THE r6lE of reflex INHIBITION 589 



the motor neurone. The term syyiaptic conduction is reserved lor 

 this process, by which the excitatory state is propagated from 

 neurone to neurone ; the word cell-conduction is restricted to 

 the process of propagation of the excited state (nerve-impulse) 

 along the length of the neurone (nerve-cell) itself, e.g. from its 

 one end to its other. S3maptic conduction is intercellular, cell- 

 conduction is intracellular. The propagation of a nervous 

 impulse along a reflex arc involves both these processes, since a 

 reflex arc is always a chain of separate neurones. As to synaptic 

 conduction and cell-conduction, both are electrical processes 

 involving movement of ions in the dilute solutions which lie 

 within the neurones (Macdonald) and also bathe the neurone 

 surfaces outside. Of the essential nature of inhibition and whether 

 its action-point lies at the synapse or in the perikaryon, is 

 intercellular or intracellular, in spite of ingenious researches 

 (Verworn, F. Frohlich) we remain still largely ignorant. 



But while much is still obscure in regard to nervous in- 

 hibition, certain types of its action have in recent years 

 become familiar to us and enable us to decipher certain of 

 the biological purposes which it serves. 



2. Grading of Muscular Contraction by Inhibition 



Experiment shows that by appropriate grading of the reflex 

 stimulus exciting its motor centre a muscle can be made to 

 contract weakly or strongly according as the stimulus applied 

 is v^^eak or strong. Other conditions remaining unchanged, 

 the intensity of the motor response follows the grading of the 

 stimulus with exquisite fidelity. This gradation of reflex 

 response is of fundamental importance in securing that the 

 degree of muscular action shall be suited to the circumstance 

 evoking it. It is an obvious element in the co-ordination of 

 the motor reactions of the animal. Much of the varied intensity 

 of contraction exhibited in natural actions of the musculature 

 is doubtless a simple consequence of the degree of intensity, 

 mild or strong, of the stimulus that evokes a reaction. 



But there is another way in which the grading of muscular 

 response is obtained, and this latter is probably the more 

 common under the complexity of natural conditions. A weak 

 discharge of impulses from the motor centre does not invariably 

 mean weak stimulation of the excitatory afferents. The motor 



