COORDINATION AND CONTROL: (1) THE NERVOUS SYSTEM 107 



glands, blood vessels, visceral organs, etc. The motor nerves of the smooth 

 muscles, blood vessels, and viscera constitute the so-called autonomic 

 nervous system. 



Each nerve is composed of an enormous number of nerve fibers lying 

 side by side like the separate wires in a telephone cable. The fibers in a 

 nerve are collected into bundles and are joined together by a little con- 

 nective tissue. As the nerve extends away from its roots and begins to 

 branch, the fiber bundles and eventually the individual fibers part com- 

 pany and disperse to their various destinations. Some of them are con- 

 nected to sense organs and carry information to the spinal cord and 

 brain; some go to voluntary muscles causing them to contract; and still 

 others go to, or come from, the smooth muscles and internal organs. Each 

 nerve fiber is a living thread of astonishing delicacy and variable length, 

 attaining in some instances a length of several feet. 



STRUCTURAL AND FUNCTIONAL ELEMENTS 



The nerve cell or neuron. The fundamental unit of all nervous struc- 

 ture and function is the nerve cell, which, because of its unique nature, 

 has been given the special name neuron. These cells differ from other cells 

 in that they project into long, slender fibers of living substance. Each 

 neuron is composed of a cell body containing a nucleus and of one or 

 more filamentous processes. In the typical motor neuron, usually cited 

 as an example, the processes are of two sorts — at one side an elongated 

 axon, which is usually unbranched except near its tip, and at the other 

 side several shorter, branched dendrites. Axons and dendrites may be 

 exceedingly long (several feet in the case of numerous spinal nerves), 

 and many of them are encased in a sheath of whitish, fatty material 

 (myelin) that distinguishes the myelinated white matter of the nervous 

 system from the nonmyelinated gray matter. The cell bodies themselves 

 and the extreme tips of the axons and dendrites are never covered with 

 this white sheath, and many parts of the central nervous system and of the 

 autonomic portion of the peripheral system appear to lack it altogether. 



The nerve impulse. When a neuron is stimulated at any point on its 

 surface, a nerve impulse results and spreads to all parts of the cell. Two 

 different processes seem to be at work in the production of such an im- 

 pulse. One involves a small amount of physiological activity on the part 

 of the cell and is capable of causing gradual fatigue of the neuron. The 

 other, which requires the presence of oxygen, is apparently a purely 

 physical, reversible change that can be indefinitely repeated without 

 fatigue. Theory and experiment strongly suggest that it depends upon 

 the formation of a thin colloidal membrane over the surface of the neuron. 

 Destruction of this membrane at any point (stimulation) results in a dif- 

 ference in electrical potential between the exposed (excited) and filmed 



