THE HEMISPHERES. 487 



also so numerous that the physiological question cannot be regarded as 

 settled. We are only certain that aphasia is most frequently produced 

 by lesions occupying the left hemisphere, in the convolutions about 

 the bottom and edges of the fissure of Sylvius ; that is, in the parts 

 nourished by branches of the middle cerebral artery. 



Special Centres of Motion in the Cerebral Hemispheres. As a rule, 

 both the white and gray substance of the hemispheres are found to be 

 both insensible and inexcitable under the application of ordinary artifi- 

 cial stimulus ; neither sensation nor motion being produced in the living 

 animal by mechanical irritation or injury of these parts. In man, also, 

 it has been repeatedly observed that the substance of the brain, when 

 exposed by accident or disease, gives no indication of sensibility or of 

 motor endowments, if subjected to external irritation. 



More careful and extended observations, however, upon the lower 

 animals, have shown that under the influence of galvanic stimulus of a 

 low degree of intensity certain points on the surface of the cerebral con- 

 volutions will give rise to definite movements in the muscles of the head, 

 body, and limbs. The fact was first discovered by Fritsch and Hitzig 

 in 1870, 1 and has been subsequently fully confirmed by other observers. 

 The experiments were performed first and most frequently on dogs ; 

 afterward on cats, guinea pigs, rabbits, and monkeys ; the animals being 

 sometimes stupefied by ether, chloroform, or morphine, sometimes not 

 subjected to any anaesthetic influence. 



The general results derived from these experiments, as given by 

 Hitzig, are as follows : 



I. One portion of the convexity of the cerebrum, in the dog, is motor ; 

 another portion is not motor. 



II. The motor portion lies, in general terms, more anteriorly; the 

 non-motor portion more posteriorly. 



III. Electrical stimulation of the motor portion produces co-ordinated 

 muscular contraction on the opposite side of the body. 



IY. With very weak electric currents, the contractions produced are 

 distinctly limited to particular groups of muscles; with stronger cur- 

 rents, the stimulus is communicated to other muscles of the same or 

 neighboring parts. 



Y. The portions of the brain intervening between these motor centres 

 are inexcitable by similar means. 



These conclusions have been verified by a variety of observations in 

 Germany, France, England, and the United States.' 2 The experiments, 

 which are most readily performed on dogs, owing to the large size of 

 the cerebrum, and the comparatively little injury suffered from hemor- 



1 Archiv fur Anatomie, Physiologic und Wissenschaftliche Medicin. Leipzig, 

 1870, p. 300. Hitzig, Untersuchungen liber das Gehirn. Berlin, 1874. 



2 Report of a Committee of the New York Society of Neurology and Electro- 

 logy on the Existence and Localization of Motor Centres in the Cerebral Con- 

 volutions. New York Medical Journal, March, 1875, p. 225. 



