posterior side is a flat or slightly depressed area called the popliteal surface.

4. The patella, or kneecap, is a large sesamoid bone; it is cartilaginous at birth and ossifies at 3 to 6 years of age.

a. It has a broad, superior base, a pointed inferior apex, and a pair of shallow articular facets on its posterior surface where it articulates with the femur.

b. The quadriceps femoris tendon extends from the anterior muscle of the thigh to the patella, and it continues as the patellar ligament from the patella to the tibia.

i. This change of terminology only reflects that tendons connect muscle to bone, whereas ligaments connect bone to bone.

5. The tibia is on the medial side of the leg and is the only weight-bearing bone of the crural region. (Fig. 8.39)

a. The head of the tibia has two articular surfaces, the medial and lateral condyles, which articulate with the condyles of the femur.

i. The condyles are separated by a ridge called the intercondylar eminence.

b. The anterior surface of the tibia, the tibial tuberosity, can be felt just below the patella; the thigh muscles that straighten the knee attach here.

c. The shaft has a sharply angular anterior crest, which can be palpated in the shin.

6. The two prominent bony knobs on each side of the ankle are the medial malleolus and lateral malleolus.

a. The medial malleolus is part of the tibia; the lateral malleolus is part of the fibula.

7. The fibula is a slender strut that helps stabilize the ankle; it does not bear any weight (Fig. 8.39)

a. The proximal head is thicker and broader; the point of the head is called the apex.

b. The distal expansion is the lateral malleolus.

8. Like the radius and ulna, the tibia and fibula are joined by an interosseous membrane.

9. The ankle and foot consist of bones in proximal and distal groups similar to those of the wrist and hand, but because of their weight-bearing role, the bones’ shapes and arrangement are very different. (Fig. 8.40)

a. The largest tarsal bone is the calcaneus, which forms the heel.

i. Its posterior end is the point of attachment for the calcaneal (Achilles) tendon.

b. The second largest and most superior tarsal bone is the talus, which has three articular surfaces.

i. The inferoposterior surface articulates with the calcaneus.

ii. The superior trochlear surface articulates with the tibia.

iii. The anterior surface articulates with the navicular tarsal bone.

c. The talus, calcaneus, and navicular make up the proximal row of tarsal bones.

d. The distal group forms a row of four bones: the medial cuneiform, intermediate cuneiform, lateral cuneiform, and cuboid, which is the largest.

e. The remaining bones are similar in arrangement and name as those of the hand: metatarsals I–V, numbered from medial to lateral, with metatarsal I being proximal to the hallux (great toe).

i. Metatarsals I to III articulate with the first through third cuneiforms.

ii. Metatarsals IV and V both articulate with the cuboid.

f. Bones of the toes, like those of the fingers, are called phalanges.

i. The hallux contains only two bones, the proximal and distal phalanx I.

ii. The other toes each contain three bones, a proximal, middle, and distal phalanx, and are numbered II through V from medial to lateral; they each have a base, body, and head.

g. Note that roman numeral I represents the medial group of bones in the foot, but the lateral group in the hand.

i. During the seventh week of embryonic development, the limbs extend anteriorly, and the future thumb and great toe are both directed superiorly. (Fig. 8.41a)

ii. Each limb then rotates about 90° in opposite directions: The upper limbs rotate laterally, and the lower limbs rotate medially.

iii. The result is that even though the thumb and great toe (both digit I) start out facing the same direction, they end up on opposite sides. (Fig. 8.41b)

iv. This rotation also explains why the elbow flexes posteriorly and the knee flexes anteriorly, and why the muscles that flex the elbow are anterior while those that flex the knee are posterior.

h. The sole of the foot has three springy arches that absorb the stress of walking. (Fig. 8.42)

i. The medial longitudinal arch that extends from heel to hallux is formed from the calcaneus, talus, navicular, cuneiforms, and metatarsals I to III.

ii. The lateral longitudinal arch extends from heel to little toe and includes the calcaneus, cuboid, and metatarsals IV and V.

iii. The transverse arch includes the cuboid, cuneiforms, and proximal heads of the metatarsals.

i. These arches are held together by short, strong ligaments.

i. Excessive weight or stress, or congenital weakness, can stretch these ligaments resulting in flat feet (pes planus).

ii. Apes, our closest relatives, are flat footed.

Insight 8.5 Skeletal Adaptations for Bipedalism (Fig. 8.43)

Cross References

Additional information on topics mentioned in Chapter 8 can be found in the chapters listed below.

Chapter 10: Muscles that flex the elbow and knee

Chapter 14: The meninges

Chapter 16: Hearing and the auditory ossicles

Chapter 20: Venous sinuses of the occipital grooves.

Chapter 25: The teeth

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