AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The dorsal root-fibres among the spinal and cranial nerves, together with 

 their homologues in the retina and the olfactory region, are the only channels for 

 the entrance of impulses into 

 the central system. Once having 

 arrived there, the impulses cause 

 other cells to discharge, and these 

 in turn still others, through an 

 indefinite series. The original 

 impulse may thus arouse many 

 other impulses within the system, 

 and these spread until some of 

 them reach cell-bodies which give 

 rise to efferent fibres and which 

 discharge away from the central 

 system. The efferent fibres pass 

 out mainly by the ventral roots, but 

 in part by the lateral (when pre- 

 sent) or by the dorsal roots (Fig. 

 163). Such efferent fibres end 

 either directly in striated muscle 

 tissue, or in the neighborhood 



D.c 



D.P 



FIG. 162. Schema of the arrangement of the 

 human spinal cord as seen in cross section ; for 

 clearness the afferent fibres are shown on the 

 left side only, efferent and central cells on the 

 right side only (von Lenhossek) : D. K, dorsal 

 root ; V. .R, ventral root ; D. P, direct pyramidal 

 fibres ; C. P, crossed pyramidal fibres ; C, direct 

 cerebellar tract ; A. L, antero-lateral tract ; D. C, 

 dorsal columns. The various classes of cell- 

 bodies are indicated by the manner of draw- 

 ing. 



FIG. 163. Schema of the distribution of the 

 efferent fibres of the spinal roots. A, afferent 

 fibres in the dorsal root only ; E, E, efferent fibres 

 in both dorsal and ventral roots. In the ventral 

 root one group of efferent fibres goes to M, the 

 striped muscles ; another group to ganglion cells, 

 S, forming a single sympathetic ganglion, or to S', 

 cells located in more than one sympathetic gan- 

 glion, but all connected with one efferent fibre by 

 means of its collaterals ; P, peripheral plexuses 

 into which the neurons of some sympathetic cells 



of ganglia (sympathetic ganglia). The fibres from the ganglia, in turn, 

 very often connect with a peripheral plexus, such as the double plexus of 

 Meissner and Auerbach, or the plexuses about the blood-vessels. 



The evidence for the foregoing statements is briefly the following : The 



