FUNCTIONS OF THE CEREBRAL HEMISPHERES 505 



is representation of visual impressions in the occipital lobes, but that 

 there is a certain amount of localisation within the visual area itself, 

 as is represented in the diagram (Fig. 230). 



These conclusions are fully borne out by the results of ablation. 

 While extirpation of the whole occipital lobe on one side in animals 

 causes crossed hemianopia, i.e. has the same effect as division of 

 the corresponding optic tract, extirpation of these lobes on both 

 sides causes complete blindness. It seems that the fovea centralis 

 the region of distinct vision is bilaterally represented, so that 



FIG. 231. Perimeter charts from right and left eye, showing the limitation of the 

 field of vision (right hemianopia) produced by a lesion of the left occipital cortex. 

 (BECHTEREW.) 



central vision is usually retained in both eyes after destruction of one 

 occipital lobe (Fig. 231). 



The area connected with vision seems to be smaller in man than 

 in the ape. and in the ape than in the dog. Thus in man complete 

 blindness has been observed as the result of localised bilateral lesions 

 of the internal surfaces of the occipital lobes, and we find the same 

 relative limitation of area as we proceed from lower to higher forms 

 in the case of the other sensory areas of the cortex. 



THE AUDITORY AREA 



Anatomical study indicates a connection of auditory sensations 

 with the superior temporal lobe. The impulses, started by the arrival 

 of sound waves at the ear, travel by the cochlear nerve to the medulla. 

 From the two auditory nuclei a well-marked set of fibres passes 

 across to the opposite side in the corpus trapezoides, then turns up 

 into the tegmentum of the opposite side to form the tract known 



