CHAPTER XVIII. 



THE CENTRAL NERVOUS SYSTEM. 



THE functional part, or parenchyma, of the central nervous 

 system is composed of ganglion- cells with their processes. Some 

 of these processes are of cytoplasmic nature, and, as explained in 

 the chapter on the elementary tissues, are called the protoplasmic 

 processes. From each ganglion-cell at least one process is given 

 oif which differs from the protoplasmic processes, and is called the 

 "axis-cylinder process." This in most cases becomes the axis- 

 cylinder of a nerve-fibre, and may be invested with a medullary 

 sheath and neurilemma at some point near or at some distance from 

 its exit from the cell. 



It will be convenient, for the brief description of the central 

 nervous system to which this chapter must be restricted, to adopt a 

 special terminology for the different portions of the ganglion-cell 

 and its processes, as follows : the term ganglion-cell will be restricted 

 to the nucleus and the cytoplasm surrounding it ; the protoplasmic 

 processes will be called the dendrites, and their terminations 

 the teledendrites. The axis-cylinder process will be termed the 

 neurite; the delicate branches it may give off in its course, the 

 collaterals; and the terminal filaments of the main trunk, col- 

 lectively the teleneurites. The cell, with its processes and their 

 terminations, will collectively constitute a neuron. 



A complete neuron, then, consists of (1) certain teledendrites, which 

 unite to form one or more dendrites connecting them with the gan- 

 glion-cell ; (2) the cell itself; and (3) one or more neurites, which may 

 give off collaterals and finally terminate in teleneurites (Fig. 217). 



At the present time these neurons are believed to be without 

 actual connection with each other, but to convey nervous stimuli by 

 contact. The course of the nervous impulses is from the teleden- 

 drites to the nerve-cell, and thence, by way of the neurite, to the 

 teleneurites, whence it is communicated, without a direct structural 

 union, to the next tissue-element in the chain of nervous transmis- 

 sion. Those neurites which carry stimuli from the nerve-centres 



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