98 VETERINARY PHYSIOLOGY 



mechanism connected with the cord. So much is this the case 

 that its destruction leads to inability to co-ordinate the move- 

 ments of the eyes, head and limbs, and to loss of muscular tone. 

 This is seen in the " knock-out " blow on the jaw, which forces 

 the condyles against the base of the skull, and by deranging the 

 action of the labyrinth leads to a sudden and absolute loss 

 of muscular tone. 



Just as the cerebrum is developed in connection with the 

 distance receptors of the head, so the cerebellum is developed in 

 connection with the labyrinthine receptor mechanism. And 

 just as the non-distance receptors of the cord are connected 

 up with the cerebrum, so the proprio-ceptive mechanism of 

 the cord is connected with the cerebellum. 



Thus it is that in the higher animals the head receptors and 

 the brain which is developed in connection with them domin- 

 ate the cord and control the various spinal reflexes. 



So far the reactions of the neuro-muscular mechanism have 

 been considered simply as an affair of reflexes. But the activity 

 of the cerebrum is accompanied by changes in consciousness, and 

 it is assumed by some that the state of consciousness is the 

 determining factor in the result, and hence such actions 

 have been called voluntary. But since in such conditions as 

 sleep-walking and hypnosis the most complex and selective 

 actions are performed without the intervention of conscious- 

 ness, it must be admitted that this metaphysical pheno- 

 menon is not an integral part of the response of the nervous 

 system. 



On the other hand, we know that the character of the 

 reaction to any stimulus is largely dependent upon the state 

 of the nervous centres. Just as a touch produces a different 

 effect in a frog poisoned with strychnine and in one under 

 the influence of bromide of potassium, so a sudden noise may 

 produce a totally different reaction upon a person with a 

 fatigued brain or a brain poisoned by alcohol, and upon one 

 with the brain in a good state of nutrition. 



Not only does the temporary state of nutrition thus 

 modify the result of a stimulus, but the paths of action 

 previously opened and defined through the centres also have 

 a marked influence. (1) These paths may have been formed 



