516 HANDBOOK OF PHYSIOLOGY. 



not sufficiently felt to rouse the cerebrum to a comparison of the differ- 

 ence between mere ideas or memories and sensations derived from exter- 

 nal objects. 



The Motor Centres of the Cerebral Cortex. 



The experiments upon the brains of various animals by means of elec- 

 trical stimulation have demonstrated that there are definite regions of 

 the cerebral cortex the stimulation of which produces definite movements 

 of co-ordinated groups of muscle of the opposite side of the body. It 

 had long been well-known that the cerebral hemispheres could not be 

 excited by mechanical, chemical, or thermal stimuli, but Fritsch and 

 Hitzig were the first to show that they are amenable to electric irritation. 

 They employed a weak constant current in their experiments, applying 

 a pair of fine electrodes not more than -^ in. apart to different parts of 

 the cerebral cortex. The results thus obtained have been confirmed and 

 extended by Ferrier and many others. 



The fundamental phenomena observed in all these cases may be thus 

 epitomized: 



(1). Excitation of the same spot is always followed by the same move- 

 ment in the same animal. (2). The area of excitability for any given 

 movement is extremely small, and admits of very accurate definition. (3). 

 In different animals excitations of anatomically corresponding spots pro- 

 duce similar or corresponding results. 



The various definite movements resulting from the electric stimula- 

 tion of circumscribed areas of the cerebral cortex, are enumerated in the 

 description of the accompanying figures of the dog and monkey's brain. 



In the case of the dog, the results obtained are summed up as follows, 

 by Hitzig: 



(a). One portion (anterior) of the convexity of the cerebrum is motor; 

 another portion (posterior) is non-motor. (#). Electric stimulation of 

 the motor portion produces co-ordinated muscular contraction on the 

 opposite side of the body. (c). With very weak currents, the contrac- 

 tions produced are distinctly limited to particular groups of muscles; 

 with stronger currents the stimulus is communicated toother muscles of 

 the same or neighboring parts, (d). The portions of the brain inter- 

 vening between these motor centres are inexcitable by similar means. 



With regard to the facts above mentioned/all experimenters are agreed, 

 but there is still considerable diversity of opinion as to their explanation. 



In applying the facts ascertained by these experiments to elucidate 

 the physiology of the human brain, we must remember that the method 

 of electric stimulation is an artificial one, differing widely from the ordi- 

 nary stimuli to which the brain is subject during life. 



Effects of Stimulation of Various Regions of a Monkey's Brain. 



