HUMAN BIOLOGY

92 Chapter 5

brain case Skull bones
that surround and protect
the brain.

mandible Lower jawbone.

sinus Air space in the
skull, lined with mucous
membrane.

the axial skeleton

Temporal bones form the lower sides of the cranium and

surround the ear canals, which are tunnels that lead to the

middle and inner ear. Inside the middle ear are tiny bones

that function in hearing. On the sides of your head, in front

of each temporal bone, a sphenoid bone extends inward to

form part of the inner eye socket. The ethmoid bone also

forms part of the inner socket and helps support the nose.

Two parietal bones above and behind the temporal bones

form much of the skull as they sweep upward and meet

at the top of the head. An occipital bone forms the back and

base of the skull. It also encloses an opening, the foramen

magnum (“large hole”). Here, the spinal cord emerges

from the base of the brain and enters the spinal column

(Figure 5.5B). Other openings are channels for nerves and

blood vessels. For instance, the jugular veins, which carry

blood leaving the brain, pass through openings between

the occipital bone and each temporal bone.

Facial bones support and shape the face

Figure 5.5 also shows facial bones, many of which you can

easily feel with your fingers. The largest is your lower jaw,

or mandible. The upper jaw consists of two maxillary bones,

each called a maxilla. Two zygomatic bones form the middle

of the hard bumps we call “cheekbones” and the outer

parts of the eye sockets. A small, flattened lacrimal bone

fills out the inner eye socket. Tear ducts pass between this

bone and the maxillary bones and drain into the nasal cav-

ity—one reason why your nose runs when you cry. Tooth

sockets in the upper and lower jaws also contain the teeth.

n    The axial skeleton includes bones that make up the body’s

vertical axis and protect many internal organs.

n    Link to Mucous membranes 4.7

We begin our tour of the skeleton with bones of the axial

skeleton—the skull, vertebral column (backbone), ribs, and

sternum (the breastbone).

the skull protects

the brain

Your skull consists of more than two

dozen bones that are divided into

several groups. By tradition many

of them have names derived from

Latin, but their roles are easy to

grasp. For example, the cranium, or

brain case, includes eight bones that together surround

and protect your brain. As Figure 5.5A shows, the frontal

bone makes up the forehead and upper ridges of the eye

sockets. It contains sinuses, which are air spaces lined

with mucous membrane. Sinuses make the skull lighter,

which translates into less weight for the spine and neck

muscles to support. But channels connect them to the nasal

passages, and their ability to produce mucus can mean

misery for anyone who has a cold or pollen allergies. A

bacterial infection in the nasal passages can spread to the

sinuses, causing sinusitis. Figure 5.5C shows sinuses in the

cranial and facial bones.

sphenoid bone

ethmoid bone

lacrimal bone

zygomatic bone

occipital bone

maxilla

mandible

external auditory meatus

(opening of the ear; part

of the temporal bone)

parietal bone

frontal

bone

temporal

bone

maxilla

maxilla

zygomatic

bone

occipital

bone

sphenoid

bone

jugular

foramen

foramen

magnum

palatine

bone

vomer

temporal bone

parietal bone

hard

palate

Figure 5.5 Skull bones surround the brain and support the forehead. A The jagged junctions between skull bones are called sutures.

B A bottom-up view of the skull. The large foramen magnum is situated atop the uppermost cervical vertebra. c Sinuses in bones in

the skull and face. (© Cengage Learning)

A B

5.3

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