CENTRAL NERVOUS SYSTEM. 653 



the drug must affect some other group than these efferent cells. Since, more- 

 over, a tetanus of the legs could be caused by the stimulation of the skin of 

 the arm, the application of the drug being to the brachial enlargement only, 

 it appears that the central cells, or those conducting the impulses entering by 

 the dorsal root-fibres in the brachial region to the nuclei of the lumbar en- 

 largement, are probably affected ; and further, that it is the bodies of these 

 cells on which the drug must act, since they alone were in the locality at 

 which the drug was applied. The application of the drug to the dorsal root- 

 ganglia and to the nerve-roots between the ganglia and the cord proved to 

 be without effect, so that the two parts which can possibly be influenced are 

 the terminations of the sensory afferent nerves within the cord and the bodies 

 of the central cells with which these terminations are associated. But whether 

 the change is in both these structures or only in one cannot now be determined. 



The diffusion of impulses in the central system depends anatomically not 

 only on the amount of branching among the neurons of the individual cen- 

 tral cells, but also on the association of many cells together so as to accomplish 

 this wide distribution of the impulses. In the case of the afferent elements, 

 as we have seen, the diffusion depends on the branching of the neurons 

 alone. 



Peripheral Diffusion. Turning next to the efferent system, we find the 

 conditions for diffusion dependent on the arrangement of several cells in 

 series. When a group of efferent cells discharges, we know from the arrange- 

 ment of the ventral roots that the impulses leave the cord mainly along the 

 fibres which comprise these roots, but where the lateral root is present they 

 may also pass out over it, as well as over the few efferent fibres found in the 

 dorsal roots. These neurons carrying the outgoing impulses have two desti- 

 nations : (1) The voluntary or striped muscle-fibres; (2) the sympathetic 

 nerve-cells, grouped in masses to form the vagrant ganglia (see Fig. 163). 



In the case of those neurons passing to the voluntary muscles, the impulses 

 are distributed to the muscle-fibres to which the final branches of the neuron 

 extend, but there is no evidence that in these localities .the impulses, having 

 entered a given muscle-cell, necessarily pass beyond the limits of that cell by 

 conduction through the muscle-substance. It thus happens that one part of 

 a large muscle can be innervated by one bundle of fibres and another part by 

 a different bundle, or that the same parts of a muscle may be innervated 

 by fibres which reach it through more than one ventral nerve-root, and also 

 that with a given stimulus the strength with which a muscle contracts depends 

 on the proportion of the neurons stimulated, and therefore on the proportion 

 of the muscle-fibres thrown into contraction. 1 



When the impulses are thus sent out there is in the case of motor nerves no 

 diffusion, the effect being limited to the peripheral distribution of the efferent 

 nerve-elements by way of which the impulses leave the central system. The 

 fibres going to the voluntary muscles form, however, but one portion, which 



1 Gad : " Ueber einige Beziehungen zwischen Nerv, Muskel, und Centrum," Wurzburger Feat- 

 tchrift, 1882. 



