THE CENTRAL NERVOUS SYSTEM 749 



axis-cylinder process on the other (Figs. 300, 303, 307). In the axis- 

 cylinders of nerve-fibres the fibrils (Fig. 301) appear to preserve 

 their identity down to the distribution of the fibre. In the ground 

 substance between the fibrils lie round, angular, or spindle-shaped 

 bodies (Nissl's bodies) whicl} stain with basic dyes (Fig. 311).* 

 These bodies vary in appearance in different kinds of nerve-cells, 

 and in the same nerve-cell under different conditions. According 

 to Macallum, they contain organically combined iron. In a multi- 

 polar cell, like those in the anterior horn of the spinal cord, several 

 processes it may be five or six, or even more pass off from the 

 cell-body (Frontispiece). The most complete pictures of them are 

 given by preparations impregnated according to the method of 

 Golgif (Figs. 302, 305). One of the processes 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 branches 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. 307) or 

 muscular spindles (Fig. 433, p. 983). The rest of the processes of the 

 cell, which are termed dendrites or protoplasmic processes, very rapidly 

 dimmish in diameter, as they pass away from the cell, by breaking 

 up into 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 signi- 

 ficance is unknown. The dendrites terminate at a little distance 

 from the cell, where they come into relation with the end-arboriza- 

 tions 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 way the nervous impulse may be switched like a railway- 

 train from one path to another. But there is no experimental basis 

 for this somewhat crude, if fascinating, hypothesis. Sherrington 

 has suggested that the presence of a ' membrane ' at the synapse 



* In Nissl's method the sections are stained in a solution of methylene 

 blue, and decolourized in anilin-alcohol. 



f The method depends upon the deposition of mercury, or silver, in or 

 around the cell-bodies and their processes in tissues which have been 

 hardened in bichromate of potassium and then soaked in a solution of 

 mercuric chloride or silver nitrate. In Pal's improvement of Golgi's 

 method' a solution of sodic sulphide follows the mercuric chloride. 



