806 A MANUAL OF PHYSIOLOGY 



the final common paths to antagonistic muscles must also be 

 temporarily closed. The closing of these central connections, 

 or rather the raising of their threshold sufficiently to bar the 

 impulses from passing through the door, is an inhibitory pheno- 

 menon. Inhibition and excitation go hand-in-hand in the 

 simultaneous combination of reflexes. 



The successive combination of reflexes is well illustrated by 

 the contraction of the oesophagus in deglutition. First one 

 portion of the tube and then the next below are involved in 

 the reflex action. The combination consists in the orderly 

 sequence. The manner in which this is secured in this class of 

 reflex actions has been luminously discussed by Sherrington,* 

 but details cannot be given here. While only allied reflexes 

 i.e., such as mutually reinforce and therefore harmonize with each 

 other can be simultaneously combined, and antagonistic 

 reflexes cannot, both allied and antagonistic reflexes can be 

 successively combined. An example of the successive com- 

 bination of allied reflexes is the series of scratch-reflexes caused 

 by a parasite travelling across the receptive field of the reflex. 

 An example of the successive combination of antagonistic 

 reflexes is afforded when either the scratch-reflex or the flexion 

 reflex is induced and caused to interrupt the other while it is 

 proceeding. The transition e.g., from flexion to scratch reflex 

 is made without any period of confusion. The same holds 

 good for other antagonistic reflexes. In many cases the avoidance 

 of confusion is due to the inhibition of the first reflex, or often 

 to inhibition of the set of muscles which were active in the first 

 reflex combined with excitation of their antagonists (so-called 

 interference). It is obvious that this is an adaptation of great 

 importance. 



Influence of the Brain on the Spinal Reflexes. The spinal reflexes 

 can be influenced by impulses descending from the higher centres. 

 For (a) it is a matter of common experience that a reflex movement 

 may be to a certain extent controlled, or prevented altogether by an 

 effort of the will, and it is worthy of remark that only movements 

 which can be voluntarily produced can be voluntarily inhibited. 

 A scratching reflex in the normal dog may be seen to be modified 

 in character or duration as compared with the same reflex in the 

 spinal animal, (b) An animal like a frog responds to stimuli by 

 reflex movements more readily after the medulla oblongata has been 

 divided from the spinal cord, (c) Long-continued muscular con- 

 tractions may be caused in animals after removal of the cerebral 

 hemispheres by stimulation of afferent nerves for example, by 

 scratching the mucous membrane of the mouth in a ' brainless ' 

 frog or Necturus. (d) By stimulation of certain of the higher 

 centres reflex movements which would otherwise be elicited may be 

 suppressed or greatly delayed. If the cerebral hemispheres are 



* ' Integrative Action of the Nervous System,' to which work the 

 advanced student is referred. 



