CHARACTERISTICS OF REFLEX ACTIONS 345 



inhibited by strong stimulation of the other sciatic nerve, by stimulation 

 of the spinal cord at a higher level, or by stimulation of the optic lobes. 

 Immediately after pithing the brain of the frog, the whole animal 

 becomes flaccid and motionless, and for the next few minutes it is 

 impossible to elicit any reaction by stimulation, however strong, 

 applied to the skin of the body. In the production of this condition 

 of ' shock/ the inhibition of all the spinal centres, produced by the 

 strong stimulation of the injury to the brain and medulla, plays 

 at any rate an important part. We may say that the passage 

 of an impulse through a chain of neurons diminishes the block for 

 subsequent impulses at each synapse that it traverses, but increases 

 during its passage the block in all the adjacent synapses. 



In dealing with the special reactions of the spinal cord we shall 

 have occasion to refer more fully and in greater detail to many of 

 these properties which are characteristic of all reflexes. Before 

 however treating of the functions of the separate parts of the central 

 nervous system in the higher mammals, it may be of interest to consider 

 the exact nature of the structure intervening between neuron and 

 neuron at each field of conj unction or synapse, as well as the significance 

 of the two chief elements of the central nervous system, nerve-cell and 

 nerve fibre, in the production of co-ordinated purposive reactions. 



