Page:Popular Science Monthly Volume 1.djvu/357

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with it; this stimulus calls into play the mechanism of swimming and this mechanism ceases to move when the stimulus is withdrawn by taking the frog from the water."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,

Fig. 1.

The Right Half of the Human Encephalon

Encephalon is the term applied to the entire nervous mass within the head the brain with all its parts. 1,1, 1, cerebrum, cerebral lobes, or hemispheres. In man, this part is large; in lower animals, much smaller; in the lowest, it is extremely small, or rudimentary. 2. cerebellum, or lesser brain, connected with the other parts by fibres called peduncles; 3, medulla oblongata, or bulb, which is continuous downward, as (4) the spinal cord; 5, the pons Varolii (bridge of Varoli), a mass of cross-fibres which connect the two lobes of the cerebellum; C, 6, 6, represent the great commissure, a body of cross-fibres which connects the two hemispheres and unifies the action of the brain. The lower portion of the brain consists of ganglia or centres of influence, connected with sensation, motion, and the vital processes. In man. these parts are relatively small, and are all covered in by the hemispheres; in inferior animals, like fishes and reptiles, they form the chief portion of the brain. The reader will remember, in the following experiments, that the nervous fibres, going from the head to the body, decussate or cross each other at the medulla oblongata, so that the right side of the encephalon is in relation with the left side of the body, while the right side of the body is controlled by the left side of the brain.

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