An American text-book of physiology . e masses which they far as known, the neurons of the dorsal rootsthat have entered the dorsal colunni of the cordon one side of the middle line do not cross, bytheir main stems at least, to the other side. Asregards the efferent cells, it appears that theneurons of some of these do cross in the ventralcommissure, but in the instances above given,and in the case of the greater number of fibresbelonging to the ventral commissure, the ueuronsconcerned are the outgrowths of central cells(Fig. 168). In the case of the central cellsthe decussation may
An American text-book of physiology . e masses which they far as known, the neurons of the dorsal rootsthat have entered the dorsal colunni of the cordon one side of the middle line do not cross, bytheir main stems at least, to the other side. Asregards the efferent cells, it appears that theneurons of some of these do cross in the ventralcommissure, but in the instances above given,and in the case of the greater number of fibresbelonging to the ventral commissure, the ueuronsconcerned are the outgrowths of central cells(Fig. 168). In the case of the central cellsthe decussation may be effected by the entireneuron or by a principal branch from it. Suchis the arrangement in the case of certain corticalcells which send one branch to the eallosum(Cajal). Besides these connections betweenparts lying symmetrically on either side ofthe middle line, there are of course dorso-ven-tral connections, but the neurons by which thisis effected do not run in bundles and are there-fore less obvious and probably less Fig. 168.—Illustrating the partialand complete decussation of thefibres of the third and fourth cranialnerves, 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. O. Pathway op the Impulses. Conditions of Stimulation.—In speaking of the nerve-impulses we regardthem as always initially at the periphery, using this last term in awide The conditions necessary for this arousal are an external stimulus,acting on an irritable nerve-end. While life exists, stimulation of varyingintensity is always going on, aird hence there is no moment at which thenervous system is not stimulated and no moment at which the effectivenessof this stimulus is not varied. The response to this continuous and ever-varying stimulation is not observable, but occasionally the variationin the stimuli is so wide that an evident reaction follows. Though the foregoing stat
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