NERVE 



55 



and to form a network which may be called a terminal 

 plexus. These are kept in connection with the central 

 nervous system by the outgrowths of processes from cells 

 which remain in the spinal cord (fig. 18). In certain 

 positions, e.g. in the wall of the gut, these terminal plexuses 

 control the activity of the structure in which they are 

 placed (see fig. 48, p. 109). 



It is as if the central nervous svstem had devolved 





Fig. 19.— (a) A Nerve Cell with Nissl's Granules ; {b) a similar cell 

 showing changes on section of its axon. 



the power of local government upon these emigrated 

 neurons. 



II. Structure. 



1. Cells. — The shape and characters of the cells, and 

 their position upon the processes vary greatly, but they have 

 all the following features in common : — They are nucleated 

 protoplasts, the protoplasui of which, after fixing and 

 staining, shows a well-marked network, in the meshes of 

 which a material which stains deeply with basic stains, and 

 which seems to be used up during the activity of the 

 neuron, may accumulate in granules. The granules formed 

 of this material are generally known as Nissl's granules 

 (fig. 19). ^ 



Mott has failed to find such a structure in living nerve 

 cells, and by the use of the ultra-microscope has observed 

 particles moving freely in the colloidal fluid contents, 



