Page:Popular Science Monthly Volume 1.djvu/360

348 according to the habits and education of the muscles, particular muscles can enter only with great 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 heredity, 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 regular, 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 unmutilated 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 peripheral 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 normal 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