Chapter 7- Skeletal system

Thoracic vertebrae

12 thoracic vertebrae are larger than the cervical vertebrae and grow progressively larger as we move down the vertebral column. Thoracic vertebrae are identifiable by their heart-shaped bodies, circular vertebral foramina, and long spinous processes that point inferiorly (Figure 7.20). On the lateral sides of the bodies of thoracic vertebrae, we find two small facets: the superior costal facet and inferior costal facet (cost = "rib"; note that the bodies of T10−T12 have only a single costal facet on each side). As implied by their name, the costal facets provide the points of articulation for the ribs. Two additional costal facets are located on the transverse processes of T1−T10, the transverse costal facets, which articulate with another portion of the ribs, to be discussed shortly.

facial bones

14 bones of the skull that from the framework of the face -14 facial bones include the paired maxillary, zygomatic, nasal, lacrimal, palatine, and inferior nasal conchal bones, as well as the unpaired mandible and vomer. With the exception of the mandible (the lower jaw bone), all skull bones are united in adults by immoveable joints called sutures.

Zygomatic bone

2 zygomatic bones along with the zygomatic process of the temporal bones and maxillae, form the bulk of the cheekbone/zygomatic arch. They also form the lateral wall and part of the inferior border of the orbit

Vertebral column structure

7 cervical vertebrae located in the neck; 12 thoracic vertebrae that articulate with the ribs; 5 lumbar vertebrae in the lower back; 5 fused sacral vertebrae (collectively called the sacrum) that articulate with the pelvic bones; and 3-5 fused coccygeal vertebrae (collectively called the coccyx) at the most inferior end of the vertebral column.

Thoracic cage

Aka the rib cage, consists of 12 pairs of ribs, the sternum, and part of the vertebral column. Together, these bones encase and protect the lungs, heart, and other delicate structures in the thoracic cavity.

vomer

Deepest facial bone -forms the inferior and posterior parts of the nasal septum.

vertebral column

Inferior to the skull we find the vertebral column, which in an adult consists of approximately 33 bones, called vertebrae, stacked on top of one another. The first 24 bones are individual vertebrae, which encase the spinal cord. The two inferior bones, the sacrum and the coccyx, are made up of fused vertebrae.

sternum

The flattened sternum forms the anterior, median part of the thoracic cage. It has three portions (Figure 7.24): Manubrium. The sternum's top part is the manubrium (muh-NOO-bree-um). On its superior surface, it has a notch called the suprasternal notch. Just lateral to the suprasternal notch on both sides are the two clavicular notches, which articulate with the clavicles of the pectoral girdles. Inferior to the clavicular notches we find the sites of attachment for the cartilages of the first ribs. Body. The middle body is the largest part of the sternum. The cartilages of the second ribs attach where the manubrium and the body meet, a location called the sternal angle. The lateral edges of the body have several notches where the cartilages of the third through seventh ribs attach. Xiphoid process. The most inferior portion of the sternum is the pointed xiphoid process (ZY-foyd; xiph- = "sword"), which is the site of attachment for certain abdominal muscles. Composed of hyaline cartilage initially, it is generally fully ossified by about age 40.

cavities of the skull

The orbits contain the eyeballs. The nasal cavity houses the sensory receptors for smell. The oral cavity surrounds the teeth and tongue. Other small cavities house the sense organs for hearing and balance

palatine bones

Two L-shaped bones located in the posterior nasal cavity between the maxillae and the pterygoid processes of the sphenoid. -Each bone is composed of two plates -smaller horizontal plates form the posterior part of the hard palate. -Larger perpendicular plates are vertical projections that form part of the lateral walls of the nasal cavity and a tiny piece of the orbit

Nasal

Two nasal bones form the bridge of the nose -articulate with hyaline cartilage that form most of the framework of the nose

nucleus pulposus

disc has two main components (Figure 7.23). The soft, inner, jelly-like substance called the nucleus pulposus (NOO-klee-us pull-POH-sus) is a resilient shock absorber. It is surrounded by an outer ring of fibrocartilage, the annulus fibrosus (AN-yoo-lus fy-BROH-sus; anulus = "ring"), which holds both the nucleus pulposus in place and the vertebrae together. A&P in the Real World: Herniated Disc discusses what happens when the anulus fibrosus tears.

cranial bones

eight bones of the skull that encase the brain or cranium -eight cranial bones: four single bones, which are the frontal, occipital, ethmoid, and sphenoid bones, and two paired bones, which are the temporal and parietal bones

cranial cavity

eight cranial bones form the large cranial cavity, which surrounds the brain -consists of a superior portion, called the cranial vault, or calvaria, -inferior portion, the cranial base. Internally, the cranial base is divided into three indentations, or fossae, into which the brain fits snugly: the anterior, middle, and posterior cranial fossae

openings

enclose delicate structures and allow them to travel through bones -Holes that allow blood vessels and nerves to travel through a bone -canal(meatus)-tunnel -Fissure(narrow slit) -foramen(hole)

axial skeleton

forms the longitudinal axis of the body -skull, vertebral column, and thoracic cage

three pelvic bones

fuse during childhood: the ilium, ischium, and pubis (Figure 7.34). All three bones contribute to the deep socket on the lateral side of the pelvic bone, referred to as the acetabulum which, along with the femur, forms the hip joint. In addition, two bones, the ischium and pubis, contribute to the large opening in the anterior pelvic bone, called the obturator foramen (AHB-tuh-ray-tur), through which nerves and blood vessels pass. Let's now look at the individual bones and their features.

Bones of the hand and fingers

he hand, or manus, has five long bones, the Metacarpals (numbered I-V from lateral to medial), that articulate with the distal carpal bones proximally and the bones of the fingers distally. Each metacarpal consists of three parts: The proximal epiphysis is the base, the diaphysis is the body, and the distal epiphysis is the head. You can see the heads of your metacarpals when you make a fist, as they form your "knuckles." Fractured metacarpal heads are called boxer's fractures. The bones of the fingers consist of 14 total phalanges (fuh-LAN-jeez; singular, phalanx). Each finger has three bones: a proximal, middle (or intermediate), and distal phalanx (FAY-langks). The exception is the thumb, or pollex, which has only a proximal and a distal phalanx. Like the metacarpals, the phalanges have a base, body, and head.

inferior nasal concahe

located in the lateral walls of the nasal cavity -situated inferior to the middle nasal conchae of the ethmoid bone

ischium

posteroinferior portion of the pelvic bone is formed by the ischium (ISS-kee-um; see Figure 7.35). It consists of two components that together approximate a C shape: the posterior ischial body, which, along with the ilium and pubis, forms part of the acetabulum, and the anterior ischial ramus, which, along with the pubis, forms part of the obturator foramen. Projecting posteriorly and medially from the ischial body is the ischial spine, to which a ligament from the sacrum attaches. Just inferior to the ischial spine is the lesser sciatic notch. Like the greater sciatic notch, the lesser notch provides a passageway for blood vessels and nerves. The most prominent feature of the ischium is the thick ischial tuberosity, located on its posteroinferior side. Together, the ischial tuberosities bear our weight when we sit. When someone is said to have a "bony butt," it simply means that the individual's ischial tuberosities are prominent.

radius

radius is narrow proximally and becomes progressively broader as we move distally. Its narrow proximal epiphysis is called the radial head, which is a round, flattened structure. It articulates with the capitulum of the humerus to form part of the elbow joint and with the ulna to form the proximal radioulnar joint. Just distal to the radial head is the radial neck, which ends in the radial tuberosity. This process sits on the medial side of the bone and is the site of attachment of the biceps brachii muscle. At the widened distal epiphysis of the radius, we find an indentation called the ulnar notch, which is the site of the distal radioulnar joint. The flattened end of this epiphysis articulates with the carpal bones at the wrist. The lateral tip of the radius, the radial styloid process, forms the lateral boundary of the wrist and helps stabilize the joint.

skull

skeleton's most complex structure. It has a total of 22 bones: 8 cranial bones, which encase the brain, and 14 facial bones, which form the framework for the face.

pelvic girdle

supports the lower limb and anchors it to the trunk. It is made up of the two pelvic bones and the sacrum. Each pelvic bone is itself composed of three fused bones: the ilium, ischium, and pubis.

lower limb

the thigh, which consists of the femur; the leg, which consists of the tibia and the fibula; and the ankle and foot, which contain the tarsals, metatarsals, and phalanges.

lacrimal bone

two lacrimal bones are the smallest and most delicate facial bones. -They are found in the medial wall of the orbit, and contain a depression called the lacrimal fossa through which tears drain

Pelvis and bones of pelvic girdle

two pelvic bones, sacrum, and coccyx together form the bowl-shaped Pelvis (Figure 7.33), which establishes the boundaries for the pelvic cavity that houses organs of the digestive, urinary, and reproductive systems. The pelvic girdle and sacrum form an oval opening called the Pelvic inlet. The bony ridge surrounding the inlet is the pelvic brim. As you can see in Figure 7.33a, the pelvic brim defines the boundaries between the greater or false pelvis, which is the area superior to the pelvic brim, and the lesser or true pelvis, which is inferior to the pelvic brim. At the inferior boundary of the lesser pelvis, we find the Pelvic outlet

forearm

, consists of two bones: the lateral Radius (RAY-dee-us) and the medial Ulna, held together by a fibrous structure called the interosseous membrane (Figure 7.30). The interosseous membrane distributes the force borne by the bones more equally, reducing the load on each bone. These two bones articulate with the humerus at their proximal ends and with the carpal bones at their distal ends. In addition, they also articulate with each other at two joints called the radioulnar joints.

female and pelvis

-Shape of greater pelvis. The greater pelvis is wider in females, with the anterior superior iliac spines farther apart, and with flared iliac crests. -Coccyx and sacrum. The female sacrum tends to be wider and shorter than the male sacrum. In addition, the female coccyx is generally situated more posteriorly and is more moveable than the male coccyx. A woman with a coccyx situated anteriorly could suffer coccygeal fractures during childbirth. -Pelvic inlet and outlet. The female pelvic inlet is usually wider and oval, whereas the male pelvic inlet is narrow and vaguely heart-shaped. The female pelvic outlet is also typically wider than that of the male. Acetabula. Female acetabula are generally farther apart and pointed more anteriorly than those of males. This affects gait patterns, giving many women a "swaying" walk. Pubic arch. The pubic arch in females tends to have an angle between 90° and 100°, whereas the male pubic arch has a narrower angle of 60°-70°. If an investigator has only a single pelvic bone, the angle of the pubic arch can still be estimated by holding the bone against a mirror and measuring the angle in the reflection. Ischial tuberosities. The female ischial tuberosities tend to point laterally, whereas those of the male point medially. This gives the female buttocks a more rounded appearance. Also, the female pelvis is generally lighter and less robust than the male pelvis due to the male's greater muscle mass and weight.

Frontal bone

-forehead -inside we find the frontal sinus, which are part of the paranasal sinuses -Supraorbital margins are sharp ridges that form the superior and superomedial boundaries of the orbit -supraorbital foramen found in the middle of supraorbital margin; allows the passage of blood vessels and nerves to the forehead -glabella is the smooth region between two supraorbital margins

Occipital bone

-forms the posterior part of the cranial cavity and the posterior cranial fossa -Anterior/inferior surface has the foramen magnum("big hole"), through which the spinal cord passes to enter the vertebral cavity -on either side of the foramen magnum are two occipital condyles, which articulate with the first cervical vertebrae -posterior surface of the occipital bone features two parallel ridges, the superior nuchal line and inferior nuchal line -external occipital protuberance runs through the superior nuchal line and continues inferior as the external occipital crest

parietal bones

-two bones form the superior wall and part of the lateral wall of the cranial cavity They meet at the sagittal suture and articulate with several cranial bones at other sutures -together they meet the frontal bone at the coronal suture -temporal bone at squamous suture -occipital bone at the lambdoid suture

structures of vertebrae

Body. The most anterior structure (the part of the vertebra facing the thoracic cavity) is the blocklike body, also called the centrum. The body is the vertebra's primary weight-bearing structure. Vertebral discs cushion adjacent vertebral bodies and help absorb the shock of running and jumping. Fractures of the vertebral body can be painful and debilitating (see A&P in the Real World: Vertebral Compression Fractures ). Vertebral foramen. Just posterior to the body is the large vertebral foramen, through which the spinal cord and its associated tissues pass. The vertebral foramina of the entire vertebral column form the vertebral cavity, or vertebral canal (which you read about in ​Chapter 1​). Pedicles and laminae. The vertebral foramen is bordered anteriorly by the body, laterally by two pedicles that project from the body, and most posteriorly by the laminae. Together, the pedicles and laminae form the vertebral arch. The inferior side of the pedicles curves up, creating a deep inferior vertebral notch, and the superior side curves down slightly, creating a superior vertebral notch. The inferior and superior notches of successive vertebrae stacked on top of one another form the intervertebral foramina that you saw in the previous section (Figure 7.18b). The pedicles merge into the laminae, which enclose the posterior part of the vertebral foramen. Superior and inferior articular processes. At the junctions of the pedicles and laminae we find two small processes, the superior and inferior articular processes, on the superior and inferior sides, respectively. The surfaces of both processes contain smooth regions known as facets that are covered with hyaline cartilage. The superior articular facets of one vertebra form joints with the inferior articular facets of the vertebra above it. Transverse processes. Projecting from the lateral sides of the vertebral arch are two transverse processes. These processes serve as attachment sites for muscles. Spinous process. The posterior projection from the vertebral arch is the Spinous process . Like the transverse processes, the spinous processes are a site for muscle attachment. The spinous processes are palpable along your back as your "spine." Notice in ​Figure 7.18b​ how the spinous processes help protect the posterior spinal cord, which would otherwise be fairly vulnerable.

spinal curvatures

In a newborn infant, the vertebral column is C-shaped, reflecting its position within the mother's uterus as the fetus develops. As an infant grows, the vertebral column changes shape to develop a series of curvatures that result in a vaguely S-shaped vertebral column (see Figure 7.16b). The concave cervical curvature, which extends from about C2 to T2, develops as an infant begins to lift the head and crawl. As the child ages and begins to walk, the lumbar curvature develops, which is a concave curvature extending from about T12 to L5. The remaining two curvatures are the thoracic curvature, from T2 to T12, and the sacral curvature, from the lumbosacral junction to the coccyx. Both are convex curves that are present in fetal life as part of the fetus's original C-shaped vertebral column. For this reason, the thoracic and sacral curvatures are known as primary curvatures. The cervical and lumbar curvatures, in contrast, are called secondary curvatures because they develop after the fetal period. The development of these secondary curvatures is critical to our species' ability to walk upright. The cervical curvature allows us to hold our heads up, and the lumbar curvature shifts the weight of the body onto the sacrum, which lends the balance and support needed to walk on two legs. Here we have another example of the Structure-Function Core Principle (Module 1.5.5).

ribs

In both men and women, the rib cage consists of 12 pairs of ribs and the anterior costal cartilages (cost- = "rib"). The spaces between successive ribs are the intercostal spaces (see Figure 7.24). Recall from earlier in the chapter that the costal cartilages are composed of mostly hyaline cartilage, which gives the rib cage some flexibility during movement and breathing. Each rib has a posterior attachment to a thoracic vertebra, from which it curves around anteriorly in a C shape. We group ribs into two classes based on their anterior attachment: Ribs 1-7 are called true ribs or vertebrosternal ribs because they attach to the sternum via their own costal cartilage. Ribs 8-12 are called false ribs because they do not attach to the sternum directly. There are two types of false ribs. The first, consisting of ribs 8-10, are the vertebrochondral ribs, which have costal cartilages that attach to the cartilage of the seventh rib. Their costal cartilages form a prominent rim called the costal margin. The second type, consisting of ribs 11 and 12, are referred to as floating ribs, or vertebral ribs, because they lack any attachment to the sternum. Figure 7.25 shows the structure of a typical rib. Each rib has a rounded head where it articulates with the body of a thoracic vertebra. The heads of most ribs have two articular facets: an inferior articular facet that articulates with the body of the thoracic vertebra of the same number as the rib, and a superior articular facet that articulates with the body of the vertebra superior to it. For example, the third rib articulates with T3 at its inferior articular facet and T2 at its superior articular facet. -Lateral to the rib's head is its neck, which runs along the transverse process of the vertebra. The neck is followed by a projection of bone called the tubercle (TOO-bur-kuhl), which articulates with the transverse process of the vertebra. The rib then curves anteriorly at its angle to become its shaft, or body. The concave internal surface of the shaft contains a groove called the costal groove, along which blood vessels and nerves travel. The ends of the shafts for ribs 1-10 are square and attach to their costal cartilage; the ends of the floating ribs are also capped with a small layer of hyaline cartilage in spite of their lack of sternal attachment. Ribs 2-9 contain all of the features just discussed, but ribs 1 and 10-12 have slight variations in their structure. For instance, each of these ribs attaches to the body of only one thoracic vertebra, and so their heads lack separate superior and inferior articular facets. In addition, the floating ribs have no articulation with the transverse processes of the vertebrae and so lack tubercles.

abnormal spinal curvatures

Scoliosis. Scoliosis (skoh-lee-OH-sis; scolios = "bending") is characterized by lateral curvatures in the vertebral column that give it a C or S shape when viewed from the posterior or anterior side (Figure 7.17a). Scoliosis may be congenital (caused by deformities present at birth), neuromuscular (caused by abnormalities of or trauma to the nerves and muscles around the vertebral column), or idiopathic (of unknown cause). Idiopathic cases are the most common. Mild scoliosis may cause no symptoms and require no treatment. Severe cases, however, can put pressure on the heart and lungs and so require treatment, which may include back braces, physical therapy, or surgery. Lordosis. Lordosis (lohr-DOH-sis; lordos = "bent forward"), commonly known as "swayback," is characterized by exaggerated cervical and lumbar curvatures (​Figure 7.17b​). Some degree of lordosis, particularly of the lumbar curvature, is normal in young children but generally diminishes with age. Abnormal lordosis, sometimes called hyperlordosis, is commonly seen in adults bearing extra abdominal weight, such as pregnant women or overweight people. It may also have skeletal and neuromuscular causes. For example, dancers often develop lordosis due to their training, which can lead to muscle imbalances. Irrespective of the cause, lordosis puts extra stress on the lumbar vertebrae and can lead to lower back pain. It is generally treated with weight loss (if the person is overweight) and physical therapy. Kyphosis. In kyphosis (ky-FOH-sis; kyphos = "hump"), the thoracic curvature is exaggerated, giving a "hunchback" appearance (​Figure 7.17c​). It is caused by joint conditions such as arthritis, bone conditions such as osteoporosis and vertebral fractures, and developmental abnormalities of the skeleton. Mild kyphosis generally requires no treatment, but severe cases can be debilitating, leading to heart and lung dysfunction, nerve compression, and significant pain. Such severe cases generally require surgical correction.

clavicle

The clavicle has two distinct ends that are usually easily palpable through the skin. From an anterior view the clavicle appears straight (see Figure 7.27a), but from a superior or inferior view it is actually S-shaped (see Figures 7.27b and 7.27c). Its medial sternal end articulates with the manubrium at the sternum, forming the sternoclavicular joint. Note that this joint is the only place where the pectoral girdle articulates with the axial skeleton. The lateral acromial end articulates with a process of the scapula called the acromion to form the acromioclavicular joint (discussed in the next section). Near the acromial end we find the conoid tubercle, which is the site of ligament attachment. The structure of the clavicle enables it to function like a brace in a building that supports two opposing beams (an example of the Structure-Function Core Principle, Module 1.5.5). It sits between the shoulder and thoracic cage and braces the upper limb so that it rests laterally to the trunk. Its role in supporting the upper limb is evident in clavicular fractures, in which the arm falls anteriorly and medially. Most clavicular fractures result from direct trauma or from falling onto an outstretched arm.

Fibula

The fibula is the much thinner, lateral bone of the leg (see Figure 7.38). Often incorrectly reported to bear none of the body's weight, it does, in fact, bear about one-sixth of our total weight. Proximally, the head of the fibula articulates with the lateral tibia at the proximal tibiofibular joint. Distally, it articulates with the tibia again at the distal tibiofibular joint, after which it expands to form the lateral malleolus (lateral "ankle bone"). This structure articulates with the talus and helps stabilize the ankle joint. The fibula's thin structure causes it to be the most commonly injured bone in ankle fractures.

lumbar vertebrae

The five lumbar vertebrae are the largest and heaviest vertebrae, which reflects their primary function: bearing the weight of the torso. Their large kidney-shaped bodies, thick spinous processes that point posteriorly, and vertebral foramina shaped like a flattened triangle make them easy to recognize (Figure 7.21). In addition, lumbar pedicles and laminae are thicker and shorter than in other regions of the vertebral column.

sacrum and coccyx

The five sacral vertebrae (S1−S5) form the posterior wall of the pelvic cavity. They begin as individual vertebrae but generally fuse by ages 20-25 to form the thick, triangular sacrum (Figure 7.22). Recall that the sacrum is curved as part of the concave sacral curvature. Its superior surface, the flattened base, articulates with the fifth lumbar vertebra at its superior articular processes. The inferior surface, which articulates with the coccyx, is called the apex. The sacrum's two lateral surfaces, the auricular surfaces, articulate with the two pelvic bones. The anterior sacrum (see Figure 7.22a) shows the fused bodies of the sacral vertebrae, with transverse lines demarcating where the vertebrae fused. At the anterior margin of the base is a bony projection called the sacral promontory. Lateral to the sacral promontory are two relatively smooth regions, the alae (AY-lee; singular, ala). On the posterior surface of the sacrum (see Figure 7.22b) is the opening to the sacral canal, a continuation of the vertebral canal, which contains nerve roots from the spinal cord as well as surrounding connective tissue membranes. The posterior boundary of the sacral canal is formed by the remnants of the sacral spinous processes, a ridge of bone called the median sacral crest. Flanking either side of the median sacral crest are four pairs of holes, the sacral foramina, through which exit nerves. Lateral to the sacral foramina are the lateral sacral crests; these are remnants of the transverse processes of the sacral vertebrae. Near the apex of the sacrum is the sacral hiatus, which is the termination of the sacral canal. The final region of the vertebral column, the coccyx, is generally composed of four vertebrae that begin to fuse at about age 25, although it can contain anywhere from three to five vertebrae. Like the sacrum, the coccyx has transverse ridges where the vertebrae fused. At its superior end, it has two horn like projections, each called a coccygeal cornu. In many adults, some mobility exists between the coccyx and sacrum, although this generally diminishes with age.

Bones of the Ankle and Foot: The Tarsals, Metatarsals, and Phalanges

The structure of the ankle parallels that of the wrist in many ways. The ankle consists of seven short bones, the Tarsals, that connect the leg with the foot (Figure 7.39). However, unlike the arrangement of the wrist, only a single tarsal bone articulates with the leg: the dome-shaped Talus (TAYL-us), which forms the ankle joint with the tibia and fibula. The talus rests on top of the largest tarsal bone, the Calcaneus (kal-KAYN-ee-us), which makes up the heel of the foot. The posterior side of the calcaneus attaches to a large tendon called the calcaneal (Achilles) tendon. Distally, the talus articulates with a bone called the navicular, named for its resemblance to the shape of a boat. Together, the talus, calcaneus, and navicular are the proximal tarsal bones. The distal four tarsal bones, from medial to lateral, include the medial cuneiform, intermediate cuneiform, lateral cuneiform, and cuboid. Notice in Figure 7.39 that the cuboid articulates posteriorly with the calcaneus, whereas the cuneiforms all articulate with the navicular. The bones of the foot and toes largely parallel the hand and fingers in structure. The five foot bones are called metatarsals (numbered I-V from medial to lateral), and like the metacarpals, they have a proximal base, a middle shaft, and a distal head. On the plantar surface of the first metatarsal we usually find two small sesamoid bones (often just called the sesamoids). These bones, and the tendon in which they are located, can become inflamed with repetitive activity, resulting in sesamoiditis. This condition is particularly common in athletes playing on artificial turf, giving it the common name "turf toe." As with the fingers, the bones of the toes consist of 14 phalanges. The second through fifth toes generally have three bones each: a proximal, middle, and distal phalanx. The great toe, or hallux, usually has only two, a proximal and distal phalanx (sometimes the fifth toe has only two, as well). The bones of the foot generally do not rest flat on the ground during standing or walking. This is due to the presence of three arches that are supported by ligaments and muscles. The most prominent is the medial longitudinal arch, which runs along the medial side of the foot from the calcaneus to metatarsals I-III (Figure 7.40). The less prominent lateral longitudinal arch extends from the lateral side of the calcaneus to metatarsals IV and V. Finally, the transverse arch runs along the middle of the foot, and involves the distal tarsals and the bases of all five metatarsals. The plantar arches help support the weight of the body and distribute this weight evenly during walking.

scapula

The triangular scapula (SKAP-yoo-luh; plural, scapulae) sits on the posterosuperior rib cage, extending from approximately ribs 2 to 7 (Figure 7.28). Its largest portion is the body, which has three borders: the medial, lateral, and superior borders. The apices of the scapular triangle are called angles: the superior, inferior, and lateral angles. -The anterior surface of the scapula (see Figure 7.28a) features a hook-shaped projection called the coracoid process (KOHR-uh-koyd; corac- = "crow"; be careful not to confuse this with the coronoid processes of the mandible and ulna). Inferior to the coracoid process is a broad indentation, the subscapular fossa, to which a muscle called the subscapularis muscle attaches. From a lateral view (see Figure 7.28b), you can see that the scapula's lateral angle features a shallow indentation called the Glenoid cavity (GLEN-oyd; glen- = "pit" or "cavity"). The glenoid cavity, along with the humerus, forms the shoulder joint. On the posterior side of the scapula, we find a ridge of bone, the spine, that crosses from medial to lateral along its superior border (see Figure 7.28c). The scapular spine is the "blade" of the shoulder that you can feel through your skin. The area superior to the spine is the supraspinous fossa; the area inferior to it is the infraspinous fossa. The spine terminates in the enlarged acromion (uh-KROH-mee-ahn; acr- = "point"), which articulates with the clavicle to form the acromioclavicular (AC) joint. Injuries to the AC joint are common and result in a condition often referred to as a separated shoulder.

Tibia

Tibia and fibula are united by an interosseous membrane. They articulate with each other at both the distal and the proximal tibiofibular joints. However, unlike the radioulnar joints, the tibiofibular joints allow very little movement, which makes the lower limb more stable. Figure 7.38 The larger, medial tibia is the second strongest bone in the body, which reflects its primary function as the main weight-bearing bone of the leg. At its proximal end, it features two concave depressions called the medial condyle and lateral condyle, which articulate with the femoral condyles to form the knee joint. The two condyles are separated by a ridge, the intercondylar eminence. As you'll see in the next chapter, the intercondylar eminence is an important site for ligament attachment in the knee joint. Just distal to the tibial condyles is a rough projection from the tibia's anterior surface, known as the tibial tuberosity, which is where the patellar ligament attaches to the tibia. The tibial diaphysis features a sharp ridge on its anterior side, called the anterior crest and commonly known as the shin. Near the distal epiphysis, the anterior crest curves to the medial side, where it terminates in the medial malleolus (mal-lee-OH-lus; plural, malleoli), which you can easily palpate as your medial "ankle bone." Lateral to this is a flat articular surface where the tibia articulates with a tarsal bone called the talus to form the ankle joint.

Maxillae

Two maxillary bones are the superior jaw bones. Their fusion creates a bony midline projection called the anterior nasal spine -The superior teeth reside in the dental alveoli of the alveolar process. Medial to the alveolar processes, the maxillae form two horizontal palatine processes, which meet at the intermaxillary suture. Together these two processes form the anterior part of the hard plate(roof of the mouth) -The anterior and superior surfaces of the maxillary bone form part of the inferomedial walls of the orbit. Just below the inferior margin of the orbit, arteries and nerves pass through the inferior orbital fissure and the infraorbital foramen to supply the face. Here we also find the large hollow maxillary sinuses. laterally, the maxillae articulate with the zygomatic bones via their zygomatic processes.

Temporal bones

Two temporal bones form the lateral walls of the cranium Four regions -Squamous region is the bone's broad, flat surface. it features the zygomatic process, which forms part of the zygomatic arch and the mandibular fossa, which articulates with the mandible -Tympanic region houses the external acoustic meatus, which is the entry into the canal that leads to the middle ear. Inferiorly it features the styloid process a needle-like spur of bone -The mastoid region contain the thick projection called the mastoid process, which posterior and lateral to the styloid process. The mastoid process is filled with tiny sinuses called mastoid air cells. -Petrous region is located on the internal or medial surface of the temporal bone, where it forms part of the middle cranial fossa. It contains four important features: internal acoustic meatus, a canal leading from the inner ear -jugular foramen, a posterior opening where the temporal bone meets the occipital bone, through which the internal jugular vein and nerve pass -carotid canal through which the internal juglar vein and nerve pass -foramen lacerum which allows the passage of small blood vessels and nerves

ulna

Unlike the radius, the ulna is wide proximally and narrow distally. Its wide proximal epiphysis has several prominent features. The first is a U-shaped notch called the trochlear notch, into which the trochlea of the humerus fits (see Figure 7.30c). On the posterior side of the trochlear notch is a projection, the olecranon, which is the knob of the elbow. The anterior lip of the trochlear notch is a projection known as the coronoid process, which fits into the coronoid fossa of the humerus. Just lateral to the coronoid process is a smooth area called the radial notch of the ulna. This area articulates with the radial head. Note that these notches are named for the bone with which they articulate, not the bone on which they are located. The narrow distal epiphysis of the ulna contains the ulnar head. This is another difference between the radius and ulna: The ulnar head is at the distal epiphysis, and the radial head is at the proximal epiphysis. The medial side of the ulnar head has a small styloid process. The styloid processes of both the radius and the ulna are palpable through the skin. The proximal radius and ulna fit together with the distal humerus to form the elbow joint. Figure 7.31 shows how the structures fit snugly together, which gives this joint a great deal of stability. The articulations chapter discusses the elbow in more detail (see Chapter 8). Figure 7.31

Depressions

allow blood vessels and nerves to travel along a bone, or provide a place where two bones can articulate (form a joint). -clefts of varying depth in bone;located where a bone meets another structure, such as another bone or a blood vessel -facet(shallow surface) -fossa(indention) -fovea(shallow pit) -Groove(long indentation)

sphenoid bone

a deep cranial bone that is unique in that it articulates with every other cranial bone. It forms part of the anterior and middle cranial fossae. From an anterior view it resembles a bat; from a superior view it resembles a sting gray it has four main components: -central body is largely hollow, contains air filled sphenoidal sinuses. On the anterior surface of the sphenoid body we find the two optic foramina leading to two optic canals, through which the nerves that transmit vision pass. On its superior surface, a saddle-shaped depression is called the sella turcica. -greater wings extend laterally from the body. With the temporal bone,they form part of the middle cranial fossae. With the frontal bone, they form part of the posterior walls of the orbit. They contain three foramina that allow passage of blood vessels and nerves: Anterior foramen rotundum, middle foramen ovale, and posterior foramen spinosum Lesser wings- wings of the stingray that project from the superior surface of the sphenoid body. In the anterior view, we can see the superior orbital fissure, a slit between the lesser and greater wings. the nerves that control eye movement pass through this fissure -pterygoid processes are the inferior projections of the sphenoid bone. These processes form part of the posterior wall of the oral and nasal cavities, and are the site of attachment muscles of mastication(chewing)

intervertebral disc

a fibrocartilage pad between two vertebrae that absorbs shock, binds the vertebral column together, and helps support the weight of the body. There are 23 intervertebral discs; the first one is between C2 and C3, and the final one is between L5 and S1 (the sacrum).

carpals

consists of eight short bones—known collectively as the Carpals—arranged in two rows containing four bones each (Figure 7.32). The four proximal carpal bones are the following, from lateral to medial (see the Study Boost for help with remembering their names): the boat-shaped scaphoid (SKAF-oyd; scaph- = "boat"), which articulates with the radius; the slightly moon-shaped lunate, which articulates primarily with the radius but also contacts the ulna; the triangular triquetrum (try-KWEE-trum), which articulates with the ulna; and the small, round pisiform (PY-zih-form; "pea-shaped"), which articulates with the anterior surface of the triquetrum (note that it is visible in the anterior view only). The distal carpal bones, from lateral to medial, are as follows: the trapezium, which articulates proximally with the scaphoid; the trapezoid, which also articulates primarily with the scaphoid proximally; the rounded capitate, which articulates proximally with the scaphoid and lunate; and the hamate, which articulates proximally with the triquetrum and is named for its anterior hooklike projection (ham- = "hook").

appendicular skeleton

consists of the bones of the upper and lower limbs and the pectoral and pelvic girdles

Pectoral girdle

contains two bones—the clavicle and the scapula—that support the upper limb and anchor it to the trunk.

orbit

houses the eyeball; its associated blood vessels, muscles, and nerves; and the lacrimal gland, which produces tears. Each orbit is a complex structure formed by seven bones (Figure 7.11): the frontal bone, which forms the superior and posterosuperior walls; the maxilla, which forms the posteroinferior wall with a small contribution from the palatine bone; the zygomatic bone, which forms the anterolateral wall; the sphenoid bone, which forms the posterior wall; and the ethmoid, lacrimal, and palatine bones, which together form the medial wall.

oral cavity

houses the teeth, the tongue, and structures such as certain salivary glands. It is the first part of the gastrointestinal tract, the passages of the digestive system that digest food and absorb nutrients. The roof of the oral cavity is formed by the same structure as the floor of the nasal cavity (the hard palate), and the anterior and lateral walls of the oral cavity are formed by the maxillae and the mandible (refer back to Figure 7.8). Unlike the other cavities, the oral cavity has no bony floor or posterior wall. Instead, the floor and wall are formed by soft tissues, including muscle and connective tissue.

Mandible

inferior jaw bone and only moveable bone of the adult skull -consists of a central mandibular body, and the right and left mandibular rami -Mandibular body , which forms the chin meets the two rami at the right and left mandibular angles. -mental foramen also in the mandibular body are the inferior teeth, housed in deep sockets called dental alveoli within a ridge known as the alveolar process -Each mandibular ramus is topped by the U-shaped mandibular notch, inferior to which we find the mandibular foramen. On either side of the mandibular notch are two processes: -on the anterior side is the coronoid process, which serves as the attachment site for a major muscle of mastication(chewing) -on the posterior side is the condylar process, at the top of which is the mandibular condyle. The mandibular condyle articulates with the mandibular fossa of the temporal bone. Together form the temporomandibular joint

femur

largest and strongest bone in the body is the Femur, the only bone of the thigh. Its proximal epiphysis features the spherical head, which articulates with the acetabulum to form the hip joint Notice there is a pit in the center of the head, the fovea capitis (FOH-vee-uh KAP-uh-tiss; "pit of the head"); a ligament attaches from the acetabulum to this spot to help stabilize the hip joint. As with other long bones, distal to the head we find the neck. This is clinically important, as it is the weakest part of the bone and the area most likely to fracture Lateral to the neck is a large projection called the greater trochanter (troh-KAN-tuhr). Medially and distally we find a similar projection, the lesser trochanter. The two are connected by a bony ridge on the anterior side, the intertrochanteric line, which continues on the posterior surface of the bone as the intertrochanteric crest. The femoral diaphysis, also known as the shaft, is fairly smooth and featureless on the anterior side. On the posterior side, a prominent line called the linea aspera (LIN-ee-uh ASP-er-uh; "rough line") runs down the shaft. Near the distal epiphysis, it splits into two lines that lead to two projections, the medial epicondyle and lateral epicondyle, which are the widest points of the femur. From the epicondyles, the femur tapers into the medial condyle and lateral condyle, which articulate with the tibia to form the knee joint. On the posterior side (see Figure 7.37b) is the intercondylar fossa, which is an indentation between the two condyles. Anteriorly, the space between the condyles is smooth; this area is known as the patellar surface. The bone that articulates with the femur's patellar surface is the triangular patella (see Figure 7.37c). The patella is a sesamoid bone located within the common tendon of the quadriceps femoris muscle group, a group of four muscles in the anterior thigh. The patellar ligament is a continuation of this tendon that inserts into the tibia and secures the patella over the anterior knee (see the next section). The proximal end of the patella is the base, and its pointed distal end is the apex. Even though the patella is not technically a thigh bone, we include it here because it articulates with the femur.

humerus

largest and strongest bone of the upper limb, and the only bone of the brachium, or arm, is the humerus (HYOO-mur-us; Figure 7.29). Like all long bones, it consists of two epiphyses that articulate with other bones flanking a long diaphysis. The proximal epiphysis of the humerus features a ball-shaped humeral head on its medial side that articulates with the glenoid cavity to form the shoulder joint. Surrounding the head is a groove called the anatomical neck of the humerus. Just lateral to the neck we find a projection known as the greater tubercle. It is separated from the smaller and more medial lesser tubercle by a groove called the intertubercular sulcus or bicipital groove. This second name stems from the fact that the tendon of the biceps brachii muscle passes through this bony groove. -The junction between the proximal epiphysis and the diaphysis is a region called the surgical neck, so named because it is a frequent site of fractures (which may require surgical repair). The humeral diaphysis is fairly featureless compared to the two epiphyses. It contains only one major projection in about the middle of the bone on the lateral side, which is known as the deltoid tuberosity. As its name implies, this provides the site of attachment for the deltoid muscle. The only other feature of note on the diaphysis is the radial groove, located on the posterior side of the bone, along which travels the nerve that gives this groove its name. The distal epiphysis has two flared ends, the medial epicondyle and lateral epicondyle, which are sites of muscle attachment. The remaining features are involved in the humeral articulation with the ulna and radius at the elbow joint. On the anterior side, we find two rounded knobs: the lateral capitulum (kah-PIT-yoo-lum; capit- = "head"), named for its spherical shape and resemblance to a head, and the medial trochlea (TROH-klee-uh; troch- = "wheel"), also named for its shape, which resembles a wheel or spool of thread. Just proximal to the capitulum and trochlea are two small indentations: the lateral radial fossa and the medial coronoid fossa. On the posterior side of the distal epiphysis, we see the continuation of the trochlea and a deep indentation called the olecranon fossa (oh-LEK-ruh-nahn).

illium

largest bone of the pelvis -forms the top part of the pelvic bone. Its superior portion is the flared ala (AY-luh), named for its resemblance to a wing; its inferior portion is the body of the ilium The superior boundary of the ala is the iliac crest, which you feel every time you put your hands on your hips. At its anterior end, the iliac crest terminates in a bony projection known as the anterior superior iliac spine (ASIS), which can be palpated in some people on the anterior pelvis. Just inferior to the ASIS is another, smaller ridge called the anterior inferior iliac spine. the iliac crest terminates posteriorly as the posterior superior iliac spine, which has a smaller projection inferior to it, the posterior inferior iliac spine. The ilium indents sharply at the posterior inferior iliac spine to form a deep groove called the greater sciatic notch (sy-AT-ik), through which the large sciatic nerve passes. Also looking at the ilium's medial surface, notice the iliac fossa, which is the large, smooth, anterior part, and the auricular surface, which is the smaller, rough, posterior part. Finally, on this side, we can see the arcuate line, which marks where the ilium forms part of the pelvic brim. It runs from the auricular surface along the body of the ilium. The smooth lateral surface of the ilium is known as its gluteal surface It is marked by three lines: the posterior, anterior, and inferior gluteal lines, to which the gluteal muscles attach.

Ethmoid bone

located deep in the anterior cranium between the eyes and posterior to the nasal bones. it superior surface -cribriform plate forms the roof of the nasal cavity. tiny nerves that detect smell pass through the cribriform(olfactory) foramina from the nasal cavity to the brain -crista galli is the superior projection which membranes that surround the brain attach. Extending inferior from the crista galli is the perpendicular plate, which forms part of the nasal septum that divides the nasal cavity into two sides. Ethmoidal sinuses two lateral masses. Lateral side of each lateral mass contributes to the orbit. The medial side of each lateral mass features two projection into the nasal cavity called the superior and middle nasal conchae so named because their curved shape resembles a conch shell

fetal skull

ou may have heard that infants have "soft spots" in their skulls. These soft spots, membranous areas called fontanels, result from the ossification process that cranial bones undergo. In the previous chapter, you learned about intramembranous ossification, in which bone develops from a membrane of fibrous connective tissue. Fontanels are places where ossification has not yet completed, and they remain until the cranial bones have ossified when a child is 18-24 months old. The skull of a fetus or infant has several fontanels, including (Figure 7.14): the anterior fontanel, which is located between the developing frontal and parietal bones where the coronal and sagittal sutures meet; the posterior fontanel, which is between the developing parietal and occipital bones at the apex of the lambdoid suture; the two sphenoid fontanels, which are located in the temple on the right and left sides where the sphenoid bone meets several other cranial bones; and the two mastoid fontanels, which are located at the junction of the lambdoid and squamous sutures where the developing parietal, temporal, and occipital bones meet. -ou may have heard that infants have "soft spots" in their skulls. These soft spots, membranous areas called fontanels, result from the ossification process that cranial bones undergo. In the previous chapter, you learned about intramembranous ossification, in which bone develops from a membrane of fibrous connective tissue. Fontanels are places where ossification has not yet completed, and they remain until the cranial bones have ossified when a child is 18-24 months old. The skull of a fetus or infant has several fontanels, including (Figure 7.14): the anterior fontanel, which is located between the developing frontal and parietal bones where the coronal and sagittal sutures meet; the posterior fontanel, which is between the developing parietal and occipital bones at the apex of the lambdoid suture; the two sphenoid fontanels, which are located in the temple on the right and left sides where the sphenoid bone meets several other cranial bones; and the two mastoid fontanels, which are located at the junction of the lambdoid and squamous sutures where the developing parietal, temporal, and occipital bones meet.

nasal cavity

part of the respiratory tract, the series of passages in the respiratory system that brings air into the lungs. As you have seen in the preceding sections, the nasal cavity is formed by several bones lined with mucous membranes (Figure 7.12): Posterior wall. The posterior boundary is formed by the sphenoid body and pterygoid processes. Lateral walls. The lateral walls of the nasal cavity are composed of several bones, including the ethmoid bone, the perpendicular plate of the palatine bones, the inferior nasal conchae, and the maxilla. You can only see these bones clearly in a parasagittal section of the skull (a section that is lateral to the nasal septum; see ​Figure 7.12a​). Roof and floor. The roof of the nasal cavity is formed by the cribriform plate of the ethmoid bone. The nasal cavity's floor is the hard palate, which is composed of the palatine bones and the palatine processes of the maxillae. Anterior wall. The nasal bones and maxillae form the anterior boundary of the nasal cavity. The cartilage and connective tissues of the nose attach to the margins of these bones. We examine these cartilages more closely in the respiratory system chapter (see ​Chapter 21​). Nasal septum. The two sides of the nasal cavity, called the nasal fossae, are divided by the nasal septum (see Figure 7.12b). Anteriorly, the nasal septum is composed of hyaline cartilage. Posteriorly, it is made up of the perpendicular plate of the ethmoid bone and the vomer. Occasionally, the nasal septum is shifted to one side, resulting in a deviated septum, which may make it difficult to breathe through the nose.

projections

provide sites to which ligaments and tendons attach or where bones articulate. -bony extensions of varying shapes and sizes; some provide locations for attachment of muscles, tendons, and ligaments -condyle -crest -head -tubercle/tuberosity -epicondyle -process -spine -protuberance -trochanter -line

cervical vertebrae

r most easily identifiable features are holes in their transverse processes, called transverse foramina. The transverse foramina allow the passage of the vertebral arteries and vertebral veins. Vertebrae C3−C7 generally have oval bodies, large triangular vertebral foramina, and short straight spinous processes that are forked like the tongue of a snake (see Figure 7.19a). However, C7 has a longer spinous process that is not forked. This long process is easily palpable and is used clinically as a landmark, which has led to the common name for C7: vertebra prominens, or "prominent vertebra." The first and second cervical vertebrae, known as the atlas and the axis, respectively, are notably different from C3−C7. C1: Atlas The first cervical vertebra (C1) is the Atlas (see Figure 7.19b). Its name stems from the fact that it holds the head, just as Atlas, the Titan from Greek mythology, supported the world. The atlas is immediately recognizable by its unusual shape and unique features. It has no vertebral body or spinous process, and instead has two anterior arches that meet at the anterior tubercle, and two posterior arches that meet at the posterior tubercle. The anterior and posterior arches enclose a large, teardrop-shaped vertebral foramen. Where the anterior and posterior arches meet, we find the lateral masses that contain the superior and inferior articular facets. Its superior articular facets articulate with the occipital condyles at the atlanto-occipital joint, the articulation that allows you to nod your head "yes." The inferior facets articulate with the second cervical vertebra. C2: Axis The second cervical vertebra (C2) is the axis (see Figure 7.19c). Its most notable feature is a superior projection from the body known as the dens or odontoid process. Both names derive from the fact that the process is shaped like a tooth (den/dont = "tooth"). Figure 7.19d shows how the dens fits up into the vertebral foramen of the atlas where the body of the atlas would be located, and is held snugly in place by the transverse ligament. Together, dens and atlas form the atlantoaxial joint. The way the dens fits into the atlas allows this joint to perform its function of rotational motion of the neck, such as shaking your head to indicate "no." Here we see another example of the Structure-Function Core Principle (​p. 28​).

paranasal sinuses

sinuses within the frontal, sphenoid, ethmoid, and maxillary bones—known collectively as the Paranasal sinuses—are located around the nasal cavity and connect to it through small bony openings (Figure 7.13). Like the nasal cavity, they are lined with mucous membranes. Air flowing through the nasal cavity passes through the openings into the sinuses, where the mucous membranes filter, warm, and humidify the air. The paranasal sinuses also lighten the skull considerably and enhance voice resonance.

Hyoid bone

small, C-shaped bone in the superior neck (Figure 7.15). It is actually not a skull bone, but we discuss it with the skull bones because of its close proximity to them. However, it does not articulate with any skull bones or, indeed, with any bones at all. Instead, the hyoid bone is suspended in the superior neck by muscles and ligaments that attach it to the styloid processes of the temporal bone and to the larynx. It serves as the attachment point for numerous muscles, including those involved in swallowing and speech. It is also an important bone in forensics, as a broken hyoid bone can indicate that a crime victim was strangled.

pubiss

smallest component of the pelvic bone is the anterior pubis (PYOO-biss), also called the pubic bone (see Figure 7.35). It consists of three parts that together approximate a C shape: the superior pubic ramus, the pubic body, and the inferior pubic ramus. The superior pubic ramus contributes to the acetabulum, and both rami form parts of the boundary of the obturator foramen. The two pubic bodies meet at a joint called the pubic symphysis (SIM-fih-sis), where they are united by a pad of fibrocartilage.

upper limb

three parts: the arm, which consists of the humerus; the forearm, which consists of the radius and ulna; and the wrist and hand, which contain the carpals, metacarpals, and phalanges.