504 



PHYSIOLOGY 



LE.FT RETINA 



RIGHT RETINA 



to be distributed to the central convolutions. Their area of distribution 

 is, however, much wider than the area of origin of the pyramidal fibres. 

 We may therefore conclude that tactile and muscular sensibility are 

 chiefly subserved by the central convolutions, including the motor 

 area, but are especially dependent on the integrity of the post-central 

 gyrus. Flechsig has shown that fibres from the thalamus, which may 

 probably be regarded as continuations of the fillet system, are also 



distributed to other portions 

 of the cortex, i.e. the temporal, 

 the frontal, and the occipital 

 lobes. It is therefore not 

 surprising that the hemi- 

 ansesthesia produced by lesions 

 in the central convolutions is 

 rarely or never complete. 



VISUAL IMPRESSIONS 

 Each optic tract, carrying 

 impulses arising as a result of 

 events occurring in the oppo- 

 site field of vision, ends in the 

 pulvinar of the optic thalamus, 

 the external geniculate body, 

 and the superior corpora quad- 

 rigemina. 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 



and the visual area of the cortex. (SCHAFER. ) 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 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 concluded not only that there 



FIG. 230. Diagram showing the probable 

 relations between the parts of the retinse 



