148 THE INVOLUNTARY NERVOUS SYSTEM 



vascular system is suggested by the behaviour of the colossal 

 adrenaline cell, which also sends efferent fibres into the anterior 

 and posterior nerves, but the relationship between the three 

 adrenaline containing cells is not yet known, nor indeed is it quite 

 certain that the axons of the other two cells behave in the same 

 way as that of the colossal cell. One striking fact is common to 

 the axons of cells which split in the ganglion to form two fibres : 

 after splitting one fibre is invariably bigger than the other. 

 This fact is very suggestive when considered in conjunction with 

 Biedermann's statement that the nerves to the antagonistic 

 muscles of the cray-fish claw show, near their termination in the 

 muscle, the existence of two nerve fibres in the same sheath, 

 which always branch simultaneously in their path to the muscle : 

 the one fibre is invariably larger than the other. The larger of 

 these two fibres is usually held to be the motor fibre and the 

 smaller the inhibitory fibre to the muscle. 



If such an excitor neuron has travelled out of the central 

 nervous system towards the periphery in one of the outflows of 

 the involuntary nervous system, it would by itself effect the re- 

 ciprocal innervation of antagonistic muscles in that system, and 

 give an efficient explanation of the numerous instances, already 

 given, of the presence in the same ganglion of neurons motor to 

 one group of muscles and inhibitory to the other. 



Considerable difficulties beset the view, that a single nerve 

 cell supplies a motor nerve fibre to one muscle, and an in- 

 hibitory nerve fibre to the opposing muscle, in those cases 

 where there are only two outgoing nerves, into which these 

 fibres respectively pass. It would follow that the one nerve 

 would contain both the motor and inhibitory fibres to the one 

 muscle and the other nerve both the motor and inhibitory fibres 

 to the opposing muscle, but Celesia's evidence in the cray-fish 

 makes us believe that stimulation of the one nerve excites the 

 one muscle and simultaneously inhibits the opposing muscle, so 

 that the inhibitory nerves of the one muscle leave the central 

 nervous system with the motor nerves of the opposing muscle. 

 My son has seen indications of the same phenomenon in the case 

 of the longitudinal and circular muscles of the leech. At present 

 we have not sufficient evidence to enable us to explain the mean- 

 ing of this striking phenomenon of the invertebrate central 

 nervous system, viz. : the origin from one nerve cell of two nerve 



