Page:Popular Science Monthly Volume 1.djvu/366

354 tonic contraction of the muscular groups corresponding to the wounded side. The nearer the lesion is situated to the bulb, the more pronounced are the phenomena, and the more the frog leans to one side.

In geese and ducks, on pricking or cutting the peduncles of the cerebellum, we can well observe phenomena analogous to these seen in frogs. When we remove the cerebellum of the two sides, there is no movement of rotation, but the animal plunges deeply into the water. In the uninjured duck, for example, the thorax penetrates the water but a little way; the duck, deprived of its cerebellum, penetrates it twice as far. The duck, represented at Fig. 6, is wounded in the right side of the cerebellum, and the animal has a movement of rotation toward the left. side.

We think these phenomena are due to an irritation of the locomotive centres, and, with Brown-Séquard, we believe that the lesions of certain points of the encephalon engender a state of irritation, whence arises, whether directly or by reflex action, a tonic contraction of certain muscular groups, and chiefly of muscles of the thorax.

It is very easy to account for this influence of the muscles of the thorax in producing the movements of swimming. This easy experiment shows us, at the same time, that simple paralysis, or the loss of function of one side, does not occasion the movements of rotation. If we swim with one arm and one leg of the same side, we do not deviate, but advance in a right line; but, if we contract the muscles of the thorax of one side, at once we lean in the water on that side. If we increase this contraction, we draw over a little more in the same direction, until we come to lie completely on one side, and at this moment there supervenes a movement of rotation, almost instinctive, which makes us spasmodically take the normal position. It is something like this which occurs in animals that have movements of rolling.

By encephalic lesion, animals are led to lean strongly to one side, an attitude which they take even in repose. At the least movement, they are drawn over upon the back; at once, spasmodically, all the limbs concur in the effort to regain the former state; but, as soon as taken, the compulsion of the injured side is reproduced, and, as the animal has already acquired motion, this carries it beyond the normal attitude, and leads it on to its side and back; immediately it seeks to get on its feet, rises, is again drawn over on one side, and so on. When the cerebral lobes are removed, animals cannot remain lying on the back. They always seek to recover their normal attitude, and, consequently, when they have deviated from it, all their efforts go to recover equilibrium. It is at this moment that the four limbs concur to execute the gyratory movement. There are two factors in the motion of rotation: the first is the contraction of the muscles of the injured side which makes the animal lean over and bear down on that side; the second is the cooperation of all the members, as soon as the animal is reversed, to produce a half-revolution and recover the normal forced