Page:Popular Science Monthly Volume 1.djvu/357
NERVOUS CONTROL OF ANIMAL MOVEMENTS.
pressions from without (excito-motor stimulation), or excitements from the sensorial centres (sensori-motor stimulation). In the frog, for example, the contact of the body with the earth makes him take his normal attitude, and when it is put in the water, says Vulpian, "the liquid produces a particular stimulation of all the surface of the body in contact with it; this stimulus calls into play the mechanism of swimming and this mechanism ceases to move when the stimulus is withdrawn by taking the frog from the water."
The explanation of Vulpian is exact only within certain limits, for the frog remains motionless in the water when it encounters an obstacle, even when the stimulus of the water on its body is kept up; and, on the other hand, the surface of the pigeon's body is stimulated in the same way by the air, whether the wings are open or shut, and yet it is obliged to fly when it loses its point of support. There are,
then, other causes of stimulation besides the impression upon cutaneous nerves. These are—first, the combination or solidarity of the movements which exist among animals deprived of the cerebral lobes; and, second, the necessity of maintaining an equilibrium.
What do we mean by solidarity of movement? When a brainless frog is swimming, and we apply a solid body to one of his fore-feet, immediately the corresponding hind-foot bends and touches the body in contact with the fore-foot. It is the same if we stop the motion of the fore-foot. Reciprocally, if the frog is motionless on the surface of