A&P Chapter 7

general features and functions

-Besides forming the large cranial cavity, the skull also forms several smaller cavities, including the nasal cavity and orbits (eye sockets), which open to the exterior. Certain skull bones also contain cavities called paranasal sinuses that are lined with mucous membranes and open into the nasal cavity. Also within the skull are small middle ear cavities in the temporal bones that house the structures that are involved in hearing and equilibrium (balance) -Other than the auditory ossicles (tiny bones involved in hearing), which are located within the temporal bones, the mandible is the only movable bone of the skull. Joints called sutures attach most of the skull bones together and are especially noticeable on the outer surface of the skull -The skull has many surface markings, such as foramina (rounded passageways) and fissures (slitlike openings) through which blood vessels and nerves pass. You will learn the names of important skull bone surface markings as we describe each bone -In addition to protecting the brain, the cranial bones stabilize the positions of the brain, blood vessels, lymphatic vessels, and nerves through the attachment of their inner surfaces to meninges (membranes). The outer surfaces of cranial bones provide large areas of attachment for muscles that move various parts of the head. The bones also provide attachment for some muscles that produce facial expressions such as the frown of concentration you wear when studying this book. The facial bones form the framework of the face and provide support for the entrances to the digestive and respiratory systems. Together, the cranial and facial bones protect and support the delicate special sense organs for vision, taste, smell, hearing, and equilibrium (balance)

surface markings

-Bones have characteristic surface markings, structural features adapted for specific functions. Most are not present at birth but develop in response to certain forces and are most prominent in the adult skeleton. In response to tension on a bone surface from tendons, ligaments, aponeuroses, and fasciae, new bone is deposited, resulting in raised or roughened areas. Conversely, compression on a bone surface results in a depression. -There are two major types of surface markings: (1) depressions and openings, which allow the passage of soft tissues (such as blood vessels, nerves, ligaments, and tendons) or form joints, and (2) processes, projections or outgrowths that either help form joints or serve as attachment points for connective tissue (such as ligaments and tendons).

intervertebral discs

-Intervertebral discs (inter- = between) are found between the bodies of adjacent vertebrae from the second cervical vertebra to the sacrum and account for about 25% of the height of the vertebral column. Each disc has an outer fibrous ring consisting of fibrocartilage called the annulus fibrosus (annulus = ringlike) and an inner soft, pulpy, highly elastic substance called the nucleus pulposus (pulposus = pulplike). The superior and inferior surfaces of the disc consist of a thin plate of hyaline cartilage. The discs form strong joints, permit various movements of the vertebral column, and absorb vertical shock. Under compression, they flatten and broaden. -During the course of a day the discs compress and lose water from their cartilage so that we are a bit shorter at night. While we are sleeping there is less compression and rehydration occurs, so that we are taller when we awaken in the morning. With age, the nucleus pulposus hardens and becomes less elastic. Decrease in vertebral height with age results from bone loss in the vertebral bodies and not a decrease in thickness of the intervertebral discs -Since intervertebral discs are avascular, the annulus fibrosus and nucleus pulposus rely on blood vessels from the bodies of vertebrae to obtain oxygen and nutrients and remove wastes. Certain stretching exercises, such as yoga, decompress discs and increase general blood circulation, both of which speed up the uptake of oxygen and nutrients by discs and the removal of wastes

fact

-Spaces between ribs, called intercostal spaces, are occupied by intercostal muscles, blood vessels, and nerves. Surgical access to the lungs or other structures in the thoracic cavity is commonly obtained through an intercostal space. Special rib retractors are used to create a wide separation between ribs. The costal cartilages are sufficiently elastic in younger individuals to permit considerable bending without breaking. -In summary, the posterior portion of the rib connects to a thoracic vertebra by its head and the articular part of a tubercle. The facet of the head fits into either a facet on the body of one vertebra (T1 only) or into the demifacets of two adjoining vertebrae. The articular part of the tubercle articulates with the facet of the transverse process of the vertebra.

fact

-The cribriform plate (cribri- = sieve) of the ethmoid bone lies in the anterior floor of the cranium and forms the roof of the nasal cavity. The cribriform plate contains the olfactory foramina (olfact- = to smell) through which the olfactory nerves pass. Projecting superiorly from the cribriform plate is a triangular process called the crista galli (crista = crest; galli = cock), which serves as a point of attachment for the falx cerebri, the membrane that separates the two sides of the brain. Projecting inferiorly from the cribriform plate is the perpendicular plate, which forms the superior portion of the nasal septum -The lateral masses of the ethmoid bone compose most of the wall between the nasal cavity and the orbits. They contain 3 to 18 air spaces called ethmoidal cells. The ethmoidal cells together form the ethmoidal sinuses. The lateral masses contain two thin, scroll-shaped projections lateral to the nasal septum. These are called the superior nasal concha (= shell) or turbinate and the middle nasal concha (turbinate). The plural form is conchae. A third pair of conchae, the inferior nasal conchae, are separate bones (discussed shortly). The conchae greatly increase the vascular and mucous membrane surface area in the nasal cavity, which warms and moistens (humidifies) inhaled air before it passes into the lungs. The conchae also cause inhaled air to swirl; as a result, many inhaled particles become trapped in the mucus that lines the nasal cavity. This action of the conchae helps cleanse inhaled air before it passes into the rest of the respiratory passageways. The superior nasal conchae are near the olfactory foramina of the cribriform plate where the sensory receptors for olfaction (smell) terminate in the mucous membrane of the superior nasal conchae. Thus, they increase the surface area for the sense of smell.

maxillae

-The paired maxillae (= jawbones; singular is maxilla) unite to form the upper jawbone. They articulate with every bone of the face except the mandible (lower jawbone). The maxillae form part of the floors of the orbits, part of the lateral walls and floor of the nasal cavity, and most of the hard palate. -Each maxilla contains a large maxillary sinus that empties into the nasal cavity. The alveolar process (alveol- = small cavity) of the maxilla is a ridgelike arch that contains the alveoli (sockets) for the maxillary (upper) teeth. The palatine process is a horizontal projection of the maxilla that forms the anterior three-quarters of the hard palate. The union and fusion of the maxillary bones normally is completed before birth. If this fusion fails, this condition is referred to as a cleft palate. -The infraorbital foramen (infra- = below; -orbital = orbit), an opening in the maxilla inferior to the orbit, allows passage of the infraorbital blood vessels and nerve, a branch of the maxillary division of the trigeminal (V) nerve. Another prominent foramen in the maxilla is the incisive foramen (= incisor teeth) just posterior to the incisor teeth. It transmits branches of the greater palatine blood vessels and nasopalatine nerve. A final structure associated with the maxilla and sphenoid bone is the inferior orbital fissure, located between the greater wing of the sphenoid and the maxilla

fact

-The shape of the sphenoid resembles a butterfly with outstretched wings. The body of the sphenoid is the hollowed cubelike medial portion between the ethmoid and occipital bones. The space inside the body is the sphenoidal sinus, which drains into the nasal cavity. The sella turcica (sella = saddle; turcica = Turkish) is a bony saddle-shaped structure on the superior surface of the body of the sphenoid. The anterior part of the sella turcica, which forms the horn of the saddle, is a ridge called the tuberculum sellae. The seat of the saddle is a depression, the hypophyseal fossa, which contains the pituitary gland. The posterior part of the sella turcica, which forms the back of the saddle, is another ridge called the dorsum sellae -The greater wings of the sphenoid project laterally from the body and form the anterolateral floor of the cranium. The greater wings also form part of the lateral wall of the skull just anterior to the temporal bone and can be viewed externally. The lesser wings, which are smaller, form a ridge of bone anterior and superior to the greater wings. They form part of the floor of the cranium and the posterior part of the orbit of the eye -Between the body and lesser wing just anterior to the sella turcica is the optic foramen or canal (optic = eye), through which the optic (II) nerve and ophthalmic artery pass into the orbit. Lateral to the body between the greater and lesser wings is a triangular slit called the superior orbital fissure. This fissure may also be seen in the anterior view of the orbit in Figure 7.12. Blood vessels and cranial nerves pass through this fissure -The pterygoid processes (= winglike) project inferiorly from the points where the body and greater wings of the sphenoid bone unite; they form the lateral posterior region of the nasal cavity. Some of the muscles that move the mandible attach to the pterygoid processes. At the base of the lateral pterygoid process in the greater wing is the foramen ovale (= oval hole). The foramen lacerum (= lacerated), covered in part by a layer of fibrocartilage in living subjects, is bounded anteriorly by the sphenoid bone and medially by the sphenoid and occipital bones. It transmits a branch of the ascending pharyngeal artery. Another foramen associated with the sphenoid bone is the foramen rotundum (= round hole) located at the junction of the anterior and medial parts of the sphenoid bone. The maxillary branch of the trigeminal (V) nerve passes through the foramen rotundum

hyoid bone

-The single hyoid bone (= U-shaped) is a unique component of the axial skeleton because it does not articulate with any other bone. Rather, it is suspended from the styloid processes of the temporal bones by ligaments and muscles. Located in the anterior neck between the mandible and larynx, the hyoid bone supports the tongue, providing attachment sites for some tongue muscles and for muscles of the neck and pharynx. The hyoid bone consists of a horizontal body and paired projections called the lesser horns and the greater horns. Muscles and ligaments attach to the body and these paired projections -The hyoid bone and the cartilages of the larynx and trachea are often fractured during strangulation. As a result, they are carefully examined at autopsy when manual strangulation is a suspected cause of death

Each pyramid-shaped orbit has four regions that converge posteriorly:

1. Parts of the frontal and sphenoid bones comprise the roof of the orbit. 2. Parts of the zygomatic and sphenoid bones form the lateral wall of the orbit. 3. Parts of the maxilla, zygomatic, and palatine bones make up the floor of the orbit. 4. Parts of the maxilla, lacrimal, ethmoid, and sphenoid bones form the medial wall of the orbit.

Associated with each orbit are five openings:

1. The optic foramen (canal) is at the junction of the roof and medial wall. 2. The superior orbital fissure is at the superior lateral angle of the apex. 3. The inferior orbital fissure is at the junction of the lateral wall and floor. 4. The supraorbital foramen is on the medial side of the supraorbital margin of the frontal bone. 5. The lacrimal fossa is in the lacrimal bone.

black eye

A black eye is a bruising around the eye, commonly due to an injury to the face, rather than an eye injury. In response to trauma, blood and other fluids accumulate in the space around the eye, causing the swelling and dark discoloration. One cause might be a blow to the sharp ridge just superior to the supraorbital margin that fractures the frontal bone, resulting in bleeding. Another is a blow to the nose. Certain surgical procedures (face lift, eyelid surgery, jaw surgery, or nasal surgery) can also result in black eyes.

deviated nasal septum

A deviated nasal septum is one that does not run along the midline of the nasal cavity. It deviates (bends) to one side. A blow to the nose can easily damage, or break, this delicate septum of bone and displace and damage the cartilage. Often, when a broken nasal septum heals, the bones and cartilage deviate to one side or the other. This deviated septum can block airflow into the constricted side of the nose, making it difficult to breathe through that half of the nasal cavity. The deviation usually occurs at the junction of the vomer bone with the septal cartilage. Septal deviations may also occur due to developmental abnormality. If the deviation is severe, it may block the nasal passageway entirely. Even a partial blockage may lead to infection. If inflammation occurs, it may cause nasal congestion, blockage of the paranasal sinus openings, chronic sinusitis, headache, and nosebleeds. The condition usually can be corrected or improved surgically.

fact

A socket called the mandibular fossa is located on the inferior posterior surface of the zygomatic process of each temporal bone. Anterior to the mandibular fossa is a rounded elevation, the articular tubercle. The mandibular fossa and articular tubercle articulate with the mandible (lower jawbone) to form the temporomandibular joint (TMJ).

suture

A suture (= seam) is an immovable joint (in most cases in an adult skull) that holds most skull bones together. Sutures in the skulls of infants and children, however, often are movable and function as important growth centers in the developing skull. The names of many sutures reflect the bones they unite. For example, the frontozygomatic suture is between the frontal bone and the zygomatic bone. Similarly, the sphenoparietal suture is between the sphenoid bone and the parietal bone. In other cases, however, the names of sutures are not so obvious.

long bones

Almost all bones of the body can be classified into five main types based on shape: long, short, flat, irregular, and sesamoid. As you learned in Chapter 6, long bones have greater length than width, consist of a shaft and a variable number of extremities or epiphyses (ends), and are slightly curved for strength. A curved bone absorbs the stress of the body's weight at several different points, so that it is evenly distributed. If bones were straight, the weight of the body would be unevenly distributed, and the bone would fracture more easily. Long bones consist mostly of compact bone tissue in their diaphyses but have considerable amounts of spongy bone tissue in their epiphyses. Long bones vary tremendously in size and include those in the femur (thigh bone), tibia and fibula (leg bones), humerus (arm bone), ulna and radius (forearm bones), and phalanges (finger and toe bones).

fontanels (continued)

Although an infant may have many fontanels at birth, the form and location of six are fairly constant: • The unpaired anterior fontanel, the largest fontanel, is located at the midline among the two parietal bones and the frontal bone, and is roughly diamond-shaped. It usually closes 18 to 24 months after birth. • The unpaired posterior fontanel is located at the midline among the two parietal bones and the occipital bone. Because it is much smaller than the anterior fontanel, it generally closes about 2 months after birth. • The paired anterolateral fontanels, located laterally among the frontal, parietal, temporal, and sphenoid bones, are small and irregular in shape. Normally, they close about 3 months after birth. • The paired posterolateral fontanels, located laterally among the parietal, occipital, and temporal bones, are irregularly shaped. They begin to close 1 to 2 months after birth, but closure is generally not complete until 12 months.

sutural bones

An additional type of bone is classified by location rather than shape. Sutural bones (sutur- = seam) are small bones located in sutures (joints) between certain cranial bones. Their number varies greatly from person to person.

caudal anesthesia

Anesthetic agents that act on the sacral and coccygeal nerves are sometimes injected through the sacral hiatus, a procedure called caudal anesthesia. While this approach is not as common as lumbar epidural block, it is preferred when sacral nerve spread of the anesthetics is preferred over lumbar nerve spread. Because the sacral hiatus is between the sacral cornua, the cornua are important bony landmarks for locating the hiatus. Anesthetic agents also may be injected through the posterior sacral foramina. Since the hiatal and foraminal injection sites are inferior to the lowest portion of the spinal cord, there is little danger of damaging the cord. The lumbar approach is preferred because there is considerable variability in the anatomy of the sacral hiatus, and with advancing age the dorsal ligaments and cornua thicken, making it difficult to identify the hiatal margins.

fact

At the floor of the cranial cavity is the petrous portion (petrous = rock) of the temporal bone. This triangular part, located at the base of the skull between the sphenoid and occipital bones, houses the internal ear and the middle ear, structures involved in hearing and equilibrium (balance). It also contains the carotid foramen, through which the carotid artery passes. Posterior to the carotid foramen and anterior to the occipital bone is the jugular foramen, a passageway for the jugular vein.

Bones of the adult skeleton are grouped into two principal divisions:

Bones of the adult skeleton are grouped into two principal divisions: the axial skeleton and the appendicular skeleton (appendic- = to hang onto). Table 7.1 presents the 80 bones of the axial skeleton and the 126 bones of the appendicular skeleton. Figure 7.1 shows how both divisions join to form the complete skeleton (the bones of the axial skeleton are shown in blue). You can remember the names of the divisions if you think of the axial skeleton as consisting of the bones that lie around the longitudinal axis of the human body, an imaginary vertical line that runs through the body's center of gravity from the head to the space between the feet: skull bones, auditory ossicles (ear bones), hyoid bone, ribs, sternum (breastbone), and bones of the vertebral column.

dislocated ribs

Dislocated ribs, which are common in body contact sports, involve displacement of a costal cartilage from the sternum, with resulting pain, especially during deep inhalations.

regions of the vertebral column

Exhibits 7.H through 7.K present the five regions of the vertebral column, beginning superiorly and moving inferiorly. The regions are the cervical, thoracic, lumbar, sacral, and coccygeal. Note that vertebrae in each region are numbered in sequence, from superior to inferior. When you actually view the bones of the vertebral column, you will notice that the transition from one region to the next is not abrupt but gradual, a feature that helps the vertebrae fit together.

fact

Figure 7.23a shows the parts of a typical (third through ninth) rib. The head is a projection at the posterior end of the rib that contains a pair of articular facets (superior and inferior). The facet of the head may fit either into a facet on the body of a single vertebra or into the demifacets of two adjoining vertebrae to form a vertebrocostal joint. The neck is a constricted portion of a rib just lateral to the head. A knoblike structure on the posterior surface, where the neck joins the body, is called a tubercle. The nonarticular part of the tubercle attaches to the transverse process of a vertebra by a ligament (lateral costotransverse ligament). The articular part of the tubercle articulates with the facet of a transverse process of a vertebra to form vertebrocostal joints. The body (shaft) is the main part of the rib. A short distance beyond the tubercle, an abrupt change in the curvature of the shaft occurs. This point is called the costal angle. The inner surface of the rib has a costal groove that protects the intercostal blood vessels and a small nerve.

flat bones

Flat bones are generally thin and composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy bone tissue. Flat bones afford considerable protection and provide extensive areas for muscle attachment. Flat bones include the cranial bones, which protect the brain; the sternum (breastbone) and ribs, which protect organs in the thorax; and the scapulae (shoulder blades).

facial bones

Fourteen facial bones form the face: two nasal bones, two maxillae (or maxillas), two zygomatic bones, the mandible, two lacrimal bones, two palatine bones, two inferior nasal conchae, and the vomer.

irregular bones

Irregular bones have complex shapes and cannot be grouped into any of the previous categories. They vary in the amount of spongy and compact bone present. Such bones include the vertebrae (backbones), hip bones, certain facial bones, and the calcaneus.

fact

Note the frontal squama, a scalelike plate of bone that forms the forehead of the skull. It gradually slopes inferiorly from the coronal suture, on the top of the skull, then angles abruptly and becomes almost vertical above the orbits. At the superior border of the orbits, the frontal bone thickens, forming the supraorbital margin (supra- = above; -orbi= circle). From this margin, the frontal bone extends posteriorly to form the roof of the orbit, which is part of the floor of the cranial cavity. Within the supraorbital margin, slightly medial to its midpoint, is a hole called the supraorbital foramen. Sometimes the foramen is incomplete and is called the supraorbital notch. As you read about each foramen associated with a cranial bone, refer to Table 7.3 to note which structures pass through it. The frontal sinuses lie deep to the frontal squama. Sinuses, or more technically paranasal sinuses, are mucous membrane-lined cavities within certain skull bones that will be discussed later.

temporomandibular joint (TMJ) syndrome

One problem associated with the temporomandibular joint is temporomandibular joint (TMJ) syndrome. It is characterized by dull pain around the ear, tenderness of the jaw muscles, a clicking or popping noise when opening or closing the mouth, limited or abnormal opening of the mouth, headache, tooth sensitivity, and abnormal wearing of the teeth. TMJ syndrome can be caused by improperly aligned teeth, grinding or clenching the teeth, trauma to the head and neck, or arthritis. Treatments include application of moist heat or ice, limiting the diet to soft foods, administration of pain relievers such as aspirin, muscle retraining, use of a splint or bite plate to reduce clenching and teeth grinding (especially when worn at night), adjustment or reshaping of the teeth (orthodontic treatment), and surgery.

fact

Recall from Chapter 6 that in adults, red bone marrow is restricted to flat bones such as the ribs, sternum (breastbone), and skull; irregular bones such as vertebrae (backbones) and hip bones; long bones such as the proximal epiphyses of the femur (thigh bone) and humerus (arm bone); and some short bones.

rib fractures

Rib fractures are the most common chest injuries. They usually result from direct blows, most often from impact with a steering wheel, falls, or crushing injuries to the chest. Ribs tend to break at the point where the greatest force is applied, but they may also break at their weakest point—the site of greatest curvature, just anterior to the costal angle. The middle ribs are the most commonly fractured. In some cases, fractured ribs may puncture the heart, great vessels of the heart, lungs, trachea, bronchi, esophagus, spleen, liver, and kidneys. Rib fractures are usually quite painful. Rib fractures are no longer bound with bandages because of the pneumonia that would result from lack of proper lung ventilation.

separated ribs

Separated ribs involve displacement of a rib and its costal cartilage; as a result, a rib may move superiorly, overriding the rib above and causing severe pain.

sesamoid bones

Sesamoid bones (= shaped like a sesame seed) develop in certain tendons where there is considerable friction, tension, and physical stress, such as the palms and soles. They may vary in number from person to person, are not always completely ossified, and typically measure only a few millimeters in diameter. Notable exceptions are the two patellae (kneecaps), large sesamoid bones located in the quadriceps femoris tendon that are normally present in everyone. Functionally, sesamoid bones protect tendons from excessive wear and tear, and they often change the direction of pull of a tendon, which improves the mechanical advantage at a joint.

orbit

Seven bones of the skull join to form each orbit (eye socket) or orbital cavity, which contains the eyeball and associated structures. The three cranial bones of the orbit are the frontal, sphenoid, and ethmoid; the four facial bones are the palatine, zygomatic, lacrimal, and maxilla

processes

Seven processes arise from the vertebral arch. At the point where a lamina and pedicle join, a transverse process extends laterally on each side. A single spinous process (spine) projects posteriorly from the junction of the laminae. These three processes serve as points of attachment for muscles. The remaining four processes form joints with other vertebrae above or below. The two superior articular processes of a vertebra articulate (form joints) with the two inferior articular processes of the vertebra immediately above them. In turn, the two inferior articular processes of that vertebra articulate with the two superior articular processes of the vertebra immediately below them, and so on. The articulating surfaces of the articular processes, which are referred to as facets (= little faces), are covered with hyaline cartilage. The articulations formed between the vertebral bodies and articular facets of successive vertebrae are termed intervertebral joints.

short bones

Short bones are somewhat cube-shaped and are nearly equal in length and width. They consist of spongy bone tissue except at the surface, which has a thin layer of compact bone tissue. Examples of short bones are most carpal (wrist) bones and most tarsal (ankle) bones.

sinusitis

Sinusitis is an inflammation of the mucous membrane of one or more paranasal sinuses. It may be caused by a microbial infection (virus, bacterium, or fungus), allergic reactions, nasal polyps, or a severely deviated nasal septum. If the inflammation or an obstruction blocks the drainage of mucus into the nasal cavity, fluid pressure builds up in the paranasal sinuses, and a sinus headache may develop. Other symptoms may include nasal congestion, inability to smell, fever, and cough. Treatment options include decongestant sprays or drops, oral decongestants, nasal corticosteroids, antibiotics, analgesics to relieve pain, warm compresses, and surgery.

fact

The adult human skeleton consists of 206 named bones, most of which are paired, with one member of each pair on the right and left sides of the body. The skeletons of infants and children have more than 206 bones because some of their bones fuse later in life. Examples are the hip bones and some bones (sacrum and coccyx) of the vertebral column (backbone).

fact

The amount of closure in fontanels helps a physician gauge the degree of brain development. In addition, the anterior fontanel serves as a landmark for withdrawal of blood for analysis from the superior sagittal sinus (a large midline vein within the covering tissues that surround the brain).

upper and lower limbs

The appendicular skeleton consists of the bones of the upper and lower limbs (extremities or appendages), plus the bones forming the girdles that connect the limbs to the axial skeleton. Functionally, the auditory ossicles in the middle ear, which vibrate in response to sound waves that strike the eardrum, are not part of either the axial or appendicular skeleton, but they are grouped with the axial skeleton for convenience

cervical vertebrae

The bodies of the cervical vertebrae (C1-C7) are smaller than all other vertebrae except those that form the coccyx. Their vertebral arches, however, are larger. All cervical vertebrae have three foramina: one vertebral foramen and two transverse foramina. The vertebral foramina of cervical vertebrae are the largest in the spinal column because they house the cervical enlargement of the spinal cord. Each cervical transverse process contains a transverse foramen through which the vertebral artery and its accompanying vein and nerve fibers pass. The spinous processes of C2 through C6 are often bifid—that is, they branch into two small projections at the tips.

coccyx

The coccyx, like the sacrum, is triangular in shape. It is formed by the fusion of usually four coccygeal vertebrae, indicated in Figure 7.21 as Co1-Co4. The coccygeal vertebrae fuse somewhat later than the sacral vertebrae, between the ages of 20 and 30. The dorsal surface of the body of the coccyx contains two long coccygeal cornua that are connected by ligaments to the sacral cornua. The coccygeal cornua are the pedicles and superior articular processes of the first coccygeal vertebra. They are on the lateral surfaces of the coccyx, formed by a series of transverse processes; the first pair are the largest. The coccyx articulates superiorly with the apex of the sacrum. In females, the coccyx points inferiorly to allow the passage of a baby during birth; in males, it points anteriorly

fact

The concave anterior side of the sacrum faces the pelvic cavity. It is smooth and contains four transverse lines (ridges) that mark the joining of the sacral vertebral bodies. At the ends of these lines are four pairs of anterior sacral foramina. The lateral portion of the superior surface of the sacrum contains a smooth surface called the sacral ala (= wing; plural is alae), which is formed by the fused transverse processes of the first sacral vertebra (S1).

vertebral canal

The convex, posterior surface of the sacrum contains a median sacral crest, the fused spinous processes of the upper sacral vertebrae; a lateral sacral crest, the fused transverse processes of the sacral vertebrae; and four pairs of posterior sacral foramina. These foramina connect with anterior sacral foramina to allow passage of nerves and blood vessels. The sacral canal is a continuation of the vertebral cavity. The laminae of the fifth sacral vertebra, and sometimes the fourth, fail to meet. This leaves an inferior entrance to the vertebral canal called the sacral hiatus (= opening). On either side of the sacral hiatus is a sacral cornu (cornu = horn; plural is cornua), an inferior articular process of the fifth sacral vertebra. They are connected by ligaments to the coccyx.

coronal suture

The coronal suture (coron- = relating to the frontal or coronal plane) unites the frontal bone and both parietal bones

cranial bones

The cranial bones (crani- = brain case) form the cranial cavity, which encloses and protects the brain. The eight cranial bones are the frontal bone, two parietal bones, two temporal bones, the occipital bone, the sphenoid bone, and the ethmoid bone.

ethmoid bone

The ethmoid bone (= like a sieve) is a delicate bone located in the anterior part of the cranial floor medial to the orbits and is spongelike in appearance. It is anterior to the sphenoid and posterior to the nasal bones. The ethmoid bone forms (1) part of the anterior portion of the cranial floor; (2) the medial wall of the orbits; (3) the superior portion of the nasal septum, a partition that divides the nasal cavity into right and left sides; and (4) most of the superior sidewalls of the nasal cavity. The ethmoid bone is a major superior supporting structure of the nasal cavity and forms an extensive surface area in the nasal cavity.

fact

The external occipital protuberance is the most prominent midline projection on the posterior surface of the bone just above the foramen magnum. You may be able to feel this structure as a bump on the back of your head, just above your neck. A large fibrous, elastic ligament, the ligamentum nuchae (nucha- = nape of neck), extends from the external occipital protuberance to the seventh cervical vertebra to help support the head. Extending laterally from the protuberance are two curved ridges, the superior nuchal lines, and below these are two inferior nuchal lines, which are areas of muscle attachment

fact

The feature of the thoracic vertebrae that distinguishes them from other vertebrae is that they articulate with the ribs. Except for T11 and T12, the transverse processes of thoracic vertebrae have costal facets that articulate with the tubercles of the ribs. Additionally, the vertebral bodies of thoracic vertebrae have articular surfaces that form articulations with the heads of the ribs. The articular surfaces on the vertebral bodies are called either facets or demifacets. A facet is formed when the head of a rib articulates with the body of one vertebra. A demifacet is formed when the head of a rib articulates with two adjacent vertebral bodies. As you can see in Figure 7.19, on each side of the vertebral body T1 has a superior facet for the first rib and an inferior demifacet for the second rib. On each side of the vertebral body of T2-T8, there is a superior demifacet and an inferior demifacet as ribs two through nine articulate with two vertebrae, and T10-T12 have a facet on each side of the vertebral body for ribs 10-12. These articulations between the thoracic vertebrae and ribs, called vertebrocostal joints, are distinguishing features of thoracic vertebrae. Movements of the thoracic region are limited by the attachment of the ribs to the sternum.

fact

The fetus has a single anteriorly concave curve throughout the length of the entire vertebral column. At about the third month after birth, when an infant begins to hold its head erect, the anteriorly convex cervical curve develops. Later, when the child sits up, stands, and walks, the anteriorly convex lumbar curve develops. The thoracic and sacral curves are called primary curves because they retain the original curvature of the embryonic vertebral column. The cervical and lumbar curves are known as secondary curves because they begin to form later, several months after birth. All curves are fully developed by age 10. However, secondary curves may be progressively lost in old age.

fact

The first through seventh pairs of ribs have a direct anterior attachment to the sternum by a strip of hyaline cartilage called costal cartilage (cost- = rib). The costal cartilages contribute to the elasticity of the thoracic cage and prevent various blows to the chest from fracturing the sternum and/or ribs. The ribs that have costal cartilages and attach directly to the sternum are called true (vertebrosternal) ribs. The articulations formed between the true ribs and the sternum are called sternocostal joints. The remaining five pairs of ribs are termed false ribs because their costal cartilages either attach indirectly to the sternum or do not attach to the sternum at all. The cartilages of the eighth, ninth, and tenth pairs of ribs attach to one another and then to the cartilages of the seventh pair of ribs. These false ribs are called vertebrochondral ribs. The eleventh and twelfth pairs of ribs are false ribs designated as floating (vertebral) ribs because the costal cartilages at their anterior ends do not attach to the sternum at all. These ribs attach only posteriorly to the thoracic vertebrae. Inflammation of one or more costal cartilages, called costochondritis, is characterized by local tenderness and pain in the anterior chest wall that may radiate. The symptoms mimic the chest pain (angina pectoris) associated with a heart attack.

atlas

The first two cervical vertebrae differ considerably from the others. The atlas (C1), named after the mythological Atlas who supported the world on his shoulders, is the first cervical vertebra inferior to the skull. The atlas is a ring of bone with anterior and posterior arches and large lateral masses. It lacks a body and a spinous process. The superior surfaces of the lateral masses, called superior articular facets, are concave. They articulate with the occipital condyles of the occipital bone to form the paired atlanto-occipital joints. These articulations permit you to move your head to signify "yes." The inferior surfaces of the lateral masses, the inferior articular facets, articulate with the second cervical vertebra. The transverse processes and transverse foramina of the atlas are quite large.

frontal bone

The frontal bone forms the forehead (the anterior part of the cranium), the roofs of the orbits (eye sockets), and most of the anterior part of the cranial floor. Soon after birth, the left and right sides of the frontal bone are united by the metopic suture, which usually disappears between the ages of six and eight.

hard palate

The hard palate is the bony roof of the mouth, and is formed by the palatine processes of the maxillae and horizontal plates of the palatine bones. The hard palate separates the nasal cavity from the oral cavity.

fact

The junction of the manubrium and body forms the sternal angle. The manubrium has a depression on its superior surface, the suprasternal notch. Lateral to the suprasternal notch are clavicular notches that articulate with the medial ends of the clavicles to form the sternoclavicular joints. The manubrium also articulates with the costal cartilages of the first and second ribs. The body of the sternum articulates directly or indirectly with the costal cartilages of the second through tenth ribs. The xiphoid process consists of hyaline cartilage during infancy and childhood and does not completely ossify until about age 40. No ribs are attached to it, but the xiphoid process provides attachment for some abdominal muscles. Incorrect positioning of the hands of a rescuer during cardiopulmonary resuscitation (CPR) may fracture the xiphoid process, driving it into internal organs. During thoracic surgery, the sternum may be split along the midline and the halves spread apart to allow surgeons access to structures in the thoracic cavity such as the thymus, heart, and great vessels of the heart. After surgery, the halves of the sternum are held together with wire sutures.

lambdoid suture

The lambdoid suture unites the two parietal bones to the occipital bone. This suture is so named because of its resemblance to the capital Greek letter lambda (Λ), as can be seen in Figure 7.6 (with the help of a little imagination). Sutural bones may occur within the sagittal and lambdoid sutures.

lumbar vertebrae

The lumbar vertebrae (L1-L5) are the largest and strongest of the unfused bones in the vertebral column because the amount of body weight supported by the vertebrae increases toward the inferior end of the backbone. Their various projections are short and thick. The superior articular processes are directed medially instead of superiorly, and the inferior articular processes are directed laterally instead of inferiorly. The spinous processes are quadrilateral in shape, are thick and broad, and project nearly straight posteriorly. The spinous processes are well adapted for the attachment of the large back muscles.

mandible

The mandible (mand- = to chew), or lower jawbone, is the largest, strongest facial bone. It is the only movable skull bone (other than the auditory ossicles, the small bones of the ear). In the lateral view, you can see that the mandible consists of a curved, horizontal portion, the body, and two perpendicular portions, the rami (= branches; singular is ramus). The angle of the mandible is the area where each ramus meets the body. Each ramus has a posterior condylar process that articulates with the mandibular fossa and articular tubercle of the temporal bone to form the temporomandibular joint (TMJ), and an anterior coronoid process to which the temporalis muscle attaches. The depression between the coronoid and condylar processes is called the mandibular notch. The alveolar process is the ridgelike arch containing the alveoli (sockets) for the mandibular (lower) teeth.

mastoiditis

The mastoid portion (mastoid = breast-shaped) of the temporal bone is located posterior and inferior to the external auditory meatus (meatus = passageway), or ear canal, which directs sound waves into the ear. In an adult, this portion of the bone contains several mastoid air cells that communicate with the hollow space of the middle ear. These tiny air-filled compartments are separated from the brain by thin bony partitions. Middle ear infections that go untreated can spread into the mastoid air cells, causing a painful inflammation called mastoiditis

fact

The mastoid process is a rounded projection of the mastoid portion of the temporal bone posterior and inferior to the external auditory meatus. It is the point of attachment for several neck muscles. The internal auditory meatus is the opening through which the facial (VII) nerve and vestibulocochlear (VIII) nerve pass. The styloid process (styl- = stake or pole) projects inferiorly from the inferior surface of the temporal bone and serves as a point of attachment for muscles and ligaments of the tongue and neck. Between the styloid process and the mastoid process is the stylomastoid foramen, through which the facial (VII) nerve and stylomastoid artery pass

fact

The mental foramen (ment- = chin) is approximately inferior to the second premolar tooth. It is near this foramen that dentists reach the mental nerve when injecting anesthetics. Another foramen associated with the mandible is the mandibular foramen on the medial surface of each ramus, another site often used by dentists to inject anesthetics. The mandibular foramen is the beginning of the mandibular canal, which runs obliquely in the ramus and anteriorly to the body. Through the canal pass the inferior alveolar nerves and blood vessels, which are distributed to the mandibular teeth.

fact

The narrow inferior portion of the sacrum is known as the apex. The broad superior portion of the sacrum is called the base. The anteriorly projecting border of the base, called the sacral promontory, is one of the points used for measurements of the pelvis. On both lateral surfaces the sacrum has a large ear-shaped auricular surface that articulates with the ilium of each hip bone to form the sacroiliac joint. Posterior to the auricular surface is a roughened surface, the sacral tuberosity, which contains depressions for the attachment of ligaments. The sacral tuberosity unites with the hip bones to form the sacroiliac joints. The superior articular processes of the sacrum articulate with the inferior articular processes of the fifth lumbar vertebra, and the base of the sacrum articulates with the body of the fifth lumbar vertebra to form the lumbosacral joint.

nasal septum

The nasal cavity is a space inside the skull that is divided into right and left sides by a vertical partition called the nasal septum, which consists of bone and cartilage. The three components of the nasal septum are the vomer, septal cartilage, and the perpendicular plate of the ethmoid bone. The anterior border of the vomer articulates with the septal cartilage, which is hyaline cartilage, to form the anterior portion of the septum. The superior border of the vomer articulates with the perpendicular plate of the ethmoid bone to form the remainder of the nasal septum. The term "broken nose," in most cases, refers to damage to the septal cartilage rather than the nasal bones themselves.

occipital bone

The occipital bone (occipit- = back of head) forms the posterior part and most of the base of the cranium. Also view the occipital bone and surrounding structures in the inferior view of the skull in Figure 7.7. The foramen magnum (= large hole) is in the inferior part of the bone. The medulla oblongata (inferior part of the brain) connects with the spinal cord within this foramen, and the vertebral and spinal arteries also pass through it along with the accessory (XI) nerve. The occipital condyles, oval processes with convex surfaces on either side of the foramen magnum, articulate with depressions on the first cervical vertebra (atlas) to form the atlanto-occipital joint, which allows you to nod your head "yes." Superior to each occipital condyle on the inferior surface of the skull is the hypoglossal canal (hypo- = under; -glossal = tongue)

lacrimal bones

The paired lacrimal bones (lacrim- = teardrops) are thin and roughly resemble a fingernail in size and shape. These bones, the smallest bones of the face, are posterior and lateral to the nasal bones and form a part of the medial wall of each orbit. The lacrimal bones each contain a lacrimal fossa, a vertical tunnel formed with the maxilla, that houses the lacrimal sac, a structure that gathers tears and passes them into the nasal cavity

nasal bones

The paired nasal bones are small, flattened, rectangular-shaped bones that form the bridge of the nose. These small bones protect the upper entry to the nasal cavity and provide attachment for a couple of thin muscles of facial expression. For those of you who wear glasses, they are the bones that form the resting place for the bridge of the glasses. The major structural portion of the nose consists of cartilage.

temporal bones

The paired temporal bones (tempor- = temple) form the inferior lateral aspects of the cranium and part of the cranial floor. In Figure 7.4a, note the temporal squama (= scale), the thin, flat part of the temporal bone that forms the anterior and superior part of the temple (the region of the cranium around the ear). Projecting from the inferior portion of the temporal squama is the zygomatic process, which articulates (forms a joint) with the temporal process of the zygomatic (cheek) bone. Together, the zygomatic process of the temporal bone and the temporal process of the zygomatic bone form the zygomatic arch.

paranasal sinuses

The paranasal sinuses (para- = beside) are cavities within certain cranial and facial bones near the nasal cavity. They are most evident in a sagittal section of the skull. The paranasal sinuses are lined with mucous membranes that are continuous with the lining of the nasal cavity. Secretions produced by the mucous membranes of the paranasal sinuses drain into the lateral wall of the nasal cavity. Paranasal sinuses are quite small or absent at birth, but increase in size during two periods of facial enlargement—during the eruption of the teeth and at the onset of puberty. They arise as outgrowths of the nasal mucosa that project into the surrounding bones. Skull bones containing the paranasal sinuses are the frontal, sphenoid, ethmoid, and maxillae. The paranasal sinuses allow the skull to increase in size without a change in the mass (weight) of the bone. The paranasal sinuses increase the surface area of the nasal mucosa, thus increasing the production of mucus to help moisten and cleanse inhaled air. In addition, the paranasal sinuses serve as resonating (echo) chambers within the skull that intensify and prolong sounds, thereby enhancing the quality of the voice. The influence of the paranasal sinuses on your voice becomes obvious when you have a cold; the passageways through which sound travels into and out of the paranasal sinuses become blocked by excess mucus production, changing the quality of your voice.

sacrum

The sacrum is a triangular bone formed by the union of five sacral vertebrae (S1-S5). The sacral vertebrae begin to fuse in individuals between 16 and 18 years of age, a process usually completed by age 30. Positioned at the posterior portion of the pelvic cavity medial to the two hip bones, the sacrum serves as a strong foundation for the pelvic girdle. The female sacrum is shorter, wider, and more curved between S2 and S3 than the male sacrum

sagittal suture

The sagittal suture (sagitt- = arrow) unites the two parietal bones on the superior midline of the skull. The sagittal suture is so named because in the infant, before the bones of the skull are firmly united, the suture and the fontanels (soft spots) associated with it resemble an arrow.

axis

The second cervical vertebra (C2), the axis, does have a vertebral body. A peglike process called the dens (= tooth) or odontoid process projects superiorly through the anterior portion of the vertebral foramen of the atlas. The dens makes a pivot on which the atlas and head rotate. This arrangement permits side-to-side movement of the head, as when you move your head to signify "no." The articulation formed between the anterior arch of the atlas and dens of the axis, and between their articular facets, is called the atlanto-axial joint. In some instances of trauma, the dens of the axis may be driven into the medulla oblongata of the brain. This type of injury is the usual cause of death from whiplash injuries.

fact

The shape of the face changes dramatically during the first two years after birth. The brain and cranial bones expand, the first set of teeth form and erupt (emerge), and the paranasal sinuses increase in size. Growth of the face ceases at about 16 years of age. The 14 facial bones include two nasal bones, two maxillae (or maxillas), two zygomatic bones, the mandible, two lacrimal bones, two palatine bones, two inferior nasal conchae, and the vomer.

unique features of the skull

The skull exhibits several unique features not seen in other bones of the body. These include sutures, paranasal sinuses, and fontanels.

skull

The skull is the bony framework of the head. It contains 22 bones (not counting the bones of the middle ears) and rests on the superior end of the vertebral column (backbone). The bones of the skull are grouped into two categories: cranial bones and facial bones.

fontanels

The skull of a developing embryo consists of cartilage and mesenchyme arranged in thin plates around the developing brain. Gradually, ossification occurs, and bone slowly replaces most of the cartilage and mesenchyme. At birth, bone ossification is incomplete, and the mesenchyme-filled spaces become dense connective tissue regions between incompletely developed cranial bones called fontanels (= little fountains), commonly called "soft spots". Fontanels are areas where unossified mesenchyme develops into the dense connective tissues of the skull. As bone formation continues after birth, the fontanels are eventually replaced with bone by intramembranous ossification, and the thin collagenous connective tissue junctions that remain between neighboring bones become the sutures. Functionally, the fontanels serve as spacers for the growth of neighboring skull bones and provide some flexibility to the fetal skull, allowing the skull to change shape as it passes through the birth canal and later permitting rapid growth of the brain during infancy.

sphenoid bone

The sphenoid bone (= wedge-shaped) lies at the middle part of the base of the skull. This bone is called the keystone of the cranial floor because it articulates with all the other cranial bones, holding them together. View the floor of the cranium superiorly and note the sphenoid articulations. The sphenoid bone joins anteriorly with the frontal and ethmoid bones, laterally with the temporal bones, and posteriorly with the occipital bone. The sphenoid lies posterior and slightly superior to the nasal cavity and forms part of the floor, side walls, and rear wall of the orbit

sternum

The sternum, or breastbone, is a flat, narrow bone located in the center of the anterior thoracic wall that measures about 15 cm (6 in.) in length and consists of three parts. The superior part is the manubrium (= handlelike); the middle and largest part is the body; and the inferior, smallest part is the xiphoid process (= sword-shaped). The segments of the sternum typically fuse by age 25, and the points of fusion are marked by transverse ridges.

thorax

The term thorax refers to the entire chest region. The skeletal part of the thorax, the thoracic cage, is a bony enclosure formed by the sternum, ribs and their costal cartilages, and the bodies of the thoracic vertebrae. The costal cartilages attach the ribs to the sternum. The thoracic cage is narrower at its superior end and broader at its inferior end and is flattened from front to back. It encloses and protects the organs in the thoracic and superior abdominal cavities, provides support for the bones of the upper limbs, and, as you will see in Chapter 23, plays a role in breathing.

fact

The third through sixth cervical vertebrae (C3-C6), represented by the vertebra in Figure 7.18c, correspond to the structural pattern of the typical cervical vertebra previously described. The seventh cervical vertebra (C7), called the vertebra prominens, is somewhat different. It has a large, nonbifid spinous process that may be seen and felt at the base of the neck, but otherwise is typical.

palatine bones

The two L-shaped palatine bones form the posterior portion of the hard palate, part of the floor and lateral wall of the nasal cavity, and a small portion of the floors of the orbits. The posterior portion of the hard palate is formed by the horizontal plates of the palatine bones

inferior nasal conchae

The two inferior nasal conchae, which are inferior to the middle nasal conchae of the ethmoid bone, are separate bones, not part of the ethmoid bone. These scroll-like bones form a part of the inferior lateral wall of the nasal cavity and project into the nasal cavity. All three pairs of nasal conchae (superior, middle, and inferior) increase the surface area of the nasal cavity and help swirl and filter air before it passes into the lungs. However, only the superior nasal conchae of the ethmoid bone are involved in the sense of smell.

parietal bones

The two parietal bones (pariet- = wall) form the greater portion of the sides and roof of the cranial cavity. The internal surfaces of the parietal bones contain many protrusions and depressions that accommodate the blood vessels supplying the dura mater, the superficial connective tissue (meninx) covering of the brain.

squamous sutures

The two squamous sutures (squam- = flat, like the flat overlapping scales of a snake) unite the parietal and temporal bones on the lateral aspects of the skull

zygomatic bones

The two zygomatic bones (zygo- = yokelike), commonly called cheekbones, form the prominences of the cheeks and part of the lateral wall and floor of each orbit. They articulate with the frontal, maxilla, sphenoid, and temporal bones. The temporal process of the zygomatic bone projects posteriorly and articulates with the zygomatic process of the temporal bone to form the zygomatic arch

vertebral body

The vertebral body, the thick, disc-shaped anterior portion, is the weight-bearing part of a vertebra. Its superior and inferior surfaces are roughened for the attachment of cartilaginous intervertebral discs. The anterior and lateral surfaces contain nutrient foramina, openings through which blood vessels deliver nutrients and oxygen and remove carbon dioxide and wastes from bone tissue.

vertebral column

The vertebral column, also called the spine, backbone, or spinal column, makes up about two-fifths of your total height and is composed of a series of bones called vertebrae (singular is vertebra). The vertebral column, the sternum, and the ribs form the skeleton of the trunk of the body. The vertebral column consists of bone and connective tissue; the spinal cord that it surrounds and protects consists of nervous and connective tissues. At about 71 cm (28 in.) in an average adult male and about 61 cm (24 in.) in an average adult female, the vertebral column functions as a strong, flexible rod with elements that can move forward, backward, and sideways, and rotate. In addition to enclosing and protecting the spinal cord, it supports the head and serves as a point of attachment for the ribs, pelvic girdle, and muscles of the back and upper limbs.

vomer

The vomer (= plowshare) is a roughly triangular bone on the floor of the nasal cavity that articulates superiorly with the perpendicular plate of the ethmoid bone and sphenoid bone and inferiorly with both the maxillae and palatine bones along the midline. It forms the inferior portion of the bony nasal septum, the partition that divides the nasal cavity into right and left sides.

thoracic vertebrae

Thoracic vertebrae (T1-T12) are considerably larger and stronger than cervical vertebrae. In addition, the spinous processes on T1 through T10 are long, laterally flattened, and directed inferiorly. In contrast, the spinous processes on T11 and T12 are shorter, broader, and directed more posteriorly. Compared to cervical vertebrae, thoracic vertebrae also have longer and larger transverse processes. They are easily identified by their costal facets (cost- = rib), which are articular surfaces for the ribs.

ribs

Twelve pairs of ribs, numbered 1-12 from superior to inferior, give structural support to the sides of the thoracic cavity. The ribs increase in length from the first through seventh, and then decrease in length to rib 12. Each rib articulates posteriorly with its corresponding thoracic vertebra.

vertebral arch

Two short, thick processes, the pedicles (= little feet), project posteriorly from the vertebral body and then unite with the flat laminae (= thin layers) to form the vertebral arch. The vertebral arch extends posteriorly from the body of the vertebra; together, the vertebral body and the vertebral arch surround the spinal cord by forming the vertebral foramen. The vertebral foramen contains the spinal cord, adipose tissue, areolar connective tissue, and blood vessels. Collectively, the vertebral foramina of all vertebrae form the vertebral (spinal) canal. The pedicles exhibit superior and inferior indentations called vertebral notches. When the vertebral notches are stacked on top of one another, they form an opening between adjoining vertebrae on both sides of the column. Each opening, called an intervertebral foramen, permits the passage of a single spinal nerve carrying information to and from the spinal cord.

cleft palate and cleft lip

Usually the palatine processes of the maxillary bones unite during weeks 10 to 12 of embryonic development. Failure to do so can result in one type of cleft palate. The condition may also involve incomplete fusion of the horizontal plates of the palatine bones. Another form of this condition, called cleft lip, involves a split in the upper lip. Cleft lip and cleft palate often occur together. Depending on the extent and position of the cleft, speech and swallowing may be affected. In addition, children with cleft palate tend to have many ear infections, which can lead to hearing loss. Facial and oral surgeons recommend closure of cleft lip during the first few weeks following birth, and surgical results are excellent. Repair of cleft palate typically is completed between 12 and 18 months of age, ideally before the child begins to talk. Because the palate is important for pronouncing consonants, speech therapy may be required, and orthodontic therapy may be needed to align the teeth. Recent research strongly suggests that supplementation with folic acid (one of the B vitamins) during early pregnancy decreases the incidence of cleft palate and cleft lip. The mechanism behind this is not yet understood.

abnormal curves

Various conditions may exaggerate the normal curves of the vertebral column, or the column may acquire a lateral bend, resulting in abnormal curves of the vertebral column. Three such abnormal curves—kyphosis, lordosis, and scoliosis—are described in the Disorders: Homeostatic Imbalances section at the end of this chapter.

parts of a typical vertebra

Vertebrae in different regions of the spinal column vary in size, shape, and detail, but they are similar enough that we can discuss the structures (and the functions) of a typical vertebra. Vertebrae typically consist of a vertebral body, a vertebral arch, and several processes.

foramina

We mentioned most of the foramina (openings for blood vessels, nerves, or ligaments; singular is foramen) of the skull in the descriptions of the cranial and facial bones that they penetrate. As preparation for studying other systems of the body, especially the nervous and cardiovascular systems, these foramina and the structures passing through them are listed in Table 7.3. For your convenience and for future reference, the foramina are listed alphabetically.

normal curves

When viewed from the anterior or posterior, a normal adult vertebral column appears straight. But when viewed from the side, it shows four slight bends called normal curves. Relative to the front of the body, the cervical and lumbar curves are convex (bulging out); the thoracic and sacral curves are concave (cupping in). The curves of the vertebral column increase its strength, help maintain balance in the upright position, absorb shocks during walking, and help protect the vertebrae from fracture.

age-related changes in the vertebral column

With advancing age the vertebral column undergoes changes that are characteristic of the skeletal system in general. These changes include reduction in the mass and density of the bone along with a reduction in the collagen-to-mineral content within the bone, changes that make the bones more brittle and susceptible to damage. The articular surfaces, those surfaces where neighboring bones move against one another, lose their covering cartilage as they age; in their place rough bony growths form that lead to arthritic conditions. In the vertebral column, bony growths around the intervertebral discs, called osteophytes, can lead to a narrowing (stenosis) of the vertebral canal. This narrowing can lead to compression of spinal nerves and the spinal cord, which can manifest as pain and decreased muscle function in the back and lower limbs.

The total number of vertebrae during early development is 33. As a child grows, several vertebrae in the sacral and coccygeal regions fuse. As a result, the adult vertebral column typically contains 26 vertebrae. These are distributed as follows:

• 7 cervical vertebrae (cervic- = neck) in the neck region. • 12 thoracic vertebrae (thorax = chest) posterior to the thoracic cavity. • 5 lumbar vertebrae (lumb- = loin) supporting the lower back. • 1 sacrum (= sacred bone) consisting of five fused sacral vertebrae. • 1 coccyx (= cuckoo, because the shape resembles the bill of a cuckoo bird) usually consisting of four fused coccygeal vertebrae. The cervical, thoracic, and lumbar vertebrae are movable, but the sacrum and coccyx are not.