852 



THE CENTRAL NERVOUS SYSTEM 



cesses of most nerve-cells is distinguished from the rest by the fact 

 that it maintains its original diameter for a comparatively great 

 distance from the cell, and gives off comparatively few branches. 

 This process, which in favourable preparations can be traced on till it 

 becomes the axis-cylinder of a nerve-fibre, is called the axis-cylinder 

 process, or more shortly the axon. The few slender brai ches that come 

 off from it, usually at right angles, are called collaterals. The collaterals 

 consist essentially of one or more fibrils of the axon. Both the main 

 thread of the axon and the collaterals end by breaking up into an 

 arborescent system of fibrils or telodendrion. The telodendrions vary 

 greatly in appearance from simple end -brushes to far-branching 

 thickets, or such special end-organs as motor plates (Fig. 335) or 

 muscular spindles. The rest of the processes of the cell, which are 

 termed dendrites or protoplasmic processes, very rapidly diminish in 

 diameter, as they pass away from the cell by breaking up into 



Fig- 33O. Multipolar Nerve-Cell: Golgi Preparation (Barker, after Kolliker). 

 , axon; c, collaterals. 



fibrils like the branches of a tree. The Nissl bodies extend for some 

 distance into the dendrites, but not into the axon. The dendrites 

 of some cells, especially the pyramidal cells of the cerebral, and 

 the Purkinje's cells of the cerebellar cortex, have small swellings, 

 the so-called lateral buds or gemmules, on their course. Their signifi- 

 cance is unknown. The dendrites terminate at a little distance from 

 the cell, where they come into relation with the end -arborizations of 

 the axons of other neurons. In this way two or more neurons are 

 linked together to form a nervous path. According to the view most 

 commonly held (neuron hypothesis), the relation is not one of actual 

 anatomical continuity, but the processes come so close together that 

 nerve impulses are able to pass across from the terminal brush of the 

 axon of one nervous element to the dendrites or cell-body of another. 

 This kind of junction is called a synapse. 



It has been suggested that the contact may be rendered more or less 

 close through amoeboid movements of the dendrites, and that in this 



