CHAPTER XIII 

 NERVE TISSUES 



THE nerve cells are the cells that put an organism into communica- 

 tion and correlation with outer chemical, physical, and mechanical 

 conditions. In order to perform this duty they must be able to do three 

 things : 



Firstly, to perceive or be stimulated by the outer conditions directly, 

 or indirectly through the stimulus of another nerve cell, cell-product, 

 or foreign substance, function of perception. 



Secondly, to transfer the stimulus, so received, as an impulse through 

 the cell substance to some other surface of its cytoplasm which is in 

 nervous contact with the cell or cells that are to be communicated with, 

 function of conduction. 



Thirdly, to discharge the impulse to this other cell or cells as a stim- 

 ulus, function of stimulation. 



It is probable that all cells of an unspecialized character in the lower 

 animals have more or less of these three powers, and it is only when the 

 cell is modified to perform the function specifically that we recognize 

 them as nerve cells. The specialization consists of the acquisition of 

 three different kinds of cell-organs by the cytoplasm of the respective 

 cells:- 



1. Of the development in the cytoplasm of a perceptory organ to 

 receive the stimulus. This organ is a modification of the cytoplasm 

 at some favorably situated point on the surface, and consists in different 

 cells of a great variety of rods, hairs, plates, cones, fibrils, protoplasmic 

 processes, etc., which are modified to suit the conditions met with (see 

 Fig. 155, upper arrows). 



2. Of the development in the cytoplasm of a number of fine fibrils, 

 the neuro-frbrils and other structures to be used in carrying the resulting 

 impulse to the other end or pole of the cell. This pole is in contact with 

 some other cell or cells with which it is intended to communicate. 

 The distance traversed causes the communicating cytoplasm to form 

 a longer or shorter fiber (Fig. 155, B, C, D, E, and F). 



3. Of the formation, at the surface of this point, of an end-organ or 

 end-plate that is used to discharge the impulse as a stimulus to the other 



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