Have you ever studied the names of towns or cities printed on a road map or travel atlas? You might have noticed that the names of some municipalities offer interesting clues to important geographic features in the surrounding area. A town named Big Bend turns out to be situated along a sweeping curve in a river. If you’re driving to Chapel Hill, you might expect to find a church on a bluff overlooking the town. And in Desert Hot Springs, well…you shouldn’t expect to find a landscape covered with lush forests.
Just as names on a map can tell you something worth knowing about the geography of a place that you might like to visit, learning the names of the bones of the skeleton will make it easier for you to locate and remember many other structures in the human body. For instance, the frontal lobe of your brain, where you think about anatomy and physiology, is deep to the frontal bone of your skull. Your radial artery, where you can feel your pulse, runs alongside the radius bone in your forearm. Nerves are also named for nearby bones; for example, your tibial nerve runs alongside your tibia, one of the bones in your leg. In fact, a particular nerve in your body is sometimes called a “bone” in everyday speech. This nerve, the ulnar nerve, runs alongside your ulna, a bone that parallels the radius bone in your forearm. If you get a tingling sensation throughout your forearm and hand after hitting your elbow, it’s because you’ve hit the ulnar nerve, your “funny bone.”
OVERVIEW OF THE SKELETON
The typical adult skeleton contains 206 bones, along with an assortment of ligaments and cartilages. The qualifying phrase “typical adult” is important for two reasons: (1) The number of bones in the skeleton rises and falls from birth to adulthood. (2) The final number of bones in an adult skeleton may vary from person to person. Why is this so? Recall that a child’s skeleton has more bones than an adult skeleton since some of the child’s bones exist as segments separated by epiphyseal discs. In addition, sesamoid bones may develop in various tendons and the skull of an adult may have a varying number of sutural bones. The skeleton is divided into two parts: the axial skeleton and the appendicular skeleton.
In this chapter, you will learn the names and identify the unique features of individual bones. Apart from having a characteristic shape, each bone displays a unique assortment of surface markings that provides clues about how the bone interacts with other bones and organs. A bone may have projections (such as knobs, ridges, bumps, or spines) where it articulates (joins) with another bone or to which ligaments and tendons attach. In addition, a bone’s surface may be marked by holes or carved by depressions and grooves through which blood vessels and nerves travel. Table 10.1 lists the variety of bone features that you will learn to recognize in this chapter.
Table 10-1. Bone features
The axial skeleton is related to the entire skeleton like a trunk is related to a tree. It forms the vertical “axis” of the skeleton upon which the “limbs” attach. The axial skeleton of a typical adult contains about 80 bones (40% of the total number in the body). Of these, 28 bones are part of the skull, 1 hyoid bone is in the neck region, 26 bones are in the vertebral column (spine), and 25 bones are in the thoracic cage (Figure 10-1).
Figure 10-1. The axial skeleton
In addition to its role in maintaining homeostasis, the axial skeleton is important in several ways: (1) it surrounds and protects the brain, spinal cord, and soft visceral organs of the thorax. (2) It houses the sensory receptors for vision, smell, hearing, and taste that allow monitoring of the external environment. (3) It supports entrances and structures that function in the respiratory and digestive systems. (4) It provides attachment sites for skeletal muscles that allow you to speak, breathe, swallow, turn your head, and make facial expressions. (5) It provides attachment sites for your appendicular skeleton and the skeletal muscles that allow you to bend your back and move your shoulders and hips. Let’s begin with the bones that form the axial skeleton’s superior end, the skull.
THE SKULL
The skull is the most complex part of the skeleton, containing a wide assortment of flat and irregular bones that lock together like pieces of a 3-dimensional jigsaw puzzle (Figure 10-2). The skull of a typical adult consists of 22 bones plus 6 tiny auditory ossicles (3 in each ear) involved in hearing. For the most part, the skull is very rigid; the mandible (lower jaw) and the auditory ossicles are its only moving parts. You move your mandible up and down to grind food between your teeth. So when you can chew gum and listen to music at the same time, then all of your skull’s movable bones are in motion.
The most distinctive features of the skull are its large domed region made up of cranial bones and its flattened, elongated anterior portion made up of facial bones. Together the skull bones form six cavities: the large cranial cavity that holds the brain, the right and left orbital cavities (eye sockets) that shield the eyes, the right and left nasal cavities that facilitate breathing, and the oral cavity that houses the tongue and teeth. Visualizing these cavities while you study can help you to remember the specific structural roll of each bone in the skull as well as the relationship of each skull bone to its neighbors.
Bones of the Skull
Cranial bones are part of the cranium (KRĀ-ne-um; “helmet”), a term that can refer to different things. In this text, the cranium means only the skull bones that touch the brain’s protective coverings, and excludes the auditory ossicles. The cranial bones include:
- 1 frontal bone
- 2 parietal bones
- 2 temporal bones
- 1 occipital bone
- 1 sphenoid bone
- 1 ethmoid bone
Just as the cranial bones form the cranium, the 14 facial bones form the face, the anterior part of the skull. Equally important, the facial bones connect with one another and with the cranium to form five smaller cavities beneath the face: the inferior portions of both orbital cavities, the left and right nasal cavities, and the oral cavity. Like some cranial bones, some of your facial bones occur in pairs (mirroring one another on each side of the skull). The facial bones that you can see from the skull’s surface include:
- 2 maxillae
- 2 nasal bones
- 2 zygomatic bones
- 1 mandible
The remaining facial bones are deep to the face, and thus can’t be seen from the surface of the skull. These bones include:
- 2 lacrimal bones (orbits)
- 2 palatine bones (oral cavity)
- 2 inferior nasal conchae (nasal cavity)
- 1 vomer (nasal cavity)
Recognizing and recalling the names of the different skull bones and their features, and remembering why all of these structures are important will take some time, so be patient. To make your task a little easier, skull bones are color coded to help you learn the bones and their special features. All parts of a particular skull bone will have its own unique color throughout this chapter.
Sutures
All skull bones, except the mandible, connect tightly to adjacent skull bones at immovable joints. Most of these joints are called sutures (SŪ-cherz; sutur-, seam). The sutures’ zigzag pattern makes skull bones look like they have been stitched together with a sewing machine. Sutures contain dense regular connective tissue consisting of very short collagenous fibers that “glue” the skull bones together and prevent them from shifting. Without strong sutures, your skull could flatten out on one side if you laid your head on a pillow. The four longest sutures are also the easiest ones to locate on the cranium exterior:
- The coronal suture is named for its orientation in the coronal (frontal) plane on the superior surface of the cranium. It occurs where the frontal bone meets the parietal bones.
- The sagittal suture is also named for its orientation, which is in the sagittal plane on the superior surface of the cranium. It occurs where the right parietal bone meets the left parietal bone.
- The squamous sutures (squam-, plate or scale) may have been given their name because the temporal bones around them look like plates or scales on a fish. The squamous suture runs along the lateral sides of the cranium where the temporal bone meets the parietal bone.
- The lambdoid suture (LAM-doyd) resembles the Greek letter, lambda (Λ). This suture is found on the posterior surface of the cranium where the occipital bone meets the parietal bones. The superior point of the lambdoid suture intersects the sagittal suture. The lambdoid suture is the most common location for the occurrence of small flat bones known as sutural (or Wormian) bones.
Figure 10-2. The skull
Figure 10-2. (continued)
The Calvaria and Skull Base
Each of the four major sutures helps hold together a portion of the skull’s superior half, also known as the calvaria (kal-VĀR-ē-a; calvar-, skull) or skullcap. The calvaria consists of the frontal and parietal bones, and the superior halves of the occipital and temporal bones. In certain types of brain surgery, a neurosurgeon removes the calvaria to operate on the brain.
Whereas the calvaria forms the superior half of the skull, the base forms the inferior half. The external surface of the base consists of cranial bones and facial bones (except for the mandible), and you can see these when the skull is turned upside-down. However, removing the calvaria from a dry skull (one without a brain) reveals the skull’s internal base, which consists only of cranial bones.
The skull’s largest cavity is the cranial cavity, and it houses the brain. The calvaria encloses the cranial cavity superiorly and laterally, while the internal base encloses the cavity inferiorly and laterally. The relatively smooth, internal surfaces of the cranial bones help prevent the brain from being scratched if the head suddenly moves.
The skull’s internal base consists of three large depressions, called cranial fossae. Each fossa is at a different level, like three steps of a staircase, and supports a different region of the brain. The anterior cranial fossa lies at the base of the frontal bone and a small portion of the ethmoid bone; this fossa supports the frontal lobe of the brain. The anterior cranial fossa is the most superior of the three cranial fossae, so it is like the “top step” in the staircase. Portions of the sphenoid and the right and left temporal bones form the middle cranial fossa, so it is like the “middle step” in the staircase. This fossa primarily supports the temporal lobes of the brain. The occipital bone forms the posterior cranial fossa, which is the most inferior of the three fossae; therefore, it is like the “bottom” step in our staircase analogy. This fossa primarily supports the cerebellum.
Cranial Bones
Although the bones of the skull work together as a single, tightly knit unit, it’s useful studying them individually, too. For example, forensic specialists (professionals who gather evidence for law courts) and archeologists (scientists who study past human cultures) often need to identify bones or bone fragments in isolation from the rest of a skull. But in general, studying individual skull bones is useful because they have markings, openings, and sinuses that will prepare you to learn about the muscles, blood vessels, nerves, and respiratory structures you will study later in this book. Indeed, the skull is like a 3D jigsaw puzzle.
FRONTAL BONE
The frontal bone (FRUN-tal; front-, forehead or brow) forms the anterior portion of the cranium, the superior wall of the orbital cavities, and most of the anterior cranial fossa. The posterior margin of the frontal bone articulates with the parietal bones at the coronal suture, while the inferior margins articulate with the sphenoid, zygomatic, ethmoid, lacrimal, maxilla, and nasal bones. Notice the part of the frontal bone that articulates with the zygomatic bone of the face. This process is the zygomatic process, so named not because it is part of the zygomatic bone, but because it articulates with the zygomatic bone. The squamous part of the frontal bone forms the forehead. Muscles that wrinkle the forehead’s skin and move the eyebrows attach to the squamous part of the frontal bone.
Note: It is common in anatomy to name a bony feature after the bone with which the feature interacts, not after the bone of which it is a part. This fact can help you to learn and remember the relationship of one bone in the skeleton with another.
The frontal bone forms a ridge, the supraorbital margin, at the superior edge of each orbit; an eyebrow lies over each supraorbital margin. The region between the supraorbital margins, where the forehead meets the nose, is the glabella (gla-BEL-a). The glabella (glab-, smooth or hairless) gets its name because eyebrows do not usually grow over that region of the skin. A pointed nasal spine projects inferiorly from the glabella, but in an intact skull, it is hidden behind the nasal bones. A supraorbital foramen in each supraorbital margin marks the passageway for an artery that delivers blood to the eyebrows and upper eyelids. Often the supraorbital foramen is merely a small notch in the supraorbital margin. On the superior wall of each orbit, posterior to the supraorbital margin, is a slight depression called a lacrimal fossa. A tear gland resides within the lacrimal fossa, and secretes tears to lubricate and protect the surface of the eyeball.
Note: The frontal bone develops from a right and left half, but these segments normally fuse to form single bone. Sometimes fusion does not occur, and a metopic (frontal) suture (mē-TOP-ik; metop-, forehead) joins the two halves of the frontal bones.
Cavities lined with mucous membranes often develop within the frontal bone and these cavities are called frontal sinuses. Mucus from the frontal sinuses drains into the nasal cavity and helps trap dust and other airborne particles, preventing them from passing farther into the respiratory system.
PARIETAL BONES
with the other parietal bone at the sagittal suture; with the frontal bone at the coronal suture; with the temporal bone at the squamous suture; and with the occipital bone at the lambdoid suture. Structurally, the parietal bones are the simplest cranial bones, resembling a square bedspread with its corners turned inward. The external surface is relatively smooth, with only two slightly raised temporal lines that serve as attachment sites for muscles that move the lower jaw. The internal surface of a parietal bone resembles a roadmap drawn in the sand, because it contains numerous shallow grooves in which blood vessels lie that supply the coverings of the brain.
TEMPORAL BONES
Each of the oddly shaped right and left temporal bones (TEM-pô-ral; “temple”) look somewhat like an elephant’s ears, which is an apt comparison because these bones house the organs of hearing. The temporal bones form parts of the cranium’s lateral walls and base. Each temporal bone articulates with 3 cranial bones (sphenoid, parietal, and occipital) and 2 facial bones (zygomatic and mandible). The temporal bones are the only skull bones that articulate with the mandible.
Externally, the inferior half of the temporal bone has a coarse texture, but the superior half, the squamous part, is very smooth and is relatively thin-walled. In fact, if you hold a skull next to a light and look through the squamous part of the temporal bone, you will likely see the shadows of your fingers on the opposite side. Because a temporal bone’s squamous part is so thin, it is relatively easy to fracture. This fact should suggest why it’s important to wear a helmet when engaging in activities that could expose the skull to trauma.
The external acoustic meatus (a-KOOS-tik; “hear”; mē-Ā-tus; “passage”) is a prominent opening just inferior to the squamous part of the temporal bone. This tube-like structure directs sound to the tympanum (eardrum), located deeper inside the temporal bone. For this reason, the region around the meatus is called the tympanic part of the temporal bone, and is considered part of the external ear. Looking at the internal surface, the petrous part (PET-rus; petr-, rock) of the temporal bone is a thick, bony ridge inside the cranial cavity that separates the middle cranial fossa from the posterior cranial fossa. The petrous part contains a middle ear and an inner ear cavity. The middle ear cavity contains the auditory ossicles that pass sound vibrations deeper into the temporal bone to the inner ear. The inner ear, in turn, contains the sensory organs for hearing and equilibrium (balance). Several nerves that conduct impulses from the inner ear to the brain pass through a tube, the internal acoustic meatus, on the posterior surface of the petrous part.
Each temporal bone contains three distinctive processes that anchor skeletal muscles. The zygomatic process (zī-gō-MAT-ik) is the most conspicuous of the three. This process is so named (zygoma-, yoke) because it looks like the crossbar used to attach a horse to a plow or wagon. The zygomatic process extends anteriorly from the squamous portion to connect with the temporal process on the zygomatic bone. Together, the zygomatic process on the temporal bone and the temporal process on the zygomatic bone form the zygomatic arch. A muscle that closes the mouth extends from the zygomatic arch to the mandible. You can feel your own zygomatic arches as ridges running between your ears and cheeks. The mandibular fossa lies at the posterior, inferior end of the zygomatic process, just anterior to the external acoustic meatus. This fossa articulates with the mandible’s condyle and this is the only movable joint between skull bones.
The mastoid process (MAS-toyd) is the most posterior of the three processes, and is so-named because it resembles a breast (mast-, “breast”). You can feel the mastoid process as a bony knob immediately posterior to your ear. A large neck muscle pulls on the mastoid process to turn your head to the side. Although externally the mastoid process may appear to be a solid knob, it contains numerous air-filled spaces called mastoid air cells that help lighten the skull.
The styloid process (STĪ-loyd) looks like a sharp spike (styl-, “pillar”) and extends anteriorly from the base of the mastoid process. The styloid process anchors skeletal muscles that move the tongue, pharynx (throat), and hyoid bone. Because of its slender shape and exposed position, the styloid process is often broken off and missing from dry skulls in the lab.
Externally, several openings are visible on the temporal bone’s inferior surface. As its name implies, the stylomastoid foramen lies between the styloid process and the mastoid process. A nerve passing through this foramen stimulates certain facial muscles to contract. Lying posterior and medial to the styloid process is the jugular foramen (JUG-ū-lar; jugul-, throat). Three nerves and the large jugular vein pass through this opening; the jugular vein transports blood from the brain toward the heart. Slightly anterior and medial to the jugular foramen is the carotid canal (ka-ROT-id). The carotid artery, which delivers most of the brain’s blood supply, passes through this canal.
OCCIPITAL BONE
The occipital bone (ok-SIP-i-tal; occiput-, “back of head”) forms most of the cranium’s posterior wall and much of its posterior base; internally this bone forms most of the posterior cranial fossa. The occipital bone articulates with the parietal bones at the lambdoid suture, and with the right and left temporal bones at the occipitomastoid sutures, which appear to be a continuation of the lambdoid suture. The occipital bone’s most anterior part, the basilar part (BĀ-si-lar; also called the basioccipital process; bā-ē-ok-SIPP-i-tal), slopes superiorly and articulates with the sphenoid bone and vomer. The smooth sloping surface of the occipital bone’s basilar part and sphenoid bone in the posterior cranial fossa is called the clivus (KLĪ-vus; “slope”). The anterior portion of the brain stem that connects to the spinal cord rests on the clivus.
The occipital bone is the only skull bone that articulates with the spine. Two large, rounded knobs on the occipital bone’s inferior surface, the occipital condyles, articulate with the first cervical vertebra (the atlas). Occipital condyles enable you to nod your head to make the “yes” sign (but do not allow you to shake your head from side-to-side to make the “no” sign).
There are several distinguishing projections on the occipital bone’s external surface. The external occipital protuberance is the site where the long nuchal ligament (NOO-kul; nuch-, back of neck), also called the ligamentum nuchae (lig-a-MEN-tum), attaches the skull to all cervical vertebrae (neck bones). You can feel this protuberance as a rounded bump on the posterior part of your head, where the skull begins to curve inward. Two horizontal ridges, the nuchal lines, are near the occipital protuberance and serve as attachment sites for skeletal muscles extending from the neck and back; these muscles keep your head in a raised position.
The occipital bone contains several foramina and forms a portion of two others. Between the occipital condyles is the foramen magnum (MAG-num; “large”), the skull’s largest foramen, where the brain and spinal cord connect. Flanking the foramen magnum on the right and left side is a hypoglossal canal, through which a nerve passes that controls tongue movement. Some skulls have a condylar canal at the posterior end of each occipital condyle; this hole serves as a passageway for a small vein that drains blood from the brain. The occipital bone forms the posterior rim of the right and left jugular foramen, while the temporal bones form the anterior rims.
SPHENOID BONE
The sphenoid bone (SFĒ-noyd) has the most elaborate shape of all skull bones. From an anterior view it looks like a swooping bird of prey with its legs extended. The sphenoid bone lives up to its name (sphen-, wedge), because it is wedged in among most of the other skull bones. It articulates with all of the other cranial bones and 7 facial bones (maxillae, zygomatic, and palatine bones on both sides, and the single vomer). The sphenoid contains two air-filled sphenoid sinuses. Mucous membranes that line the sphenoid sinuses contribute mucus to the nasal cavity. The sphenoid sinuses also lighten the skull, and affect the sound of the voice. The sella turcica (SEL-a, “saddle”; TER-si-ka, “Turkish”) is a saddle-shaped structure on the superior surface of the sphenoid that supports the hypophysis (pituitary gland) of the brain. The sphenoid bone contains three prominent processes: the greater wings, the lesser wings, and the pterygoid processes. The greater wings extend laterally from the inferior portion of the sphenoid’s body, and then curve slightly superiorly. Externally, the greater wings are visible as a rectangular shaped bone in the temple region, between the frontal bone and temporal bone. The greater wings also form a portion of each orbit’s posterior wall. Anterior to the sella turcica, the lesser wings extend laterally from the sphenoid’s body and taper sharply to meet the frontal bone. The lesser wings form a ridge that marks the anterior, superior border of the middle cranial fossa. They also form part of each orbit’s superior wall.
Despite the fact that their name means “wing,” two pterygoid processes (TER-i-goyd; ptery-, wing) look more like extended legs. These processes extend inferiorly from the sphenoid’s body and from the medial portion of each greater wing. The pterygoid processes are attachment sites for the pterygoid muscles that move the mandible in a horizontal plane (side-to-side).
Intricate bone that it is, the sphenoid contains a host of openings, including canals, fissures, and foramina. The optic canals (OP-tik; “eye”) pass through the superior portion of the sphenoid body, and each canal supports an optic nerve and blood vessel from the eyeball. Within the cranial cavity, the optic canal opens just anterior to the sella turcica. The optic canal is also visible on the sphenoid’s orbital surface, inside the superior, medial wall of each orbit. Also within the orbit, and lateral to the optic canal, is the superior orbital fissure. This slit-like groove is the space between the sphenoid’s greater and lesser wings, and it serves as a passageway for several nerves and blood vessels leading to the face.
On the sphenoid’s superior surface, three foramina are aligned in a diagonal row on each greater wing. The most anterior of these, the foramen rotundum (ro-TUN-dum; “round”), is a round opening inferior and slightly lateral to the sella turcica. It serves as a passageway for a nerve leading to the maxilla. Looking posteriorly and laterally, you can see the foramen ovale (ō-VĀ-lē; “egg”), an egg-shaped (oval) opening near the posterior edge of the greater wing. It serves as a passageway for a nerve leading to the mandible. Slightly posterior and lateral from the foramen ovale is the foramen spinosum (spi-NO-sum; “thorn”), so-named because it is next to a short spine on the external surface of the cranial base. The foramen spinosum supports small blood vessels that lead to the membranous coverings that surround the brain.
The temporal bone and sphenoid bone each form part of the foramen lacerum (LA-ser-um; lacer-, to tear). This foramen’s irregular edges look as if they were torn like paper; hence, the name “lacerum” is appropriate. The carotid artery emerges from the carotid canal and passes through this foramen before entering the cranial cavity.
ETHMOID BONE
The ethmoid bone (ĒTH-moyd; ethm-, sieve) has a strange, irregular shape that is difficult to compare with anything else, and much of the bone is hidden behind other bones. Its name relates to a sieve, a device with small holes used to strain or sift materials. Numerous ethmoid air cells inside the ethmoid bone resemble the holes of a sieve.
The ethmoid bone forms the superior portion and much of the lateral portion of the nasal cavity, most of the nasal septum, most of an orbit’s medial wall, and a small portion of the anterior cranial fossa. Although the ethmoid is a cranial bone, it articulates with only two other cranial bones (the frontal and sphenoid), while articulating with more facial bones than any other cranial bone. The ethmoid articulates with the right and left nasal bones, lacrimal bones, maxillae, palatine bones, inferior nasal conchae, and the vomer.
The midline portion of the ethmoid bone consists of the crista galli and the perpendicular plate. The crista galli (KRIS-ta, “crest;” GAL-ē; “rooster”) is a pyramid-like ridge resembling a rooster’s comb on the ethmoid’s superior surface. It serves as an attachment site for one of the membranous coverings of the brain. Extending inferiorly from the crista galli is the broad, flat perpendicular plate. The plate forms the superior half of the bony nasal septum, a vertical partition that divides the nasal cavity into right and left halves. The posterior portion of the perpendicular plate articulates with the vomer, which forms the inferior half of the nasal septum. In the living body (not a dried skull), a vertical plate of hyaline cartilage connects the perpendicular plate to the vomer to complete the nasal septum.
The lateral portions of the ethmoid bone consist of the cribriform plate, lateral masses, and nasal conchae. The cribriform plate (KRIB-ri-form; cribri-, sieve) is a narrow, horizontal shelf on either side of the crista galli. Numerous olfactory foramina perforate the cribriform plate and mark the passageways for the olfactory nerves leading from the nasal cavity; these nerves function in smelling. The lateral masses contain the ethmoid air cells.
Parts of the ethmoid bone extend inferiorly as two plate-like projections called nasal conchae (KON-kē, shells; singular is concha, KON-ka). The smaller, more superior conchae are the superior nasal conchae; the larger more inferior conchae are the middle nasal conchae. In the living body, the nasal conchae are enveloped in a cartilaginous structure and covered with a mucous membrane, which warms and moistens the air flowing through the nasal cavity on its way to the lungs. The mucus also helps trap airborne particles, thereby acting like an air filter.
Table 10-2. Summary of CRANIAL BONES and their major features
Facial Bones
Compared to the cranial bones, some of the skull’s facial bones are small and less easy to locate. A helpful way to keep track of the facial bones is to group them according to the three sets of facial cavities with which they are associated. For example, the maxillae, palatine bones, and mandible form the entire oral cavity. The maxillae, vomer, nasal bones, inferior nasal conchae, and lacrimal bones contribute to the nasal cavity. Finally, the maxillae, zygomatic bones, and lacrimal bones contribute to the orbits. (Notice the maxillae and lacrimal bones contribute to several facial cavities.)
MAXILLAE
The maxillae (mak-SIL-ē, “jaws;” singular is maxilla) form the anterior and superior portions of the oral cavity, but they touch almost every other facial bone and contribute to the orbits and nasal cavities, too. In fact, just as the sphenoid bone is said to “anchor” the cranial bones, the maxillae anchor the skull’s facial bones. The maxillae form the upper jaw, and their inferior surfaces articulating at the intermaxillary suture form the anterior two-thirds of the hard palate (PAL-et, “roof of the mouth”). Near the posterior end of the hard palate, the maxillae articulate with the palatine bones, which form the remainder of the hard palate. The right and left maxillae articulate with all other facial bones on the same side of the face except the mandible. Each maxilla also articulates with the frontal bone and the ethmoid bone of the cranium.
Each maxilla forms several parts of an orbit. The superior portion of the maxilla, its orbital surface, forms most of the orbit’s inferior surface, and the medial, inferior part of the orbit’s inferior edge, the infraorbital margin. On the floor of the orbit, the maxilla forms the anterior edge of a slit-like groove, the inferior orbital fissure; the sphenoid bone’s greater wing forms the posterior edge. Inferior to the infraorbital margin on the maxilla’s anterior surface is a small opening, the infraorbital foramen. A nerve and a blood vessel leading to the face pass through this foramen. The maxilla also forms the lateral wall of the nasal cavity.
The most superior portions of the maxillae extend as wing-like frontal processes that articulate with the frontal bone. The medial edges of the frontal processes articulate with the nasal bones. The medial, anterior edge of each maxilla encloses the inferior half of the nasal aperture, the large opening into the nasal cavity. At the base of the nasal aperture, the maxillae join to form the anterior nasal spine.
Lateral to the nasal cavity and inferior to the orbit, each maxilla contains a large air-filled space, the maxillary sinus. Mucus secretions from the maxillary sinuses drain into the nasal cavity. Extending laterally to articulate with the zygomatic bone is the zygomatic process. (Recall that the temporal bone also has a zygomatic process.)
Making a V-shape around the anterior and lateral edges of the hard palate are alveolar processes (al-VĒ-ō-lar; alveoli-, socket), which form the rim around the alveolar sockets holding the teeth. Along the midline immediately posterior to the front teeth is the incisive fossa (in-SĪ-siv, “to cut”), named for the incisors (front teeth). This fossa leads to a foramen through which a blood vessel and nerve pass through to the soft tissues covering the hard palate.
PALATINE BONES
Two additional facial bones complete the roof of the oral cavity. The palatine bones (PAL-a-tīn; palat-, roof of mouth), one on each side of a midline plane, form the posterior one-third of the hard palate, as well as a tiny portion of each orbit. Each palatine bone articulates with four other facial bones: a maxilla, inferior nasal concha, vomer, and the other palatine bone. Each one also articulates with two cranial bones: the sphenoid and the ethmoid. Viewed anteriorly, each palatine bone is L-shaped, but the two bones together resemble a “crooked U.”
MANDIBLE
The mandible (MAN-di-bul, “jaw”) is the lower jaw forming the floor of the oral cavity. The mandible is the only movable skull bone (excluding the middle ear bones). It can move inferiorly, superiorly, and laterally to allow you to chew food, speak, and even make different facial expressions. A mandible of a typical adult contains 16 teeth. The mandible has three distinct landmarks on both of its superior, posterior ends: the condylar process, the coronoid process, and the mandibular notch.
The condylar process is a rounded knob that articulates with the mandibular fossa of the temporal bone. Anterior to the condylar process is the pointed coronoid process (KOR-ō-noyd; coron-, crow’s beak). As the term suggests, this process looks like a bird’s beak. The large temporalis muscle extends from the lateral side of the skull through the zygomatic arch to attach to the coronoid process. When the muscle pulls on the process, the anterior end of the mandible moves superiorly and the mouth closes. The U-shaped gap between the condylar process and coronoid process is the mandibular notch.
Extending inferiorly from the mandibular notch is a vertical portion of the mandible, the ramus (RĀ-mus, “branch”). The mandible’s angle at the inferior, posterior end of each ramus is where the ramus meets the more horizontal part of the mandible, the body. The superior portion of the mandible’s body contains alveolar sockets, which normally contain teeth. The rim around each socket is the alveolar margin. Superior projections along the alveolar margin are called alveolar processes. The inferior half of the mandible body’s anterior tip curves outward, forming the mental protuberance (MEN-tul), or “chin.” The midline region of the mandible’s body is the mandibular symphysis.
The mandible displays four major foramina (two on each side). The mandibular foramen is found on the medial surface of the ramus. Blood vessels and nerves supplying the mandible’s teeth pass through this foramen. The mental foramen is on the mandible body’s external surface, slightly lateral and posterior to the mandibular symphysis. Nerves and blood vessels serving the chin and lower lip pass through this foramen.
NASAL BONES
Turning next to the facial bones that solely border the nasal cavity, the nasal bones form the bridge (superior portion) of the nose. The right and left nasal bones articulate with one another medially at the internasal suture, along the midline of the face. In addition, each nasal bone articulates with the frontal bone, superiorly; with a maxilla, laterally; and with the perpendicular plate of the ethmoid bone, posteriorly. The inferior edges of the nasal bones form the superior rim of the large nasal aperture leading to the nasal cavity.
VOMER
The vomer (VŌ-mer, “plow”) is a long, thin bone that resembles the blade of a plow or knife. The vomer’s vertical plate forms the inferior one-third of the bony nasal septum. The superior portion of the vomer articulates with the perpendicular plate of the ethmoid bone. Beginning anteriorly and moving posteriorly along the midline, the inferior edge of the vomer articulates with the maxillae and palatine bones. The superior, posterior end of the vomer displays two alae (Ā-lē; “wings”), each of which articulates with the sphenoid bone.
INFERIOR NASAL CONCHAE
Unlike the superior and middle nasal conchae, which are processes of the ethmoid bone, the inferior nasal conchae are independent bones that protrude from the lateral walls of the nasal cavity. Most of an inferior nasal concha’s lateral surface articulates with a maxilla, while a small portion near its posterior end articulates with a palatine bone. A very tiny portion articulates with a lacrimal bone, superiorly.
Each inferior nasal concha tapers on its anterior and posterior ends. The medial surface is slightly convex and the lateral surface is slightly concave, like the blade of an airplane propeller. This shape causes inhaled air to “spiral,” which enables it to be more evenly warmed by the nasal mucosae.
ZYGOMATIC BONES
Of the facial bones that contribute solely to the orbits, the zygomatic bones (zī’-gō-MAT-ik, “bar”) are the largest. On each side of the face, a zygomatic bone forms the superior, protruding part of the cheeks and lateral walls of the orbits. The superior portion of each zygomatic bone articulates with the frontal bone at the frontozygomatic suture, while the inferior portion articulates with the maxilla at the zygomaticomaxillary suture. The medial, posterior portion of the zygomatic bone articulates with the sphenoid bone, and the lateral, posterior portion, called the temporal process, articulates with the zygomatic process of the temporal bone to complete the zygomatic arch. Viewing the skull inferiorly, the zygomatic arches look like handles of a kitchen pot.
LACRIMAL BONES
The lacrimal bones (LAK-ri-mal, tear) are the smallest facial bones and form a portion of the anterior, medial wall of the orbits. Each of these small, rectangular-shaped bones articulates with the frontal bone, superiorly; with a maxilla anteriorly and inferiorly; and with the ethmoid bone, posteriorly. At the medial edge of the infraorbital margin, the maxilla and lacrimal bone together form a narrow tube, the nasolacrimal canal. This canal surrounds a membranous nasolacrimal duct that drains tears from the lower eyelid into the nasal cavity. The anterior portion of a lacrimal bone forms half of the lacrimal groove (the maxilla forms the other half), which feeds into the nasolacrimal canal.
THE ORBITS AND NASAL CAVITY
As you studied the cranial and facial bones, you probably noticed that the orbits and nasal cavity are amazingly intricate complexes of bones. Why such complexity? As with the cranial cavity, the many bony plates and splinters that make up the facial cavities allow these oddly shaped chambers to maintain their proportions as the skull grows. Because the orbits and nasal cavities will figure prominently in your study of the eyes and the respiratory tract, it’s worth taking a second look at these structures individually and summarizing the bones that form them.
Orbits. Staring into the depths of an orbit is perhaps the closest you might get to experiencing what a hatching chick “sees” as it breaks free from its shell – a fractured assembly of curved plates. The orbits house the eyeballs. Between the orbital walls and the eyeball are skeletal muscles that move the eye, a cushioning layer of adipose tissue, as well as blood and lymphatic vessels, nerves, and a lacrimal (tear) gland.
Parts of three cranial bones (frontal, sphenoid, and ethmoid) and four facial bones (maxilla, zygomatic, lacrimal, and palatine) form the orbital walls. The protruding rim of each orbit (formed by the frontal bone, maxilla, and zygomatic bone) is quite sturdy, but the orbital floor is relatively fragile (Figure 10-3).
Figure 10-3. Bones of the orbit
Table 10-3. Summary of FACIAL BONES and their major features
The nasal cavity. A vertical partition, the nasal septum, divides the nasal cavity into right and left sides. From a medial view, the nasal septum looks like three relatively flat plates. Anteriorly, the nasal septum consists of a hyaline cartilage plate, the septal cartilage. The septal cartilage tapers to a point near the posterior end of the nasal septum. The anterior end of the septal cartilage forms the medial portion of the nose that is covered by skin. The perpendicular plate of the ethmoid bone forms most the nasal septum’s superior half, while the vomer forms the septum’s inferior half (Figure 10-4).
Figure 10-4. Bones of the nasal cavity
The walls of each nasal cavity are formed, in part, by five facial bones (maxilla, palatine, inferior nasal concha, nasal, and lacrimal bones) and three cranial bones (ethmoid, frontal, and sphenoid). Three ridge-like nasal conchae protrude from the lateral walls of the nasal cavity. The two smaller ridges, the superior nasal conchae and middle nasal conchae, are part of the ethmoid bone. The inferior nasal conchae are separate bones and the largest nasal conchae. They are located inferior to the middle nasal conchae and extend anteriorly to meet the nasal spine of the maxilla.
The cribriform plate of the ethmoid bone forms most of the nasal cavity’s superior border (roof), while the frontal bone and the nasal bones form a small portion of the roof near the anterior end. Nerves that function in the sense of smell pass from the superior portion of the nasal cavity through numerous tiny olfactory foramina of the cribriform plate and enter the brain in the anterior cranial fossa. The maxillae and palatine bones form the inferior border (the floor) of the nasal cavity, while the sphenoid bone forms most of the posterior wall.
PARANASAL SINUSES
As described earlier, five skull bones contain internal cavities called sinuses, and since these sinuses border the nasal cavity, they are collectively called paranasal sinuses (para-, alongside). The paranasal sinuses include the frontal sinus, the sphenoid sinus, the ethmoid air cells, and the maxillary sinuses (one in each maxilla). The paranasal sinuses lighten the skull and affect the sound of the voice. Each sinus is lined with a mucous membrane that secretes mucus, which drains into the nasal cavity. Normally a paranasal sinus is filled with air, but excessive mucus or watery tissue fluid in the sinus can change the resonance of the voice. This is similar to the difference in sounds that is apparent if you tap a spoon on an empty water glass, and then tap the glass again after filling it with water.
FONTANELS
So far, we have been describing the anatomy of the adult skull. A human infant, however, is born without fully developed cranial bones, a situation which is actually helpful. First, a partially formed skull is flexible, which helps an infant’s head squeeze through its mother’s birth canal. Second, a partially formed skull accommodates the explosive growth of the brain that occurs during the first few years after birth. An infant’s cranial bones are relatively thin, and they aren’t ossified to the edges of the cranial sutures. In place of sutures, an infant’s cranium has patches of fibrous membranes called fontanels (fon-ta-NELZ) or “soft spots” (Figure 10-5). The fontanels are what remain of the membrane model from which the cranial bones begin to form by intramembranous ossification. Eventually the cranial bones grow together to form sutures; the last fontanel usually ossifies by about age 2.
Figure 10-5. Fontanels
Note: The French word, fontanel, literally means “fountains,” but the sense of the word for English-speakers has a meaning closer to “wellspring,” a source or origin of something.
There are six major fontanels present in an infant’s skull at the time of birth. The anterior fontanel is the largest fontanel, and is found where the coronal suture and the sagittal suture meet. A smaller posterior fontanel is found at the intersection of the sagittal suture and the lambdoid suture. A sphenoidal fontanel is found in each temple region, where the squamous suture and coronal suture intersect. The mastoid fontanel is located superior to the mastoid process of the temporal bone, where the lambdoid suture meets the squamous suture.
THE HYOID BONE
Before turning to the rest of the axial skeleton, we will describe the only bone in the skeleton that does not articulate with another bone; this is the hyoid bone (HĪ-oyd, hy-, U-shaped). The U-shaped hyoid bone is found in the anterior cervical (neck) region, inferior to the mandible. Several muscles extend superiorly from the hyoid bone; two support the hyoid from the styloid process of each temporal bone, and the other moves the tongue. Muscles also extend inferiorly to support the larynx (voice box). In essence, the hyoid bone is a suspension device, much like a coat hanger or a curtain rod. What “hangs” from the hyoid are muscles that attach to and “lift” the larynx when a person swallows.
GENERAL STRUCTURE OF THE SPINE
Like a plant’s stem, which supports its flower and branches, the vertebral column supports the skull and appendicular skeleton. In addition, the vertebral column surrounds and protects the spinal cord, and it provides attachment sites for muscles that move the skull, back, shoulders, arms, hips, and thighs.
Regions of the Vertebral Column
The vertebral column (VER-te-bral), or spine, of a typical adult consists of 33 bones stacked vertically along the body’s midline, from the base of the skull through the pelvis (Figure 10-6). The bones of the vertebral column are irregular in shape and are called vertebrae (VER-te-brā; vertebr-, a joint; singular is vertebra).
Figure 10-6. The vertebral column
Characteristics of a “typical” Vertebra
The vertebrae in the cervical, thoracic and lumbar regions of the vertebral column are unique in several ways, but they all have certain features in common. They all have a body, vertebral arch, and several processes (Figure 10-7).
The vertebral body is the thick, anterior part of a vertebra that bears the weight of structures superior to it. In a superior view, the vertebral body appears somewhat oval-shaped, whereas in a lateral view it appears more rectangular.
Figure 10-7. Features of vertebrae
In the living body, a thin layer of compact bone tissue covers the sides of the vertebral body, while pads of fibrocartilage, called intervertebral discs, cover its superior and inferior surfaces. Spongy bone tissue comprises the internal portion of the vertebral body; this spongy tissue is easily visible on a dried vertebra that no longer has intervertebral discs attached to it. Cervical vertebrae have the smallest bodies; thoracic vertebra have medium-sized bodies; and lumbar vertebrae have the largest bodies. You can easily discern an increase in vertebral body size as you scan down the vertebral column from the skull to the sacrum. This progressive increase in size is logical since vertebrae that are more inferior in position must support more weight than vertebrae that are more superior in position.
The vertebral arch is posterior to the vertebral body and, along with the vertebral body, forms a large opening, the vertebral foramen. Within the vertebral column, vertebral foramina of the cervical, thoracic, and lumbar vertebrae align to form the vertebral canal, through which the spinal cord passes. The right and left vertebral pedicles (PED-i-kulz, pedic-, little feet) form the lateral walls of the vertebral arch and extend from the posterior, lateral sides of the vertebral body. Two prominent notches are visible on each pedicle’s superior and inferior surface, respectively. The superior notch of one vertebra lines up with the inferior notch of the next highest vertebra to form an intervertebral foramen, which serves as a passageway for a spinal nerve. Remember that a single vertebra has a vertebral foramen, but it takes two vertebrae to make an intervertebral foramen. The right and left vertebral laminae (LAM-i-nē; “thin plates,” singular is lamina) form the posterior walls of the vertebral arch.
Individual vertebrae display a number of processes, which may include a spinous process, transverse processes, and articular processes. A spinous process (SPĪ-nus; “thorny”), a tapered projection extending posteriorly from the vertebral arch, arises from the junction of the right and left laminae. The knobby projections that are visible beneath the skin covering your backbone are spinous processes of your vertebrae. Two transverse processes (one on each lateral side of the vertebra), arise at the junction of a pedicle and a lamina. The transverse processes are so named because they orient along the vertebra’s transverse (horizontal) plane. The spinous process and the transverse processes are important as attachment sites for muscles that bend the vertebral column.
Intervertebral Discs and Longitudinal Ligaments
In an adult, pads of fibrocartilage called intervertebral discs separate most vertebrae from one an other; exceptions are the first and second cervical vertebrae, and the sacral and coccygeal vertebrae. Altogether, intervertebral discs comprise about a fifth of the vertebral column’s length. An intervertebral disc does not adhere directly to a vertebra, but rests on a thin layer of hyaline cartilage that attaches to the body of the vertebra. The superior and inferior surfaces of the vertebral body are also concave, with a slightly elevated ridge around the rim. This shape allows an intervertebral disc to penetrate slightly into the body of the vertebrae. Consequently, the vertebrae can “grip” the inter vertebral disc and are less likely to experience excessive side-to-side or front-to-back sliding.
Intervertebral discs are thinnest in the thoracic region and thickest in the lumbar region, but they all have two major regions: an outer annulus fibrosus and an inner nucleus pulposus. The annulus fibrosus (AN-ū-lus, “ring;” fī-BRŌ sus) consists of a thin outer layer of dense, fibrous connective tissue surrounding a thicker layer of fibrocartilage. The nucleus pulposus (pul PŌ-sus; pulp-, flesh) at birth consists of a soft, gelatinous material, but this is gradually replaced by fibrocartilage after about age 10. Intervertebral discs cushion the vertebral bodies and allow the vertebral column to bend. Two extremely strong bands of dense regular connective tissue, the longitudinal ligaments, hold adjacent vertebral bodies together.
CERVICAL VERTEBRAE
Figure 10-8. Atlas and axis
THORACIC VERTEBRAE
LUMBAR VERTEBRAE
Like thoracic vertebrae, lumbar vertebrae have articular processes containing vertically pitched facets that articulate with adjacent vertebrae. However, the articular facets of lumbar vertebrae face each other in different planes than those of the thoracic vertebrae. Between adjacent thoracic vertebrae, an anterior-facing superior articular facet meets a posterior-facing inferior articular facet. But between adjacent lumbar vertebrae, a medial-facing superior articular facet meets a lateral-facing inferior articular facet. Consequently, very little rotational movement is possible. On the other hand, the orientation of the lumbar articular facets does not impede anterior or posterior bending of the lumbar spine. You can demonstrate this with a simple experiment: Press your medially facing palms (“superior articular facets”) firmly against the sides of an immovable object, such as a refrigerator (“inferior articular facets”). While keeping your shoulders perpendicular to your palms, try twisting your torso back and forth; it can’t be done. However, you can bend forward or back if you let your hands slide up and down along the sides of the refrigerator.
The vertebral foramen of a lumbar vertebra is diamond shaped, and it is smaller than that of a cervical or thoracic vertebra. The smaller size of the lumbar foramen reflects the fact that the spinal cord at the lumbar level of the vertebral column is relatively narrow. In an adult, the spinal cord ends at the level of L1 or L2 ; however, numerous nerves extend inferiorly through the vertebral foramina of L3-L5 , through the sacrum and to the coccyx.
THE SACRUM
Moving inferiorly from the lumbar region, we find a large, heart-shaped bone, the sacrum (SĀ-krum) (Figure 10-9).
Figure 10-9. The sacrum
In ancient cultures, the sacrum was considered a sacred bone (sacr-, sacred), one that could not be destroyed but would be part of the body after a person rises from the dead. During childhood, five sacral vertebrae, numbered S1 (most superior) through S5 (most inferior), grow as separate bones. However, during the mid-to-late teenage years, the sacral vertebrae fuse into a single bone, the sacrum. Its odd-looking wedge shape makes the sacrum a sturdy attachment point that divides the weight of the body supported by the vertebral column between the lower limbs (via the pelvic bones).
The sacrum is widest at its superior end, the base, and it is narrowest at the inferior end, the apex. You may recognize these terms as applying to a triangle. In this case, the sacrum would be like an upside-down triangle. The transverse processes of adjacent sacral vertebrae fuse together to form a sacral wing or ala (Ā-la; “wing”) on each side of the sacrum. When viewed anteriorly, the alae (Ālē) resemble the spread wings of a bat.
COCCYX
The coccyx (KOK-siks) is a triangular-shaped bone inferior to the sacrum and consists of 3-5 fused caudal vertebrae (KAW-dal; caud-, tail). The coccyx is so named (coccy-, cuckoo) because it looks like the bill of a cuckoo bird, but most people simply call it the tailbone. The small caudal vertebrae, numbered Co1 (most superior in position) through Co4 (most inferior), usually fuse when a person is in the mid-to-late twenties. The coccyx is one attachment site for the gluteus maximus muscles on which you sit, and for several muscles that support the visceral organs in the pelvic cavity. Table 10-4 summarizes the vertebrae and their features.
Table 10-4. Comparison of Cervical, Thoracic, and Lumbar Vertebrae
THE THORACIC CAGE
Whereas a treasure chest surrounds and protects valuable jewels, your body’s chest, called the thorax (THŌ-raks, “chest”), surrounds and protects vital organs such as your heart and lungs. The part of the axial skeleton that forms the thorax is the thoracic cage (thō-RAS-ik), so named because most of its bones, the ribs, appear like the bars of an animal cage. In addition to protecting internal organs, the thoracic cage supports parts of the appendicular skeleton and provides attachment sites for muscles of the neck, torso, and arm.
The thoracic cage forms four major parts: the thoracic vertebrae, the ribs, their costal cartilages, and the sternum. As we discussed earlier, thoracic vertebrae (T1-T12 ) form the posterior, midline portion of the thoracic cage, and they anchor the ribs posteriorly.
The Ribs
A rib is a flexible, curved, flat bone that forms the bony wall of the thoracic cavity and serves as an attachment site for skeletal muscles (Figure 10-10). Certain skeletal muscles pull the ribs farther apart, which causes the thoracic cage to “expand” (move outward), drawing air into the lungs. Other skeletal muscles pull the ribs closertogether, which causes the thoracic cage to “col lapse” (move inward), forcefully expelling air out of the lungs. You can easily see the out-and-in movements of your own thoracic cage when you take deep breaths.
No two ribs look exactly alike. They are all flat bones, however, consisting of a layer of spongy bone (diploë) sandwiched between two layers of compact bone. The most superior ribs on each side curve gently. The superior pair of ribs, their costal cartilages, the first thoracic vertebra, and the superior part of the sternum form the boundary of the superior thoracic aperture; this is the opening where the neck and thoracic cavity meet. The most inferior pair of ribs, along with the cartilages extending from the inferior six pairs of ribs, form the rim of the inferior thoracic aperture. The muscular diaphragm attaches to this rim and separates the thoracic cavity from the abdominal cavity.
There are twelve pairs of ribs, numbered 1 (most superior pair) through 12 (most inferior pair). The anterior end of a rib consists of varying lengths of hyaline cartilage, the costal cartilage, which may or may not articulate with the sternum. Ribs 1-7 are called the true ribs, or vertebrosternal ribs (VER-te-brō-STER-nal), because their costal cartilages articulate directly with the sternum, forming a complete or “true” cage-like structure. Ribs 8-12 are called the false ribs, because their costal cartilages do not articulate directly with the sternum. Each of the costal cartilages of ribs 8-10 attaches to the costal cartilages of the immediately superior rib; therefore, false ribs 8-10 are also called vertebrochondral ribs (VER te-brō-KON-dral). Because the costal cartilages of false ribs 11 and 12 do not attach to other costal cartilages, these ribs are often called floating ribs or vertebral ribs.
STERNUM
The sternum (STER-num; “breastbone”) is the sword-shaped, flat bone on the anterior side of the thoracic cage (see Figure 10-10). The sternum anchors the costal cartilages of ribs 1-7, and serves as attachment sites for several skeletal muscles from the head and neck. In children and young adults, the sternum consists of three individual bones: the manubrium, body, and xiphoid process; in older adults, these bones fuse to form a single bone.
The manubrium (ma-NOO-brē-um; “handle”) is the diamond-shaped, superior portion of the sternum. If the two inferior bones of the sternum look like a sword, then the manubrium looks like its handle. Each superior, lateral edge of the manubrium contains a broad depression, called a clavicular notch (kla-VIK-ū-lar), which articulates with the medial end of a clavicle (collarbone). At the superior end of the manubrium, between the clavicular notches, is a depression called the jugular notch (JUG-ū-lur; “throat”). You can feel this notch where your throat meets the top of your chest.
Each lateral edge of the manubrium contains two shallow depressions, called costal notches that articulate with costal cartilages on the ends of ribs. The costal cartilage of rib 1 articulates with a costal notch near the middle edge of the manubrium. A portion of the second rib’s costal cartilage articulates with a costal notch at the inferior, lateral end of the manubrium. Two large muscles that turn the head right or left attach to the manubrium’s superior, anterior portion. In addition, several smaller muscles that aid in swallowing extend from the throat and attach to the manubrium.
The sternum’s long, middle portion is its body. The sternal angle is the site where the inferior end of the manubrium meets the superior end of the sternal body. You can feel the sternal angle as a slight projection. The costal cartilages of rib pairs 2-7 attach to costal notches along the lateral margins of the sternal body. The xiphoid process (ZĪ-foyd; xiph, sword) is the pointed, inferior end of the sternum. A portion of the muscular respiratory diaphragm attaches to the xiphoid’s deep surface. A large abdominal muscle attaches to the xiphoid’s superficial surface.
Figure 10-10. The ribs and stemum
TOPICS TO KNOW FOR CHAPTER 10
(The Axial Skeleton)
alae
alveolar processes
alveolar sockets
annulus fibrosus
anterior arch
anterior cranial fossa
anterior fontanel
anterior nasal spine
articular facet
articular processes
atlas
axial skeleton
axis
basilar part of occipital
basioccipital process
bones of the skull
calvaria
carotid canal
caudal vertebrae
cervical curvature
clavicular notch
clivus
coccygeal vertebrae
coccyx
condylar process
condyle
coronal suture
coronoid process
costal cartilage
costal facets
costal notches
cranial bones
cranial cavity
cranium
crest
cribriform plate
crista galli
dens
epicondyle
ervical vertebrae
ethmoid air cells
ethmoid bone
external acoustic meatus
external
occipital protuberance
facet
line
facial bones
false ribs
fissure
floating ribs
fontanels
foramen
foramen lacerum
foramen magnum
foramen ovale
foramen rotundum
foramen spinosum
fossa
fovea
frontal bone
frontal processes
frontal sinus
frontozygomatic suture
glabella
greater wings
hard palate
head of rib
hyoid bone
hypoglossal canal
importance of axial skeleton
incisive fossa
inferior nasal conchae
inferior orbital fissure
inferior thoracic aperture
infraorbital foramen
infraorbital margin
intermaxillary suture
internal acoustic meatus
internasal suture
intervertebral discs
intervertebral foramen
jugular foramen
jugular notch
lacrimal bones
lacrimal fossa
lacrimal groove
lambdoid suture
lamina
lateral masses
lesser wings
ligamentum nuchae
longitudinal ligaments
lumbar vertebrae
mandible
mandible angle
mandible body
mandibular foramen
mandibular fossa
mandibular notch
mandibular symphysis
manubrium
mastoid air cells
mastoid fontanel
mastoid process
maxillae
maxillary sinus
meatus
mental foramen
mental protuberance
metopic (frontal) suture
middle cranial fossa
middle nasal conchae
nasal aperture
nasal bones
nasal cavity
nasal septum
nasal spine
nasolacrimal canal
neck
notch
nuchal ligament
nuchal lines
nucleus pulposus
occipital bone
occipital condyles
occipitomastoid sutures
odontoid process
olfactory foramina
optic canals
orbits
palatine bones
paranasal sinuses
parietal bones
pedicles
perpendicular plate
petrous part of temporal bone
posterior arch
posterior cranial fossa
posterior fontanel
process
protuberance
pterygoid processes
ramus
regions of the vertebral column
rib
true ribs
sacral canal
sacral cornua
sacral curvature
sacral foramina
sacral hiatus
sacral promontory
sacral vertebrae
sacrum
sacrum apex
sacrum base
sagittal suture
sella turcica
septal cartilage
sinus
skull
sphenoid bone
thoracic vertebrae
thorax
sphenoid sinuses
sphenoidal fontanel
spine
spine
spinous process
squamous part of frontal bone
squamous sutures
sternal angle
sternal body
sternum
structure of the spine
styloid process
stylomastoid foramen
sulcus
superior articular facet
superior nasal conchae
superior orbital fissure
superior thoracic aperture
supraorbital foramen
supraorbital margin
sutural bone
suture
temporal bones
temporal lines
temporal process
thoracic cage
thoracic curvature
transverse foramen
transverse processes
tubercle
tuberosity
tympanic part of temporal bone
typical vertebra
vertebra prominens
vertebrae
vertebral arch
vertebral body
vertebral canal
vertebral column
vertebral foramen
vertebral laminae
vertebral ribs
vertebrochondral ribs
vertebrosternal ribs
vomer
wormian bone
zygomatic arch
zygomatic bones
zygomatic process
zygomaticomaxillary suture
EOC Questions
