Page:Popular Science Monthly Volume 1.djvu/358

346 the water, and if we set the fore-foot in motion the hind-foot immediately begins to move, and the animal swims. In a word, to speak generally, when an animal is deprived of its cerebral lobes, if one limb begins to move the others immediately follow; if one comes to rest, the others tend to cease moving. Very seldom in these animals is the movement limited to a single member. This solidarity in the movements distinguishes animals deprived of the cerebral lobes, not only from animals with a brain, but also from those in which the spinal cord is severed at top.

In a frog with the cord cut near the cranial bulb, if we move a member, it produces no effect on the movements of the animal. If we excite one foot, only the subjacent muscles contract. If the impulse is stronger, the entire foot recedes, but the rest of the body is motionless. A lively excitation is needed to put the other feet in motion. In a word, each excitation, according to its energy, produces more or less extensive movement which may be limited to a single group of muscles. It is not so with a frog in which the spinal cord is unsevered; the movements which succeed a stimulus, whether it be strong or feeble, are always movements of the whole body.

If we put a drop of vinegar on the foot of a frog in which the cord is separated from the brain, the foot retires at first, then the other foot makes coordinated movements to get rid of the cause of irritation. The frog which has lost only its cerebral lobes commences, on the contrary, to make many leaps; afterward it moves only one or other of its feet. In the frog where the cord is severed, to each excitation succeed muscular contractions; these are always in proportion to the energy of the excitation. In the frog with the cord joined to the cerebellum alone, the excitation can take place without producing movement; but, be it feeble or strong, from the moment reflex action begins the result is the same—a movement of the body which produces a leap.

According to the excitation, to the kind of impression produced on the sensitive nerves of the skin, and on the nerves of muscular sense, there is formed among the different regions of the nervous centres a common purpose, which has for its regulator the pons Varolii (bridge of Varoli) and the cerebellum.

Another interesting effect now claims our notice. Among animals deprived of the cerebral lobes there is another curious and constant phenomenon, the forced and continual tendency to maintain an equilibrium. We have seen in the frog, the carp, the eel, the pigeon, the goose, mammals, etc., that every time we disturbed their centre of gravity, immediately there took place a series of coordinated movements which have the single aim of restoring the equilibrium. A decapitated insect remains always firmly posed on his feet and can take no other position. If a frog is motionless on a piece of board, and you slowly lower the board in the water so that he is immersed, in most