Cranial Bones

The cranial cavity is enclosed by the cranium4 (braincase), which protects the brain and associated sensory organs. Cranial bones are those that directly contact the meninges around the brain. There are eight of these:

1 frontal bone 1 occipital bone

2 parietal bones 1 sphenoid bone 2 temporal bones 1 ethmoid bone

The cranium is a rigid structure with an opening, the foramen magnum (literally "large hole"), where the spinal cord enters. An important consideration in treatment of head injuries is swelling of the brain. Since the cranium cannot enlarge, swelling puts pressure on the brain and results in even more tissue damage. Severe swelling may force the brainstem out through the foramen magnum, usually with fatal consequences.

The delicate brain tissue does not come directly into contact with the cranial bones but is separated from them by three membranes called the meninges (meh-NIN-jeez) (see chapter fourteen). The thickest and toughest of these, the dura mater5 (DUE-rah MAH-tur), lies loosely against the inside of the cranium in most places but is firmly attached to it at a few points.

The cranium consists of two major parts—the calvaria and the base. The calvaria6 (skullcap) forms the roof and walls (see fig. 8.6). In study skulls it is often sawed so that part of it can be lifted off for examination of the interior. This reveals the base (floor) of the cranial cavity (see fig. 8.5b), which is divided into three basins corresponding to the contour of the inferior surface of the brain (fig. 8.9). The relatively shallow anterior cranial fossa is shaped like a crescent and accommodates the frontal lobes of the brain. The middle cranial fossa, which drops abruptly deeper, is shaped like a pair of outstretched bird's wings and accommodates the temporal lobes. The posterior cranial fossa is deepest and houses a large posterior division of the brain called the cerebellum.

6calvar = bald, skull

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Cribriform plate of ethmoid bone Nasal bone

Perpendicular plate of ethmoid bone

Palatine process of maxilla

Palatine bone

Cribriform Plate

Styloid process

Mandible (b)

Figure 8.4 The Skull. (a) Right lateral view; (fa) interior of the right half.

Cribriform plate of ethmoid bone Nasal bone

Perpendicular plate of ethmoid bone

Palatine process of maxilla

Palatine bone

Squamous suture Sella turcica Lambdoid suture Internal acoustic meatus Jugular foramen Hypoglossal canal

Styloid process

Mandible (b)

Figure 8.4 The Skull. (a) Right lateral view; (fa) interior of the right half.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Incisive foramen

Zygomatic bone

Nasal choana

Zygomatic arch Vomer

Sphenoid bone Mandibular fossa Styloid process External acoustic meatus Occipital condyle Mastoid process Mastoid notch Condylar canal Temporal bone

Parietal bone Inferior nuchal line Superior nuchal line Occipital bone (a)

Mastoid Notch

External occipital protuberance

Palatine process of maxilla

Palatine bone Greater palatine foramen Medial pterygoid plate Lateral pterygoid plate

Foramen ovale Foramen spinosum Foramen lacerum Basilar part of occipital bone Carotid canal Stylomastoid foramen Jugular foramen Foramen magnum Mastoid foramen Lambdoid suture

External occipital protuberance

Cranium Superior View
Figure 8.5 Base of the Skull. (a) Inferior view; (fa) internal view of the cranial floor.

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Chapter 8 The Skeletal System 251

Superior View Skull

Figure 8.6 The Calvaria (skullcap), Superior View.

Posterior

Figure 8.6 The Calvaria (skullcap), Superior View.

Frontal Bone

Frontal section

Figure 8.7 Major Cavities of the Skull, Frontal Section.

Frontal section

Figure 8.7 Major Cavities of the Skull, Frontal Section.

We now consider the eight cranial bones and their distinguishing features.

Frontal Bone

The frontal bone extends from the forehead back to a prominent coronal suture, which crosses the crown of the head from right to left and joins the frontal bone to the parietal bones (see figs. 8.3 and 8.4). It forms the anterior wall and about one-third of the roof of the cranial cavity, and it turns inward to form nearly all of the anterior cranial fossa and the roof of the orbit. Deep to the eyebrows it has a ridge called the supraorbital margin. The center of each margin is perforated by a single supraorbital foramen (see figs. 8.3 and 8.14), which provides passage for a nerve, artery, and veins. In some people, the edge of this foramen breaks through the margin of the orbit and forms a supraorbital notch. The smooth area of the frontal bone just above the root of the nose is called the glabella.7 The frontal bone also contains the frontal sinus. You may not see this on some study skulls. It is absent from some people, and on some skulls the calvaria is cut too high to show the sinus. Along the cut edge of the calvaria, you can see the diploe—the layer of spongy bone in the middle of the cranial bones (see fig. 8.5b).

Parietal Bones

The right and left parietal (pa-RY-eh-tul) bones form most of the cranial roof and part of its lateral walls (see figs. 8.4 and 8.6). Each is bordered by four sutures that join it to the neighboring bones: (1) a median sagittal suture between the parietal bones; (2) the coronal8 suture at the anterior margin; the lambdoid9 (LAM-doyd) suture at the posterior margin; and (4) the squamous suture laterally. Small sutural (wormian) bones are often seen along the sagittal and lambdoid sutures, like little islands of bone with the suture lines passing around them. Internally, the parietal and frontal bones have markings that look a bit like aerial photographs of river tributaries (see fig. 8.4b). These represent places where the bone has been molded around blood vessels of the dura mater.

Externally, the parietal bones have few features. A parietal foramen sometimes occurs near the corner of the lambdoid and sagittal sutures (see fig. 8.6). A pair of slight thickenings, the superior and inferior temporal lines, form an arc across the parietal and frontal bones (see fig. 8.4a). They mark the attachment of the large, fan-shaped tempo-ralis muscle, a chewing muscle that passes between the zygomatic arch and temporal bone and inserts on the mandible.

Temporal Bones

If you palpate your skull just above and anterior to the ear—that is, the temporal region—you can feel the temporal bone, which forms the lower lateral wall and part of the floor of the cranial cavity (fig. 8.10). The temporal bone derives its name from the fact that people often develop glab = smooth

8corona = crown

9Shaped like the Greek letter lambda (X.)

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Physiology: The Unity of Form and Function, Third Edition

Figure 8.8 The Paranasal Sinuses.

Table 8.3 Foramina of the Skull and the Nerves and Blood Vessels Transmitted Through Them

Bones and Their Foramina*

Structures Transmitted

Frontal Bone

Supraorbital foramen or notch

Supraorbital nerve, artery, and vein; ophthalmic nerve

Parietal Bone

Parietal foramen

Emissary vein of superior longitudinal sinus

Temporal Bone

Carotid canal External acoustic meatus Internal acoustic meatus Stylomastoid foramen Mastoid foramen

Internal carotid artery Sound waves to eardrum

Vestibulocochlear nerve; internal auditory vessels Facial nerve

Meningeal artery; vein from sigmoid sinus

Temporal-Occipital Region

Jugular foramen

Internal jugular vein; glossopharyngeal, vagus, and accessory nerves

Temporal-Occipital-Sphenoid Region

Foramen lacerum

No major nerves or vessels; closed by cartilage

Occipital Bone

Foramen magnum Hypoglossal canal Condylar canal

Spinal cord; accessory nerve; vertebral arteries Hypoglossal nerve to muscles of tongue Vein from transverse sinus

Sphenoid Bone

Foramen ovale Foramen rotundum Foramen spinosum Optic foramen Superior orbital fissure

Mandibular division of trigeminal nerve; accessory meningeal artery Maxillary division of trigeminal nerve Middle meningeal artery; spinosal nerve; part of trigeminal nerve Optic nerve; ophthalmic artery

Oculomotor, trochlear, and abducens nerves; ophthalmic division of trigeminal nerve; ophthalmic veins

© The McGraw-Hill Companies, 2003

Anatomy And Physiology Textbook

Saladin: Anatomy & I 8. The Skeletal System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Table 8.3 Foramina of the Skull and the Nerves and Blood Vessels Transmitted Through Them (continued)

Bones and Their Foramina*

Structures Transmitted

Ethmoid Bone

Cribriform foramina

Olfactory nerves

Maxilla

Infraorbital foramen Incisive foramen

Infraorbital nerve and vessels; maxillary division of trigeminal nerve Nasopalatine nerves

Maxilla-Sphenoid Region

Inferior orbital fissure

Infraorbital nerve; zygomatic nerve; infraorbital vessels

Lacrimal Bone

Lacrimal foramen

Tear duct leading to nasal cavity

Palatine Bone

Greater palatine foramen

Palatine nerves

Zygomatic Bone

Zygomaticofacial foramen Zygomaticotemporal foramen

Zygomaticofacial nerve Zygomaticotemporal nerve

Mandible

Mental foramen Mandibular foramen

Mental nerve and vessels

Inferior alveolar nerves and vessels to the lower teeth

When two or more bones are listed together (for example, temporal—occipital), it indicates that the foramen passes between them.

When two or more bones are listed together (for example, temporal—occipital), it indicates that the foramen passes between them.

Posterior cranial fossa — Middle cranial fossa—

Posterior cranial fossa — Middle cranial fossa—

Cranial Fossa

Anterior cranial fossa-

Anterior cranial fossa -

Middle cranial fossa

Posterior cranial fossa

Anterior cranial fossa -

Posterior cranial fossa

Medial Cranial Fossa Mcgraw

Figure 8.9 Cranial Fossae. The three fossae conform to the contours of the base of the brain.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

254 Part Two Support and Movement

254 Part Two Support and Movement

Temporal Bone Medial
Figure 8.10 The Right Temporal Bone. (a) Lateral view; (fa) medial view. List all the bones that articulate with the temporal bone.

their first gray hairs on the temples with the passage of time.10 The relatively complex shape of the temporal bone is best understood by dividing it into four parts:

1. The squamous11 part (which you just palpated) is relatively flat and vertical. It is encircled by the squamous suture. It bears two prominent features:

  • 1) the zygomatic process, which extends anteriorly to form part of the zygomatic arch (cheekbone), and
  • 2) the mandibular fossa, a depression where the mandible articulates with the cranium.
  1. The tympanic12 part is a small ring of bone that borders the external acoustic meatus (me-AY-tus), the opening into the ear canal. It has a pointed spine on its inferior surface, the styloid process, named for its resemblance to the stylus used by ancient Greeks and Romans to write on wax tablets. The styloid process provides attachment for muscles of the tongue, pharynx, and hyoid bone.
  2. The mastoid13 part lies posterior to the tympanic part. It bears a heavy mastoid process, which you can palpate as a prominent lump behind the earlobe. It is filled with small air sinuses that communicate with the middle-ear cavity. These sinuses are subject to infection and inflammation (mastoiditis), which can erode the bone and spread to the brain. Ventrally, there is a groove called the mastoid notch medial to the mastoid process (see fig. 8.5a). It is the origin of the digastric muscle, which opens the mouth. The notch is perforated by the stylomastoid foramen at its anterior end and the mastoid foramen at its posterior end.
  3. The petrous14 part can be seen in the cranial floor, where it resembles a little mountain range separating the middle cranial fossa from the posterior fossa (fig. 8.10b). It houses the middle- and inner-ear cavities. The internal acoustic meatus, an opening on its posteromedial surface, allows passage of the vestibulocochlear (vess-TIB-you-lo-COC-lee-ur)

10tempor = time

11 squam = flat + ous = characterized by

12tympan = drum (eardrum) + ic = pertaining to

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Chapter 8 The Skeletal System 255

nerve, which carries sensations of hearing and balance from the inner ear to the brain. On the ventral surface of the petrous part are two prominent foramina named for the major blood vessels that pass through them (see fig. 8.5a): (1) The carotid canal is a passage for the internal carotid artery, a major blood supply to the brain. This artery is so close to the inner ear that you can sometimes hear the pulsing of its blood when your ear is resting on a pillow or your heart is beating hard. (2) The jugular foramen is a large, irregular opening just medial to the styloid process, between the temporal and occipital bones. Blood from the brain drains through this foramen into the internal jugular vein. Three cranial nerves also pass through this foramen.

Occipital Bone

The occipital (oc-SIP-ih-tul) bone forms the rear of the skull (occiput) and much of its base (see fig. 8.5). Its most conspicuous feature, the foramen magnum, admits the spinal cord to the cranial cavity and provides a point of attachment for the dura mater. The bone continues anterior to the foramen magnum as a thick medial plate, the basilar part. On either side of the foramen magnum is a smooth knob called the occipital condyle (CON-dile), where the skull rests on the vertebral column. At the anterolateral edge of each condyle is a hypoglossal15 canal, named for the hypoglossal nerve that passes through it to supply the muscles of the tongue. A condy-lar (CON-dih-lur) canal sometimes occurs posterior to each occipital condyle.

Internally, the occipital bone displays impressions left by large venous sinuses that drain blood from the brain (see fig. 8.5b). One of these grooves travels along the mid-sagittal line. Just before reaching the foramen magnum, it branches into right and left grooves that wrap around the occipital bone like outstretched arms before terminating at the jugular foramina. Here, the venous sinuses drain their blood into the internal jugular veins, which exit the cranial cavity and pass down the neck.

Other features of the occipital bone can be palpated on the back of your head. One is a prominent medial bump called the external occipital protuberance—the attachment for the nuchal16 (NEW-kul) ligament, which binds the skull to the vertebral column. A ridge, the superior nuchal line, can be traced horizontally from the external occipital protuberance toward the mastoid process (see fig. 8.5a). It defines the superior limit of the neck and provides attachment to the skull for several neck and back muscles. By pulling down on the occipital bone, some of these muscles help to keep the head erect. The inferior

15hypo = below + gloss = tongue 16nucha = back of the neck nuchal line provides attachment for some of the deep neck muscles. This inconspicuous ridge cannot be palpated on the living body but is visible on an isolated skull.

Sphenoid Bone

The sphenoid17 (SFEE-noyd) bone has a complex shape with a thick medial body and outstretched greater and lesser wings, which give the bone as a whole a somewhat ragged, mothlike shape. Most of it is best seen from the superior perspective (fig. 8.11a). In this view, the lesser wings form the posterior margin of the anterior cranial fossa and end at a sharp bony crest, where the sphenoid drops abruptly to the greater wings. These form about half of the middle cranial fossa (the temporal bone forming the rest) and are perforated by several foramina to be discussed shortly.

The greater wing forms part of the lateral surface of the cranium just anterior to the temporal bone (see fig. 8.4a). The lesser wing forms the posterior wall of the orbit and contains the optic foramen, which permits passage of the optic nerve and ophthalmic artery (see fig. 8.14). Superiorly, a pair of bony spines of the lesser wing called the anterior clinoid processes appear to guard the optic foramina. A gash in the posterior wall of the orbit, the superior orbital fissure, angles upward lateral to the optic foramen. It serves as a passage for nerves that supply some of the muscles that move the eyes.

The body of the sphenoid has a saddlelike prominence named the sella turcica18 (SEL-la TUR-sih-ca). It consists of a deep pit called the hypophyseal fossa, which houses the pituitary gland, a raised anterior margin called the tubercu-lum sellae (too-BUR-cu-lum SEL-lee), and a posterior margin called the dorsum sellae. In life, a fibrous membrane is stretched over the sella turcica. A stalk penetrates the membrane to connect the pituitary gland to the floor of the brain.

Lateral to the sella turcica, the sphenoid is perforated by several foramina (see fig. 8.5a). The foramen rotundum and foramen ovale (oh-VAY-lee) are passages for two branches of the trigeminal nerve. The foramen spinosum, about the diameter of a pencil lead, provides passage for an artery of the meninges. An irregular gash called the foramen lacerum19 (LASS-eh-rum) occurs at the junction of the sphenoid, temporal, and occipital bones. It is filled with cartilage in life and transmits no major vessels or nerves.

In an inferior view, the sphenoid can be seen just anterior to the basilar part of the occipital bone. The internal openings of the nasal cavity seen here are called the nasal choanae20 (co-AH-nee), or internal nares. Lateral to each choana, the sphenoid bone exhibits a pair of parallel

17 sphen = wedge + oid = resembling 18sella = saddle + turcica = Turkish 19lacerum = torn, lacerated 20choana = funnel

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

256 Part Two Support and Movement

256 Part Two Support and Movement

Sphenoid Bone Posterior View
Figure 8.11 The Sphenoid Bone. (a) Superior view; (b) posterior view.

plates—the medial pterygoid21 (TERR-ih-goyd) plate and lateral pterygoid plate (see fig. 8.5a). These provide attachment for some of the jaw muscles. The sphenoid sinus occurs within the body of the sphenoid bone.

Ethmoid Bone

The ethmoid22 (ETH-moyd) bone is located between the orbital cavities and forms the roof of the nasal cavity (fig. 8.12). An inferior projection of the ethmoid, called the perpendicular plate, forms the superior part of the nasal septum, which divides the nasal cavity into right and left nasal fossae (FOSS-ee). Three curled, scroll-like nasal con-chae23 (CON-kee), or turbinate24 bones, project into each fossa from the lateral wall (see figs. 8.7 and 8.13). The superior and middle conchae are extensions of the ethmoid bone. The inferior concha—a separate bone—is included in the discussion of facial bones in the next section. The pterygo = wing

2ethmo = sieve, strainer + oid = resembling

'3conchae = conchs (large marine snails)

4turbin = whirling, turning

Ethmoid Bone Location
Figure 8.12 The Ethmoid Bone, Anterior View. List all the bones that articulate with the ethmoid bone.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Chapter 8 The Skeletal System 257

Chapter 8 The Skeletal System 257

Lateral Wall Right Nasal Cavity
Figure 8.13 The Right Nasal Cavity, Sagittal Section.

conchae are covered with the mucous membrane of the nasal cavity. The superior concha and the adjacent region of the nasal septum also bear the receptor cells for the sense of smell (olfactory sense). The ethmoid bone also includes a large, delicate mass on each side of the perpendicular plate, honeycombed with chambers called ethmoid air cells; collectively, these constitute the ethmoid sinus.

The superior part of the ethmoid, viewed from the interior of the skull, exhibits a midsagittal crest called the crista galli25 (GAL-eye), a point of attachment for the meninges (see figs. 8.4b and 8.5b). On either side of the crista is a horizontal cribriform26 (CRIB-rih-form) plate marked by numerous perforations, the cribriform foramina. These foramina allow nerve fibers for the sense of smell to pass from the nasal cavity to the brain (see insight 8.1).

crista = crest + galli = of a rooster 6cribri = sieve + form = in the shape of

Insight 8.1 Clinical Application

Injury to the Ethmoid Bone

The ethmoid bone is very delicate and is easily injured by a sharp upward blow to the nose, such as a person might suffer by striking an automobile dashboard in a collision. The force of a blow can drive bone fragments through the cribriform plate into the meninges or brain tissue. Such injuries are often evidenced by leakage of cerebrospinal fluid into the nasal cavity, and may be followed by the spread of infection from the nasal cavity to the brain. Blows to the head can also shear off the olfactory nerves that pass through the ethmoid bone and cause anosmia, an irreversible loss of the sense of smell and a great reduction in the sense of taste (most of which depends on smell). This not only deprives life of some of its pleasures, but can also be dangerous, as when a person fails to smell smoke, gas, or spoiled food.

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Responses

  • Eino
    Do olfactory nerve fibers penetrate the cribriform plate of the eth,oid bone?
    5 years ago
  • kedija gebre
    Where cranial nerves travel through the sphenoid?
    5 years ago
  • Tanta Roper
    Do olfactory nerve fibers penetrate the cribriform plate of ethmoid bone?
    4 years ago
  • Celio Russo
    What is the deepest of the skull bones where you would find the cribriform plate?
    4 years ago
  • tanja
    Which foramen can be seen in an inferior view of the ventral skull?
    4 years ago
  • melilot
    What is cranial 8 physiological name?
    4 years ago
  • arnor
    Is the ethmoid bone pair?
    4 years ago
  • Ambessa
    Which cranial bone is one of a pair?
    4 years ago
  • peter shuster
    Is a blow to the foramen magnum ever fatal?
    4 years ago
  • leonardo
    What is the large group of bones that forms the cranial cavity?
    4 years ago
  • delma
    What are the four cavites in the bones of your skull called?
    4 years ago
  • pearl
    Which bone forms part of the floor of the cranial cavity?
    4 years ago
  • Eros Lo Duca
    Which cranial nerve is located near the frontal bone?
    4 years ago
  • Tanja
    Which part is a thick medial plate of the bone anterior to the Foramen magnum?
    8 months ago

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