4;o 



NERVOUS SYSTEM 



Nerve-cells present many peculiarities as they are found in different 

 situations. Some of these will be described here, and others will be 

 considered in connection with the study of special nerve-centres and 

 the structure of what is called the neuron. 



In the sympathetic ganglia, the nerve-cells usually are multipolar, 

 and, with their processes, are enclosed in a sheath lined with endothelium. 

 Figure 109 represents an isolated cell from a sympathetic ganglion of man. 

 It is to be noted that this cell, like most of the cells of the central ner- 

 vous system, is multipolar. Figure 1 10 represents a cell from the electric 

 organ of the torpedo, isolated and prepared without the use of staining 

 solutions. This cell shows the characteristics of a multipolar nerve-cell : 

 the axis-cylinder prolongation, or neurite, the so-called protoplasmic 



prolongations, or dendrites, 

 the nucleus and nucleolus 

 and the structure of the cell- 

 body. 



NissTs Granules. In the 

 cells of the encephalon and 

 spinal cord, staining deeply 

 with methylene blue, are 

 found small angular gran- 

 ules, first described by Nissl. 

 These extend a short distance 

 into the protoplasmic pro- 

 longations, and the stain also 

 is taken up by the nucleus and 

 nucleolus (see Fig. 1 12, page 

 472, and Plate XI, Fig. 2). 

 The Nissl granules some- 

 times called tigroid granules 

 are composed of chromo- 



Fig. 100. Sympathetic ganglion-cell from man. X 7^0 -\ , . , . , 



(Key andletzius). P laSm > whlch 1S * HUCleO-prO- 



A, cell with nucleus and nucleolus; B, sheath with tei d. The name khietoplasm 



nucleated-cell lining ; C, branched process ; D, axis-cyl- has been given to this Sub- 



inder process ' stance to indicate its probable 



function. It is thought that the potential activity of the cell is stored 

 up in the chromoplasm, which is used in the discharge of nerve- 

 impulses during the activity of the cell ; and that during periods of rest 

 the granules are formed again. It has been noted, indeed, that the 

 discharge of nerve-impulses brings about a disintegration of Nissl's 

 granules, which break down into a fine granular substance, part of 

 which temporarily disappears. This result of cell-activity is called 



