FUNCTIONS OF THE CEREBRAL HEMISPHERES 447 



VISUAL IMPRESSIONS 



Each optic tract, carrying impulses arising as a result of events occurring 

 in the opposite field of vision, ends in the pulvinar of the optic thalamus, the 

 external geniculate body, and the superior corpora quadrigemina. The last 

 named is apparently not concerned in vision, but represents a centre for 

 the co-ordination of visual impressions with those from other regions of the 

 body in influencing bodily movements. From the pulvinar and external 

 geniculate body arises a sheaf of fibres, which pass through the extreme hinder 

 end of the posterior limb of the internal capsule and diverge in the centrum 

 ovak to be distributed to the occipital lobes, being here known as the optic 

 radiations. The anatomical connection of the occipital lobes with vision 

 is confirmed by evidence derived from experiment. Movements of the 

 eyes result from stimulation of almost any part of this lobe. If the upper 

 surface of the right occipital lobe be stimulated, both eyes move downwards 

 and towards the left. Excitation of the posterior part causes movement 

 of the eyes up and to the left ; while between these two parts there is an 

 intermediate zone, most marked on the mesial surface, stimulation of which 

 evokes a purely lateral deviation of the eyes to the left. It is therefore con- 

 cluded not only that there 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. 232). 



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 central vision is usually retained in both eyes after 

 destruction of one occipital lobe (Fig. 233). 



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 sur- 

 faces 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 audi- 

 tory 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 oppsite side 

 to form the tract known as the lateral fillet. The fibres of this tract end 

 partly in the inferior corpora quadrigemina, partly in the internal geniculate 

 body. From the latter, fibres pass into the internal capsule, and thence as 

 ' auditory radiations ' directly to the superior temporal convolution. 



In the monkey stimulation of the upper two-thirds of this lobe of the 



