THE BRAIN. 429 



anatomical structure, which distinguishes it from other parts of the 

 hemispheres. In the investigations of Betz, already quoted (page 419), 

 it was found that in^the brain of the dog the motor region about the 

 frontal fissure was that which contained in its gray substance the 

 "giant pyramidal cells," similar in size to the cells of the anterior 

 horns in the spinal cord, and exclusively existing in the cortical layer 

 of this part of the brain. The microscopic structure of these convolu- 

 tions has therefore an individual character, corresponding with their 

 physiological properties. 



Secondly. Extirpation of the motor centres in the cerebral convolu- 

 tions produces more or less paralysis of voluntary motion on the oppo- 

 site side of the body. This has been shown in experiments on dogs 

 by Hitzig, Schiff, Hermann, and Carville and Duret.* 



The paralysis affects special movements, according to the particular 

 seat of the lesion ; and when a certain spot on the cerebral convolutions 

 has been found by stimulation to excite movements of flexion or exten- 

 sion ii> one of the opposite limbs, its extirpation causes paralysis of 

 the same movement. But the paralysis thus produced varies in extent 

 and duration in different animals. In the pigeon, removal of an entire 

 hemisphere hardly interferes with the acts of standing or locomotion. 

 In the dog, destruction of the motor centres on one side causes a par- 

 tial hemiplegia, which has a distinct effect on locomotion, but which 

 after some days or weeks gradually disappears, the animal recovering 

 his natural power of movement. In the monkey, according to Ferrier, 

 the hemiplegia from this cause is strongly marked; and shows no indi- 

 cation of amendment; while in man, according to numerous patho- 

 logical observations, it is absolutely complete and permanent. 



This difference is explained by supposing that in the lower animals 

 the movement of the limbs in locomotion is mainly confined to con- 

 sentaneous acts, in which direct volition takes a small share ; while 

 in the higher animals, and especially in man, the influence of imme- 

 diate volitional impulses is more essential, and preponderates in 

 importance, according to the number and variety of the muscular 

 actions. 



In man, the motor region of the cerebral hemispheres comprises in 

 general terms the convolutions about the fissure of Bolando, and espe- 

 cially the anterior and posterior central convolutions. It would hardly 

 be possible to assume this from the analogies of external configuration, 

 since the comparative size of the hemispheres and the proportion of 

 their various parts differ so widely in the dog's brain and that of man ; 

 but it is made certain by anatomical and experimental facts, as well as 

 by the result of observation in disease. In man it is the convolutions 

 surrounding the fissure of Rolando which present, like those of the 

 motor region in the dog's brain, the special structure characterized by 

 the presence of " giant pyramidal cells," which are not found elsewhere. 



* Archives de Physiologie. Paris, 1875, 2me se"rie, tome ii., p. 352. 



