THE CEREBRAL HEMISPHERES. 693 



around the posterior extremity of the fissure of Sylvius, is the supra-Sylvian convolution 

 (6, 6, 6), which is continuous with the first temporal convolution (7, 7, 7), the latter lying 

 behind the fissure of Sylvius and parallel with it. External to the posterior portion of 

 the parietal fissure, is the angular convolution (8, 8, 8), which is continuous with the mid- 

 dle temporal convolution (9, 9, 9). At the inferior border of the temporal lobe, is the 

 third temporal convolution (10). The upper parietal convolution (11, 11) lies by the side 

 of the median fissure and is the posterior continuation of the first frontal convolution. 

 12, 12, 12 in the diagram indicates the situation of the occipital convolutions. In addi- 

 tion to these convolutions upon the general surface of the cerebrum, there are convolu- 

 tions on the surface of the base of the brain and in the gray matter of the sides of the 

 great median fissure. In the fissure of Sylvius, near its ascending branch, between the 

 anterior and the posterior lobes of the brain, and beneath the third frontal convolution, 

 is a group of convolutions constituting the island of Reil. 1 



The gray matter of the cerebrum, which is external and follows the convolutions, is 

 from T ^ to i of an inch in thickness. Writers have described this substance as existing 

 in several layers, but this division is mainly artificial. In certain parts, however, par- 

 ticularly in the posterior portion of the cerebrum, the gray substance is quite distinctly 

 divided into two layers, by a very delicate intermediate layer of a whitish color. 



There is a marked difference in the appearance of the cells in the most superficial and 

 in the deepest portions of the gray -substance. The superficial cells are small and present 

 a net-work of delicate, anastomosing fibres, resembling the cells of the posterior cornua 

 of the gray substance of the cord ; while the deepest cells are large and resemble the so- 

 called motor cells of the cord. Between these two extremes, in the intermediate layers, 

 there is a gradual transition in the size of the cells. This anatomical fact points to the 

 possibility of distinct functions of the cells belonging to the superficial and the deep layers ; 

 viz., that the larger cells are for the generation of the motor stimulus, while the smaller 

 are for the reception of sensory impressions. 



The mode of connection between the cellular and the fibrous elements of the nervous 

 system has already been considered and does not demand farther mention. We shall also 

 pass over the amorphous matter, nuclei, myelocytes, etc., found in the central nervous 

 matter, as these points possess little or no physiological interest. 



General Properties of the Cerebrum. By the general properties of the cerebrum, we 

 mean the effect, or the absence of effect, observed when the gray or white substance is 

 subjected to direct stimulation. While some of the older writers state that the brain is 

 both irritable and sensible, nearly all authorities, up to a very recent date, have been 

 agreed that direct stimulation of the white or the gray substance of the greatest part 

 of the brain produces neither pain nor convulsive movements. In a number of experi- 

 ments upon pigeons, we have invariably noted complete insensibility and inexcitability 

 of both the gray and the white substance of the cerebral hemispheres. The generally- 

 accepted view has been that a great part of the substance of the cerebrum is neither 

 excitable nor sensible, in the sense in which these terms are applied to the ordinary 

 mixed nerves. There can be no doubt with regard to the conducting properties of 

 the white matter of the brain, but the nerve-fibres here seem to conduct sensory im- 

 pressions and the stimulus generated by the nerve-cells, without being capable of receiv- 

 ing or conducting artificial impressions applied directly to their substance. 



We have said that a great part of the cerebral substance seems to be neither excitable 

 nor sensible to direct stimulation ; but we must make an exception in favor of certain 

 portions of the cerebrum, which have lately been shown to possess excitability, their 

 action being confined to particular sets of muscles. In 1870, Fritsch and Hitzig, expos- 

 ing the cerebral hemispheres in dogs, found that certain parts of their anterior portion 



1 Our sketch of the fissures and convolutions of the brain is taken mainly from the description given by Dalton in 

 hia Treatise on Human Physiology, Philadelphia, 1875, p. 472, et seq. 



