224 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



chemical changes in the inhibited muscle of the heart arc peculiar and 

 different from those occurring during excitation. For instance, Gaskell ' 

 found that a 'positive variation of the muscle current occurred in the hearts 

 of both the tortoise and crocodile upon stimulating the peripheral end of the 

 vagus; while stimulation of the accelerator nerves caused the usual negative 

 variation. Further, Gaskell pointed out that during the inhibition of the 

 heart-muscle the anabolic processes were in excess, so that the cessation of 

 inhibition was followed by an increase in the strength of the heart-beats. 



If we now turn to the observations bearing on inhibition in the central 

 nervous system, there are to be found numerous experiments, of which the 

 following is a type: Let one leg of a reflex frog be stimulated by pinching 

 it, or by dipping the toe iu weak acid : a withdrawal of the stimulated leg 

 will follow. Now repeat the experiment, at the same time pinching or other- 

 wise stimulating the skin on the opposite leg. It will then lie found that 

 either the latent period of the reacting leg is much prolonged or that the 

 reaction fails completely. This is a very simple example of a type of 

 inhibition which is continually occurring. 



The inhibitory effects are, however, not limited to the motor responses of 

 the central system. It is an observation of the ancients that the greater 

 obscures the leaser pain, and, in a general way, all strong sensations prevent 

 the appreciation of the weaker ones, whether they be in the terms of the 

 same or of a different sense. 



AYithin the central nervous system very remarkable examples of inhibitory 

 phenomena have been investigated, chiefly by Sherrington. 2 Boubnoff and 

 Heidenhain 3 were the first to record the observation that under certain 

 conditions stimulation of the cerebral cortex might cause a relaxation of 

 some extensor muscles of the limbs when these were in a state of tonic 

 contraction. 



Sherrington was able to show that the stimulation of the cortical area for 

 the flexor- of the arms also gave rise to impulses leaving the cortex and 

 causing a (inhibition) relaxation of the antagonistic extensors. 



( )n stimulating the cortical ana for the extensor muscles a corresponding 

 relaxation of the flexors could be observed. Thus the cortical area for the 

 contraction of a given group of muscles coincides with the area for the inhi- 

 bition of the group antagonistic to it. Sherrington has also demonstrated the 

 important role of this inhibitory process in mediating muscular co-ordination 

 shown in movements of the eye. When all the muscles of the eye are para- 

 lyzed, the eyeball held by the connective tissues about it looks straight 

 ahead. Sherrington cut the nerves to all the muscles of the left eyeball 

 (monkey) except the external rectus. ruder these conditions the eye, when 

 ;it rest, looked toward the left. Stimulation of the cortical centers, which 

 cause a conjugate deviation of both eyes to the right, was followed by a 



1 Graskell : Journal of Physiology, vol. vii. 



'-' Sherrington : Ibid., vol. xvii. 



3 Boubnotr und Heidenhain: Pfluger'a Arckiv, xxvi. 



