104 THE INDIVIDUAL ORGANISM 



elements of nervous structure and processes are relatively simple and 

 clear-cut. 



MAJOR FEATURES OF THE NERVOUS SYSTEM 



Two main divisions of the nervous system are generally recognized: 

 the central nervous system, consisting of the brain and spinal cord ; and the 

 peripheral nervous system, which includes all the nervous tissue outside 

 the brain and spinal cord and is made up largely of the nerves that extend 

 from the central nervous system to all parts of the body. 



The brain is a large organ contained within the cranium. It weighs 

 approximately 49 ounces, on the average, and has a volume of between 

 1,200 and 1,500 cc. By far the largest and most conspicuous portion of 

 the brain is the greatly convoluted cerebrum. As seen from above, the 

 cerebrum is rounded oval in outline and is divided by a deep median 

 fissure into two lateral hemispheres. Issuing from the undersurface of 

 the cerebrum is the lower part of the brain stem. This part of the brain is 

 rather complex in structure, but we may recognize within it four regions. 

 Named from above downward, these are the thalamus, the midbrain, the 

 pons, and the medulla. Another convoluted structure, the cerebellum, is 

 attached to the posterior (dorsal) face of the brain stem. 



The spinal cord is a long, relatively slender cylinder that lies in, but 

 only partially fills, the canal formed by the vertebrae. At the upper end, 

 it merges with the brain stem by way of the medulla; in the opposite 

 direction, it terminates at the level of the first lumbar vertebra. 



Nerves arise from both the brain and the spinal cord. Those issuing 

 from the brain are called cranial nerves; they arise from the lower or ven- 

 tral surface of the brain, and in man there are 12 pairs of them. These 

 nerves chiefly supply the head and neck. Some of them, like the olfac- 

 tory, optic, and auditory nerves, carry sensations from specialized sense- 

 receiving organs (nose, eyes, ears, etc.) to the brain; others carry impulses 

 out from the brain or combine both functions. The tenth pair of cranial 

 nerves, known as the vagus nerves (Latin, vago, "to wander") arise from 

 the medulla and carry impulses to and from many of the visceral organs 

 and the blood vessels. They play an important part in the unconscious 

 reflex control of visceral and circulatory functioning, as has already been 

 noted in connection with the heart. 



The spinal nerves arise from the lateral aspects of the spinal cord and 

 supply the rest of the body. There are 31 pairs of spinal nerves, leaving 

 the cord at regular intervals. With some minor exceptions, there is one 

 pair of spinal nerves for each vertebra in the spinal column; the nerves 

 alternate with the vertebrae, each pair leaving the spinal column through 

 an intervertebral opening. These nerves differ considerably among them- 

 selves in size, those supplying the arms and legs, for example, being much 



