NERVOUS CONTROL OF ANIMAL MOVEMENTS. 345 



pressions from without (excito-motor stimulation), or excitements from 

 the sensorial centres (sensori-motor stimulation). In the frog, for ex- 

 ample, 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 

 swimmino\ 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 withdrawn by taking the frog from the water."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 froo- remains motionless in the water when it encounters an obsta- 

 cle, 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, 



4 



The Eight Half of the Httman Encephalon. 



Encepkalon 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, connect- 

 ed with the other parts by fibres called peduncles; 3, medulla oblongata, or bulb, which is con- 

 tinuous 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 hemi- 

 spheres ; 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 en- 

 cephalon 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 cutane- 

 ous 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 



