GENERAL PROPERTIES OF THE CEREBRUM. 693 



gray matter, which is external and follows the convolutions, is from ^ to 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, particularly in the posterior por- 

 tion 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. This, however, is a mere supposition, inca- 

 pable, as yet, of positive demonstration. 



The mode of connection between the cellular and the fibrous elements of the ner- 

 vous 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 cen- 

 tral 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 sub- 

 stance is subjected to direct irritation. 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 numer- 

 ous experiments upon pigeons, we have invariably noted complete insensibility and inex- 

 citability 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 ordi- 

 nary 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 impressions 

 conveyed to them by the sensory nerves and the stimulus generated by the nerve-cells, 

 without being capable of receiving 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 its anterior portion 

 responded to a feeble galvanic current. Each galvanization produced movements re- 

 stricted to particular sets of muscles; but it was difficult to say whether the contractions 

 were due to stimulation of the white or of the gray substance. Different centres for the 

 sets of muscles were accurately determined. The centre for the muscles of the neck 

 was located in the middle of the frontal convolution ; external to that, was a centre for 

 the extensor and adductor muscles of the forelegs ; and so on, other centres for sets of 

 muscles being found in the anterior portion of the hemispheres. By passing an inter- 

 rupted current through these parts, tetanus of particular muscles was produced. In 

 other observations, when the gray substance was removed at the points mentioned, there 

 was partial loss of power, but not paralysis, of the sets of muscles corresponding to the 

 centres operated upon. The authors regarded this as due to a loss of " muscular sense." 

 In these experiments the action was always crossed. It was also found that, after severe 

 haemorrhage, the excitability of the cerebrum quickly disappeared, which may account 



