THE NERVOUS SYSTEM 181 



without loss of muscular power, and thus the conclusion has 

 been drawn that the occipital lobe is the receiving mechanism 

 for stimuli from the eyes. 



Visual Centre. The way in which the fibres, coming from 

 the two retinae, are connected with each thalamus opticus and 

 occipital lobe has been already considered, and it has been 

 shown that the optic tract passes into the geniculate bodies 

 on the posterior aspect of the thalamus, and that a strong band 

 of fibres, called the optic radiation, extends from these back- 

 wards to the occipital lobes (Fig. 66, p. 127). An extensive 

 lesion of one say the right occipital lobe, especially if on 

 the inner aspect in the region of the cuneate lobe, is accom- 

 panied by no loss of muscular power but by blindness for all 

 objects in the opposite side of the field of vision i.e. the right 

 side of each retina is blind. The central spot of neither eye 

 is completely blinded because the fibres from the macula 

 lutea only partially decussate at the chiasma. Probably each 

 part of the occipital lobe is connected with definite parts of 

 the two retinse. Certain it is that there is no part of this 

 cortical mechanism connected with the blind spot, and hence 

 this is not perceived in ordinary vision. 



Colour perception seems to be a less fundamental function 

 of the visual apparatus than perception of the degree and 

 direction of illumination, and hence colour perception may be 

 lost in less extensive lesions of the occipital lobes without the 

 other functions being impaired. One of the most recently 

 acquired functions of the mechanism is the power of appre- 

 ciating the significance of the signs used in written language, 

 and it is found that in small and superficial lesions this 

 function may alone be lost. 



The cortex of the occipital lobe is rich in the smaller variety 

 of cells and poor in the larger pyramidal cells, which are found 

 in areas connected with the production of movements. 



While there is good evidence that a special localised area 

 of the cortex is connected with the reception of stimuli from 

 the eyes, the evidence of the existence of similar areas or 

 centres connected with the other organs of special sense is 

 by no means satisfactory. 



Auditory Centre. Ferrier, by removing the superior 



