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Cranial Anatomy

The Skull
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.

Cranial Bones
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.

Facial Bones
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 head.

The Brain
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 skull.

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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