CEREBRAL LOCALIZATION 845 



called the indifferent electrode and consisting of a flat plate, being 

 placed on some other part of the body, such as the skin of the back. This 

 unipolar method gives much finer results than when the ordinary bipolar 

 electrodes are employed. 



Before we describe the results which have been obtained by the use 

 of this method, a question arises which it may be well to consider 

 briefly; namely, how do we know that the electric current is really stimu- 

 lating the center present in the gray matter of the cortex, and not 

 the numerous nerve fibers that constitute the white matter of the brain 

 and along which, between the two electrodes, it is plain some of the 

 electric current must pass? The evidence that we are really stimulating 

 centers is as follows: (1) The latent period for a response produced by 

 stimulating the centers is much longer than that which follows upon di- 

 rect stimulation of the white matter. (2) Under deep narcosis, as that 

 produced by chloral or morphine, the effect of stimulation of the gray 

 matter is greatbr delayed and altered in type ; whereas stimulation of the 

 white matter gives the usual response. (3) A weaker current suffices to 

 stimulate the gray matter than that required for the exposed white 

 matter. 



In order to demonstrate the movements which follow stimulation of 

 the cerebral cortex, it is necessary, as will be inferred from the pre- 

 ceding remarks, that the animal be not too deeply anesthetized. Fur- 

 thermore, it is necessary to be very careful in adjusting the strength 

 of stimulation employed, for the results vary considerably accordingly. 

 When the stimulus is of the proper intensity, the movements are located 

 in some particular group of muscles, for example, those of the thumb 

 or of the hand, whereas, if the stimulation is strong, the movements 

 spread over much larger areas. As a result of feeble or moderate stimu- 

 lation, it is found that the muscles which move are those of the opposite 

 side of the body, and that the localization is finer the higher the position 

 of the animal in the scale of development. The movements are perfectly 

 coordinate and purposeful in character, and reciprocal innervation is 

 evident. 



There is, however, a marked difference in the reactions obtained by 

 stimulation of the motor cortex and those obtained by eliciting spinal 

 reflexes. For example, the movements produced by stimulation of the 

 cortex are much more readily modified by slight variations in the con- 

 dition of the animal, the blood supply, the degree of narcosis, etc., than 

 are those elicited by stimulation of receptor neurons. A careful study 

 of this difference has been made in recent years by Brown and Sher- 

 rington. 13 They observed the behavior of two antagonistic muscles 

 acting on the elboAV when the respective cortical centers were stimu- 



