348 THE POPULAR SCIENCE MONTHLY. 



according to the habits and education of the muscles, particular mus- 

 cles can enter only with gx-eat difficulty upon isolated movements, and 

 that the contraction of one muscle sometimes forces the contraction of 

 others, even against the will. 



There are, then, among the encephalic centres at the base of the 

 brain, centres of coordination and direction of movement, which, so to 

 speak, preside over the details of external acts (walking, swimming, 

 flying, etc.), and which receive from the cerebrum only general orders 

 to execute such or such bodily movements. 



The motions of the body produced by the locomotive centres, under 

 the influence of the brain, are of two orders : those of instinct or he- 

 redity, and those of habit. Both are inevitable, but they differ in this, 

 that the first take place in all animals, whether young or old ; while 

 the second occur only in the old. To employ the usual expression, 

 one is nature / the other, second nature. If you remove the cerebral 

 lobes of a duck that has never been in the water, and then place it in 

 water, it will swim regularly ; but it will not, like an old duck deprived 

 of its cerebrum, make certain habitual movements of the neck. The 

 old pigeon, although without its cerebrum, when asleep places the head 

 under the wing ; and he often even dresses his feathers. Young pigeons 

 have never been observed to perform these acts, while they execute 

 other movements normally like old pigeons ; their flight is very regu- 

 lar, even when they undergo the operation before leaving the nest. 



It is, then, probable that by habit there are formed in the nervous 

 centres certain connections between cellular groups, which give rise to 

 bodily movements that become as imperative as those which are due to 

 instinct. 



In animals deprived of the cerebrum, then, the locomotive centres are 

 still complete, and, as we have already said, they differ from the unmuti- 

 lated only by the impossibility of spontaneously beginning movement. 

 To act, they must receive an impulse either from without or within. 

 Exterior excitement we can produce artificially, by acting on the pe- 

 ripheral nerves ; interior excitation is produced by the cerebrum, and 

 we may say that, from the point of view of physiology, the cerebrum 

 has no other function than to put in action the different motor centres. 

 It is a simple excitant, with this important difference that external 

 impressions can determine only a certain number of movements, while 

 the brain provokes an immense variety. 



Let us now consider the movements of rotation, which follow from 

 wounds of portions of the encephalon. They are of two distinct 

 types : the one is a rotary movement round a circle, the other a motion 

 of rolling or spinning. In the first case, the animal remains in his nor- 

 mal attitude, but tends always to go to one side, and describe an orbit 

 more or less extended. In the second case, the animal can progress 

 but little in moving himself. When he attempts to move, he is forced 



