INHIBITION. 



711 



bo 



in the cortex cerebri, 1 although in itself slower than the normal rate of 

 voluntary contractions, nevertheless shows indications of its curves 

 being formed by the summations of excitation of a rate of about 10 per 

 second. 2 



Although the character of the curves obtained by excitation of the 

 cortex is similar to that obtained by excitation of the corona radiata, it 

 must not be supposed that the effect is obtained by a spread of the 

 stimulus to the subjacent fibres of the white matter. For there is a 

 distinct delay in the lost time (period of latent excitation) of the 

 muscular response, if we compare the com 

 mencemonts of the two curves ; amounting 

 according to Bubnoff and Heidenhain, 3 t 

 about - 0045 4 in the unamesthetised dog 

 (Fig. 325), while with anaesthesia it is more 

 prolonged, and eventually the response of 

 the cortex ceases before that of the corona 

 radiata. Further, as will presently appear, 

 the excitation of the cortex is apt to be 

 followed by epileptiform contractions of the 

 muscles, which is never the case with 

 stimulation of the corona radiata. 



Inhibition. — There is another function 

 of the cerebral cortex which is no less im- 

 portant than that of originating movements, 

 namely, that of inhibiting them. The rest- 

 lessness of Goltz's decerebrised dogs, and 

 perhaps that of the lunatic, may be due to 

 the loss or diminution of this function. It 

 has, moreover, been shown 5 that in mammals 

 there occurs, as an immediate result of re- 

 moval of the hemispheres, a condition of tonic 

 contraction of certain groups of muscles of 

 the trunk and limbs, the contraction being brought about by afferent 

 impulses passing along the ordinary sensory channels ; if one hemisphere 

 only be removed, the tonus or rigidity only occurs on the homonymous 

 side of the body. An increase of the inhibitory function, on the other 



Fig. 325. — Period of latent ex- 

 citation of a dog's muscle on 

 stimulation respectively of the 

 cortex cerebri, M, and of the 

 subjacent corona radiata, M'. 

 Time tracing T ^ ff of a sec- 

 ond. — Francois - Franck and 

 Pitres. 6 



1 As is shown by the experiments of Gotch and Horsley {Phil. Trans., London, 1891, 

 B, p. 267), who found that changes of electrical potential of the same rate as the clonic 

 contractions of the muscles, passed along the spinal cord during the occurrence of an 

 epileptoid attack. 



2 A similar observation has been made by Wolfenden and Dawson Williams regarding 

 the rhythm of the contractions in cases of disease accompanied by muscular tremors, which 

 frequently show a tendency to fuse, so that the apparent rhythm is only about one-half the 

 normal rate (Brit. Med. Jour*., London, 1888, vol. i. p. 1049). See also Herringham, 

 Joum. Physiol., Cambridge and London, 1890, vol. xi. p. 481. 



3 Arch./, d. ges. Physiol., Bonn, 1881, Bd. xxvi. S. 187. 



4 It is interesting to note that this reduced lost time of the cerebral cortex is precisely 

 the same as the reduced lost time of a simple reflex act, as determined by Wundt, and since 

 in all probability the delay occurs, not in transmission through the nerve cell bodies, but at 

 the synapse or conjunction of afferent fibres with them (see p. 608), it may be concluded that 

 when the cerebral surface is stimulated, the nervous impulses which are started pass to the 

 projection cells from which the descending fibres originate, through the intermediary oi 

 synapses formed between these cells or their dendrons and the more superficial cells oi the 

 cortex, which are those actually stimulated. 



5 Sherrington, "Decerebrate Rigidity, etc.," Jowrn. Physiol., Cambridge and London. 

 1898, vol. xxii. p. 379. 



6 Francois-Franck, " Lecons sur les fonctions mo trices du cerveau," Paris, 1887. 



