PHYLOGENETIC ORIGIN OF NERVOUS SYSTEM 149 



fibres which pass out from the central nervous system into two 

 separate nerves respectively innervating with motor fibres an- 

 tagonistic muscles. 



The muscular system, whose motor fibres arise from cells of 

 the sympathetic system, and which I have called shortly the 

 sympathetic musculature, appears in the first instance, judging 

 from the evidence of the annelids, to have been confined to the 

 vascular musculature ; whether it has spread from there to form 

 a sheet of muscle underlying the epidermis or whether the dermal 

 musculature has nothing to do with that of the blood vessels is 

 a question of great interest which must be left for future re- 

 search. 



I would further venture to suggest that the same kind of 

 nerve cell may be the essential factor in the reciprocal innervation 

 of muscle belonging to the voluntary system of the vertebrate, 

 with the difference that it is not situated in the excitor neurons 

 of that system. There is no evidence at present that inhibitory 

 nerve cells and fibres exist in the voluntary nervous system ; 

 reciprocal innervation of antagonistic muscles is due to the ex- 

 citation and inhibition of the motor neurons themselves, and not 

 to direct action on the muscles. 



Sherrington's work has shown clearly how this reciprocal in- 

 nervation can be maintained by the spinal cord alone, and he 

 has established the laws of this reflex action. It can be carried 

 out by the direct reflexes in the spinal cord involving the simple 

 reflex arc of receptor, connector and excitor neurons. As the 

 excitor neurons send only motor fibres to the muscles, we must 

 look for the mechanism of the reciprocal innervation in the con- 

 nector neurons, and search for evidence that they are connected 

 with motor neurons by two sets of fibres, of which the one excites 

 and the other inhibits the activity of the appropriate motor 

 neurons. 



