CENTRAL NERVOUS SYSTEM. 



647 



There are then to be expected from these cells, forming as they do the 

 great bulk of the central system, reactions of the same order as those occurring 

 among the central cells of the cord. 



Decussation. All through the central system neurons pass from one 

 lateral half to the other, witness for example the arrangements of the optic 

 chiasma, the callosum, the decussatiou of the 

 pyramidal fibres and the ventral commissure 

 in the cord itself. It is to be noted, however, 

 that the bulk of the commissures is small as 

 compared with the masses which they connect. 

 So far as known, the neurons of the dorsal roots 

 that have entered the dorsal column of the cord 

 on one side of the middle line do not cross, by 

 their main stems at least, to the other side. As 

 regards the efferent cells, it appears that the 

 neurons of some of these do cross in the ventral 

 commissure, but in the instances above given, 

 and in the case of the greater number of fibres 

 belonging to the ventral commissure, the neurons 

 concerned are the outgrowths of central cells 

 (Fig. 168). In the case of the central cells 

 the decussation may be effected by the entire 

 neuron or by a principal branch from it. Such 

 is the arrangement in the case of certain cortical 

 cells which send one branch to the callosum 

 (Cajal). Besides these connections between 

 parts lying symmetrically on either side of 

 the middle line, there are of course dorso-ven- 

 tral connections, but the neurons by which this 

 is effected do not run in bundles and are there- 

 fore less obvious and probably less important. 



IV 



VI 



FIG. 168. Illustrating the partial 

 and complete decussation of the 

 fibres of the third and fourth cranial 

 nerves, and the absence of decussa- 

 tion in the case of the sixth: ///, 

 root of the third cranial nerve ; IV, 

 of the fourth ; VI, of the sixth. 



C. PATHWAY OP THE IMPULSES. 



Conditions of Stimulation. In speaking of the nerve-impulses we regard 

 them as always initially aroused at the periphery, using this last term in a 

 wide sense. The conditions necessary for this arousal are an external stimulus, 

 acting on an irritable nerve-end. While life exists, stimulation of varying 

 intensity is always going on, and hence there is no moment at which the 

 nervous system is not stimulated and no moment at which the effectiveness 

 of this stimulus is not varied. The response to this continuous and ever- 

 varying stimulation is not necessarily observable, but occasionally the variation 

 in the stimuli is so wide that an evident reaction follows. 



Though the foregoing statements suggest that the chief variable is that 

 represented by the stimulus, the strength of which changes, yet as a matter of 



