NEEVE 83 



labyrinth of intercalated neurons in the cord, the outgoing 

 neurons and the muscles upon which these act. 



The marvel is, that in the labyrinth of neurons in the 

 cord any sufficiently definite course can be taken to secure 

 the perfectly harmonious and co-ordinated response which 

 occurs, and it is not surprising that the definite passage of 

 the impulse may be readily interfered with. Thus, the 

 administration of strychnine leads to the abolition of 

 definite reaction and to a generalised contraction of the 

 muscles. 



Only by a long process of evolution can such definite 

 paths have been marked out through this labyrinth, and 

 only on the supposition that, if once a certain response has 

 followed a given stimulus, the same response will tend to 

 follow it again, can we understand how these definite reflex 

 actions have been gradually established. The reflexes are 

 then inherited reactions and in the process of evolution only 

 those of benefit to the species have survived. 



Primarily reflex action was confined to the segment of 

 the body in which it was originated, but gradually the reflex 

 association of all segments, of different levels of the cord, 

 has been established. 



In order that the appropriate response should follow a 

 given stimulus, the structural and functional integrity of 

 all the structures involved is of course essential, and any 

 modification in the condition of any part may seriously 

 alter the result. 



Above all, the condition of the spinal cord itself is of 

 importance. 



1. The impulse in passing across the synapses in the 

 cord takes time, and the period between the application of 

 the stimulus and the resulting action is much longer than 

 the time which the impulse would take to travel up the 

 ingoing nerve and down the outgoing nerve (p. 70). This 

 is called the latent period of reflex action. 



The duratioa of the latent period varies with the strength 

 of the stimulus, and with the condition of the spinal cord. 

 It also probably varies with the number of synapses which 

 have to be crossed in the cord, and hence some simple 



