668 THE CEREBRUM 



cortex. In the rabbit, for example, the poljnnorphous layer displays a 

 thickness three times greater than that of the pyramidal layer, whereas 

 in man just the reverse relationship exists. The inference to be drawn 

 from this is that the pyramidal cells are the association units of the 

 brain, excepting, of course, the cells of Betz which are motor in their 

 function, while the polymorphous elements are concerned with the lower 

 types of function. By exclusion, we may then assign a sensory func- 

 tion to the constituents of the granular layer. 



This analysis should also take into account that the ^'psychic" 

 brain of man exhibits certain minor differences in regard to the relative 

 size and complexity of its different association areas. One or the other 

 of these may be more highly developed with the result that the mechan- 

 ism of which the area so favored forms a part, possesses a greater 

 functional adaptability. In other words, it frequently happens that 

 these association centers are not evenly balanced. It need scarcely 

 be emphasized that such differences may also be displayed by one and 

 the same association area belonging to the brains of different indi- 

 viduals, i.e., one or the other person may excel in certain motor or sen- 

 sory actions. 



Removal of the Cerebrum. — The preceding discussion may well 

 be amplified by a study of the behavior of animals which have suffered 

 a partial or complete loss of the cerebral hemispheres by disease or 

 surgical operation. While the symptoms appearing subsequent to the 

 latter procedure vary somewhat in different animals, they present 

 nevertheless the same general characteristics. The essence of these 

 changes is that an animal, the cerebrum of which has been removed, is 

 devoid of associations. Its psychic life, whether simple or complex, 

 has been destroyed. It has been converted into a simple reflex ma- 

 chine. This fact will be brought out more clearly by a brief considera- 

 tion of the functional capabilities of decerebrated fish, amphibia and 

 reptilia. These animals are selected for this purpose partly because 

 their cerebrum is sufficiently compact and easy of access to permit of 

 its quick removal, and partly because the positive results following 

 this operation are so few that they do not overshadow the principal 

 effect briefly alluded to above. As this operation is performed under 

 ether, these animals should, of course, be permitted to fully recover 

 before they are studied. 



Emphasis should be placed upon the fact that the loss of the 

 cerebrum destroys the sensorium. The decerebrated bony fish (shark) ' 

 shows the same power and manner of movement as a perfectly normal 

 animal. It tends, however, to be more inactive, assuming a rather 

 continuous position of rest which is changed to one of activity only 

 upon stimulation. But when made to move, its motor reactions show 

 no deviation from normal. More decided defects, however, appear 

 when the lesion is extended to the midbrain, because the animal then 

 is rendered blind and loses its sense of equilibrium. 



^ Bandelot, Ann. d. sc. nat., 105, 1864. 



