NERVOUS CONTROL OF ANIMAL MOVEMENTS. 347 



cases, notwithstanding the stimulus of the water on the integuments, 

 the frog will remain immovable. If, now, you slowly withdraw the 

 board below the frog without disturbing his position, he will remain 

 motionless ; but, if you tip it one side, the frog at once wakes from his 

 quietude. The loss of equilibrium acts more energetically than the 

 stimulus of the water on the skin. 



Tip a carp to the right or left, at once it recovers its normal atti- 

 tude. If you place a duck on one of its sides, either on the ground or 

 in the water, it at once corrects itself and comes upright. The cere- 

 bellum alone controls equilibrium, as is easily proved by experiment. 

 When it is wounded or destroyed, the animals He indifferently on one 

 side or the other, and make no movements to recover the lost equi- 

 librium. In certain cases, even, they cannot maintain an equilibrium, 

 but tend to fall on one side. 



Fig. 2. 



A Frog in which the Cerebrum has been removed. 



We may conclude that the movements of animals, whether superior 

 or inferior, are produced by certain special mechanisms, or by locomo- 

 tive centres, situated at the base of the brain. These centres are essen- 

 tially passive ; they have no spontaneous action, and come into activity 

 only when excited by peripheral stimulus or by the brain. 



We ought, then, no longer to admit, in the habitual movements which 

 appear perfectly voluntary, a direct action of the brain on each muscle. 

 We must remember that there exist at the base of the brain motor cen- 

 tres which serve to intermediate between the will and external acts. The 

 will calls into action such or such centres, and these immediately deter- 

 mine the action of certain muscular groups. We know besides, that, 



