The skull is
made up of 22 bones: the cranium includes eight
bones that surround and protect the brain and
14 bones that form the face. In adults, all but
one of the skull bones are fused together by immovable
joints called sutures. The sutures lock the edges
of the skull bones together, like pieces in a
puzzle, to form a structure that is both rigid
and strong. The mandible, or lower jaw, the only
moveable skull bone, allows the mouth to open
and close. In newborns, the skull bones are not
completely fused; they are linked by fibrous membrane
called fontanels. Fontanels allow the skull to
be compressed slightly during birth and accommodate
growth of the brain during early infancy. By one-and-a-half
years of age, the skull sutures have formed and
the fontanels have disappeared.
The frontal bone forms the forehead. Two parietal
bones form the sides of the cranial roof. Two
temporal bones form the lower cranial sides. The
occipital bone forms the cranial rear and floor.
The ethmoid bone forms part of the nasal cavity.
Shaped like a butterfly, the sphenoid bone forms
the middle part of the cranial floor.
The 14 facial bones provide the structure of the
face and form the openings through which food,
water, and air enter the body. Each of the following
facial bones are paired: the maxillae form the
upper jaw and front of the hard palate; the zygomatic
bones form the cheeks; the nasal bones form the
bridge of the nose; the lacrimal bones form part
of the orbit, or eye socket; the palatine bones
form the rear of the hard palate; and the inferior
nasal conchae divide the nasal cavity. The vomer
is a single bone that makes up part of the nasal
septum, which divides the nostrils, and the mandible
forms the lower jaw. The maxillae and mandible
secure the teeth. Small holes in the skull bones,
called foraminae, and canals enable blood vessels,
such as the carotid arteries and nerves, to enter
and leave the skull. The spinal cord passes through
a largest hole, called the foramen magnum, in
the base of the cranium to join the brain. The
occipital condyles on either side of the foramen
magnum articulate with the first vertebra (C1)
of the spine to permit up-and-down movement of
The brain is the control center of the nervous
system. It enables us to think, feel, and move.
The brain constantly receives information and
sends out instructions to the body through the
spinal cord and the body's vast network of nerves.
There are 12 pairs
of cranial nerves branching off the brain. These
nerves relay impulses from the sensory organs,
such as the eyes or ears. Thirty-one pairs of
spinal nerves branch off the spinal cord, exiting
between each level of vertebrae. These nerves
relay impulses to and from the rest of the body.
The largest part of the brain is the cerebrum,
which controls the most sophisticated functions,
such as thought, imagination, memory, emotion,
speech, and sensory perception. The human cerebrum
is quite large. It has two halves, or hemispheres.
A band of nerve tissue, called the corpus callosum,
links two halves to allow them to exchange information.
Each hemisphere is covered by a layer of gray
tissue, called the cerebral cortex, which is responsible
for the higher functions of the brain, including
conscious thought. The cortex is composed of sulci
(folds) and gyri (bulges), which together provide
a large surface area in the limited space inside
The cortex of each hemisphere has four lobes:
the occipital, temporal, parietal, and frontal
lobes. The occipital lobe controls vision. The
temporal lobe controls sound and speech. The parietal
lobe controls movement, touch, and recognition.
And the frontal lobe controls thinking and planning.
The brain stem and hypothalamus control automatic
processes, such as breathing and heartbeat.
The cerebellum acts as a "mini brain"
that coordinates body balance, posture, and movement.
The human brain is well
protected from injury. It is firmly surrounded
by three layers of membranes, encased in a rigid
skull (the cranium), and covered by a muscular
scalp. Each of these barriers to the brain is
important, because brain tissue is fragile and
unforgiving if injured.
Three membrane layers,
the meninges, protect the brain from injury and
infection. The dura mater, tough and fibrous,
lines the skull. The thinner pia mater, highly
vascular (containing many blood vessels), covers
the brain's surface. Between these two is another,
the arachnoid. The brain floats in a protective
cushion of cerebrospinal fluid (CSF), which flows
within the subarachnoid space-beneath the arachnoid
membrane, on top of the pia mater. It also surrounds
the spinal cord and fills open spaces (ventricles)
inside the brain. The amount of CSF that circulates
around the brain normally stays the same, replenished
by the body, and helps to maintain a constant
pressure inside the skull, known as intracranial
pressure (ICP). The largest part of the brain
is divided into two major areas, the left and
right cerebral hemispheres, which control most
of the body's thought and sensory processes. Some
sections of each hemisphere can be "mapped" to
correspond with the body functions for which they
seem to be responsible. The brain stem controls
such vital functions as breathing, heartbeat,
and eye movement. It anchors the brain to the
other part of the central nervous system, the
spinal cord, and acts as the main circuit for
all brain activity. Twelve pairs of cranial nerves,
emerging from the base of the brain and the brain
stem, transmit nerve impulses for vision, hearing,
smell, and many other important body functions.
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