THE PHYSIOLOGY OF THE BRAIN. 629 



and partly overlaps them, or rather is intermixed with them ; as 

 pointed out above, the lesion is not one of motor speech cen- 

 tres, but of the connection between these and other cerebral 

 areas in which have occurred changes accompanied by the 

 desire of verbal expression; something wrong probably in the 

 gray network. Very rarely aphasia has been known to follow 

 disease or injury of the corresponding convolution on the 

 right side; so that in it we have an example of a very definite 

 nexus between a limited area of the cortex and the expres- 

 sion of will through movements. Cases of recovery from 

 aphasia have occurred, but are extremely rare. In the ex- 

 ceptional cases it has been supposed that the right side of the 

 brain takes up the duty of connecting the material changes 

 in the gray network which accompany the origination of an 

 idea in one or more cortical areas, with the other changes 

 which result in speech. This view gains some support from 

 the fact that in certain cases of recovery due to left-side dis- 

 ease, subsequent disease in the third right frontal convolution 

 has been followed by a fresh aphasia. But however that may 

 be we have in aphasic persons definite evidence of the limita- 

 tion of definite function to a very limited area or areas of 

 the cerebral cortex. 



Much less is known as to other regions of the cortex than 

 of the motor area: most of them do not respond to electrical 

 stimulation at all, and those areas that do, only show it by 

 movements lacking in precision. We are reduced, therefore, 

 to observation on animals from whom certain cortical parts 

 have been removed, and to observations on diseased persons. 

 Certain broad regions have in this way been mapped out as 

 connected with certain main groups of sensations (Figs. 179, 

 180), probably rather with the combining and interpreting 

 of sensations, with their ideation, than with the mere raw 

 sensation itself. The latter is probably more dependent on 

 the lower brain centres; in most cases it is secondary changes 

 in these which lead to impulses which are passed on to excite 

 the cortical sensory areas. 



There is considerable evidence that removal or extensive 

 injury of the left occipital lobe causes blindness of the left 

 half of each retina, and vice versa. Also, that stimulation of 

 this region of the brain may cause movements of the eyes 

 and eyelids which have been described as such as an animal 

 would make if it thought it saw something, though obviously 



