FUNCTIONS OF THE SPINAL CORD 875 



facilitation is probably of great importance in the acquirement of 

 new reactions and in rendering these acquisitions stable. It is prob- 

 ably one of the main physiological foundations of habit, and there- 

 fore of education. In this connection it is important to note that 

 the very same repetition of stimuli which leads to facilitation leads 

 to fatigue when the stimuli are applied in too rapid succession. 

 (6) The rhythm and intensity of the reflex end-effect correspond 

 much less closely with the rhythm and intensity of the stimulus than 

 in nerve-trunks. (7) The phenomena of refractory period (p. 155), 

 inhibition and ' shock/ are much more conspicuous in the reflex arc 

 than in nerve-trunks. 



Inhibition in Reflex Action. Special emphasis must be laid upon 

 the part played by inhibition in reflex actions. For the proper 

 carrying out of many reflex movements it is necessary not only that 

 the appropriate effector organ, the appropriate muscle, or group of 

 muscles, should be caused to contract at the proper time, but that 

 their contraction, or that of other muscles, should be diminished or 

 abolished by inhibition, or even rendered for a certain period im- 

 practicable by the establishment somewhere in the reflex arc of a 

 refractory state, which is itself a phenomenon of inhibition. There is 

 good evidence that this is a central inhibition i.e., it depends on 

 some process occurring in the spinal portion of the reflex arc. 



As an example of the numerous class of reflexes in which the 

 excitation of certain muscles is accompanied by the inhibition of 

 their antagonists (reciprocal inhibition), we may take the ' flexion 

 reflex/ the flexion at the knee, hip, and ankle of the hind-limb 

 readily elicited in the spinal dog by ' nocuous ' or harmfm stimuli 

 (such as a prick, a strong squeeze, chemical agents, or excessive 

 heat), or by electrical stimuli applied to the skin of the limb or of 

 any afferent nerve of the limb. 



Sherrington has shown that when the legs of the animal are so 

 prepared that only the flexors can act on one knee, and only the 

 extensors on the other, stimulation of symmetrical points on the 

 two sides in the area of skin (receptive field) from which the flexion 

 reflex can be evoked causes contraction (excitation) of the flexors and 

 simultaneous relaxation (inhibition) of the tone of the extensors. The 

 same is true when corresponding afferent nerve-twigs are stimulated 

 on the two sides. From this it is inferred that each of the nerve- fibres 

 from the receptive field of the reflex divides in the cord into two sets 

 of end-branches (e.g., collaterals) a set which produces excitation 

 when it is stimulated, and another set which produces inhibition. 



Reversal of Reflexes. The difference in action is specific in the 

 sense that no mere change in the kind or intensity of stimulation 

 affects it. Yet there are facts which show that the specificity is not 

 absolutely immutable, and that a change of conditions in the spinal 

 cord may permit excitation of a given group of muscles to be pro- 



