158 CEREBRAL LOCALIZATION 



VISION. There is abundant evidence that visual 

 sensations are received on the mesial surfaces of the 

 occipital lobes, just above and just below the 

 calcarine fissure. Histologically, the area is mapped 

 out by the white line of Gennari ; it barely encroaches 

 posteriorly on the convexity of the hemisphere. 



The left half of each retina is represented in the 

 left cerebral cortex, and the right half of each retina 

 in the right cortex. The fovea centralis of each eye 

 has a bilateral representation. The upper half of each 

 retina is projected above the calcarine fissure ; the 

 lower half of each retina below the fissure. Therefore 

 a tumour of the left cortex above the calcarine 

 fissure would render the upper left quadrant of each 

 eye psychically blind, and the patient would be 

 unable to see objects downwards and to his right. 



A smaller lesion, however, does not produce a 

 smaller patch of blindness ; it merely reduces the 

 visual acuity over the whole of the corresponding 

 quadrant. 



CUTANEOUS SENSATION. There is some evidence 

 that the cortical representation of heat, cold, pain, 

 and tactile sense is identical. Possibly muscular 

 sense has a somewhat different location. 



It is universally agreed that the main receiving 

 platform for cutaneous sensation is situated in the 

 postcentral (ascending parietal) gyms, just behind the 

 fissure of Rolando, and that the general arrangement 

 is the same as that of the motor centres ; thus, the 

 sensory centre for the leg is nearest the vertex, 

 opposite the origin of the pyramidal fibres for the 

 leg ; next come the arm centres, and lowest of all 

 those for the face and head. 



