646 MANUAL OF PHYSIOLOGY. 



LOCALIZATION OF THE CEREBRAL FUNCTIONS. 



Whether the surface of the hemispheres can be mapped out 

 into small areas, each of which is set apart for a definite duty, or 

 whether a comparatively restricted portion of the cortex suffices 

 for the performance of all the functions of the hemispheres, are 

 questions surrounded with difficulty, and which, up to the present, 

 cannot be answered with any degree of certainty. The experi- 

 mental evidence hitherto brought forward on the subject seems, 

 in many points, to be contradictory, a fact which may be ex- 

 plained partly by the difficulties with which such experiments 

 are beset, and partly by different observers being anxious to up- 

 hold with too great fervor either the localization or non-localiza- 

 tion theory in their entirety. 



The leading experimental experiences which have been recorded 

 are the following : 



1. Extensive tracts of the cortex of the hemispheres may be re- 

 moved, by accident or experiment, without interfering with the 

 cerebral functions in any marked or tangible way. Both men 

 and animals have lived for years, after the loss of a considerable 

 quantity of brain substance, without showing impairment of either 

 mental or bodily faculties. 



2. Lesion of a certain part of the frontal lobe of the left hemi- 

 sphere of man (posterior part of the third frontal convolution) 

 has been so frequently followed by the loss of the faculty of speech 

 aphasia that pathologists now call that spot the centre of 

 speech. 



3. Destruction of the convolutions around and in the neighbor- 

 hood of the fissure of Rolando gives rise to temporary loss of 

 power in the limbs of the other side, voluntary motion being abol- 

 ished when an extensive area is destroyed. This loss of power is 

 more obvious in animals with complex brains (man and monkey) 

 than in those less highly organized (dog, cat, rabbit), which rap- 

 idly recover. 



4. Destruction of the surface of the posterior lobes interferes 

 with the reception of visual impressions, and if an area including 



