NERVE TISSUES 177 



especially of man. These inner nerve cells form the central nervous 

 system, and those remaining in direct contact with the exterior form the 

 perceptory nervous system or sense organs. 



Where large numbers of nerve cells were to be retired from the sur- 

 face at once, entire parts of this surface were invaginated and the whole 

 mass thus carried inside as in the vertebrate brain and the cephalopod 

 brain. These form communicatory and motor centers. Many large 

 sensory areas that can be reached inside the body (through specially 

 developed outer tissues) by vibrations of the ether and of air or other 

 matter are also invaginated for the protection of their delicate cells 

 (retina, organ of Corti). 



The nerve cells rarely work alone (motor cells of some jellyfish). 

 For the most part they are arranged in chain-like pathways through the 

 body. The links of such a chain are the individual neurons arranged 

 with the discharging end of each one in close proximity or contact with 

 the perceptory organ of the next, so that an impulse, beginning at the 

 perceptory end of the first cell in the line (this must be a cell specially 

 modified to take a stimulus from some outer conditions), will travel the 

 entire length of the chain, ending at the discharging end of the last 

 one as a motor stimulus to a tissue cell. The impulse may be divided 

 and be discharged from the end-organs of a number of branches at the 

 same time. 



The circuits vary from short ones composed of two or three neurons 

 to long ones composed of many. These paths are arranged, in some 

 cases, so that the longer ones may be short-circuited. As some of the 

 nerve-cells can modify and act upon the impulses that pass through them, 

 this becomes true also of the entire circuit. Some of them are very com- 

 plicated and are arranged for the performance of the mental processes 

 in the forms that possess the power of thinking. The method of this 

 performance is not understood. 



The maintenance of these closely related pathways through the nerv- 

 ous system depends upon the exact and accurate working of its units, 

 the nerve cells. These cells, called the neurons, will always, in health, 

 carry an impulse along its appointed path and deliver it at a certain 

 point or points. It will not allow it to "leak" into the neighboring cells 

 or tissues until it is discharged into the cell for which it is intended, and 

 it will always be proportional, in kind and degree, and within more or 

 less narrow limits, to the stimulus that caused it. This proportion is not 

 always a direct ratio. Some neurons can receive a wider range of stimuli 

 than others which are more highly specialized and consequently more 

 restricted in their repertory. A stimulus too weak or too strong will 

 produce no impression whatever. The aggregate of body surfaces from 

 which the nerve cells receive their perceptory stimuli is known in neu- 



