CHAPTER V. 



THE INHIBITORY NERVES. 



So far I have built up the conception of an involuntary nervous 

 system formed on the same plan as the voluntary nervous system, 

 but with its motor nerve cells outside the central nervous system. 

 We come now to a much more difficult question, the meaning of 

 inhibition in these two systems. In the voluntary nervous system 

 we are accustomed to the conception of inhibition of motor 

 activities in connexion with co-ordinated actions, and consider 

 such inhibition to be brought about by interaction between parts 

 of the central nervous system itself, i.e. through impulses passing 

 along connector nerves of the voluntary system. An exactly 

 similar inhibition might be produced in the involuntary nervous 

 system by impulses passing along connector nerves of the in- 

 voluntary system ; in other words, by stimulation of nerves like 

 the vagus, the splanchnic, and the pelvic nerves. In the volun- 

 tary nervous system we cannot go farther than this ; all the 

 fibres which pass to muscles from the cells of the anterior horn 

 are motor in function, and, what is more important, there is no evi- 

 dence of more than one nerve fibre ending in a muscle fibre, so 

 that neither histologically, nor physiologically, is there. any evi- 

 dence for the existence of nerve cells among the anterior horn 

 cells, which give origin to fibres inhibitory to the contraction of 

 voluntary muscle. 



In the involuntary system on the contrary we cannot refer all 

 cases of inhibition to the action of connector fibres, for there 

 seems to me clear evidence that the fibres passing from certain 

 nerve cells to the tissue itself are not motor but inhibitory to the 

 activity of that tissue ; we must in fact accept, in the case of the 

 involuntary nervous system, the existence of inhibitory cells and 

 fibres, as well as motor cells and fibres. 



The question is : Is there evidence that fibres which relax 

 contracted involuntary muscle pass directly into that muscle 



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