THE SPECIAL NERVOUS MECHANISMS 



117 



enters the latter via the dorsal or sensory root. In the grey 

 matter it is received by the dendrites of a nerve cell, and is then 

 transferred (after something has been done with it) to the axon 

 of the cell. This axon issues from the grey matter and passes 

 out from the cord through the ventral or motor root of the same, 

 or a different, spinal nerve, and is transmitted by the latter as 

 an efferent impulse to a muscle which it then sets in motion. 



This is merely a scheme illustrating the simplest conceivable 

 form of nervous motor apparatus. It may be regarded as the unit 



Fig. 33.- 



-DlAGRAM OP THE SiMPLKST POSSIBLE SpINAL RefLEX 



Mechanism. 



An afferent fibre is represented as entering the cord, and ending in a synapse 

 round a cell, which then sends out an axon to a muscle fibre. Through 

 an additional cell and synapse another axon goes out to the antago- 

 nistic muscle. The + sign indicates that one muscle contracts, and 

 the — sign that the antagonistic one relaxes. 



or element of the sensori-motor mechanism, but the reader must be 

 very clear in his mind that the very simplest actual nerve-muscle 

 mechanism is very much more complicated than Fig. 33 suggests. 

 So we may now proceed to elaborate it till it approximates to 

 the conditions that may be studied experimentally. First, then, 

 one muscle never, by itself, constitutes a motor mechanism. 

 For there are always two antagonistic organs, one of which 

 contracts while the other simultaneously relaxes. Secondly, the 

 nerve that supplies a muscle always contains fibres that carry 



