FUNCTIONS OF THE CEREBRAL HEMISPHERES 447 



spacial relations, (6) appreciation of .intensity of stimuli, and (c) apprecia- 

 tion of similarity and difference in external objects which are brought into 

 contact with the surface of the body. 



It is not surprising therefore that in such cases the answers of the patient, 

 when his sensibility is tested, seem to be confused, and it is this confusion 

 of judgment which is more apparent than definite loss of sensibility. With 

 regard to sensory localisation, it should be noted that the functions rather 

 than the anatomical relations of any one part of the body, are represented 

 on the cortex ; hence, as in the motor functions of the brain, those portions, 

 such as the hand, which are endowed with thft highest powers of discrimina- 

 tive sensibility, are most extensively represented, and next in order comes 

 the sole of the foot. Thus, after a cortical lesion, sensibility of the hand and 

 foot may be disturbed without there being any alteration in that of the elbow, 

 shoulder or knee. 



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 

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

 radiations. The anatomical connexion 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 f ovea 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- 



