882 CENTRAL NERVOUS SYSTEM 



out, arise in the nasal half of each retina. Complete destruction of 

 one of the visual tracts between the chiasm and the sensory area of the 

 cortex produces a condition called homonymous hemianopsia, or blindness 

 in the same half of the visual field of both eyes. Thus destruction of 

 this part of the left optic tract which contains fibers from the left half 

 of both eyes will produce blindness for the right half of their visual field. 

 The visual area of the cortex is located about the calcarine fissure on 

 the median and posterior surface of the parietal lobes. Its area is 

 sufficiently great to make it probable that a lesion will involve only cer- 

 tain parts of it, and as a result produce blindness in only part of the 

 visual field. By correlating the position of lesions with the resulting 

 loss of vision it has been possible for Holmes and Lister 10 to determine 

 what part of the visual area corresponds with each part of the retina. 

 They found that the center of each retina, or macula, is represented in 

 the posterior part of the visual area. The superior quadrant of each 

 eye is represented in the upper, and the inferior quadrant in the lower 

 half of the visual area anterior to the center for macular vision. It 

 has previously been held that the center of each retina was represented 

 in the visual area of both sides of the brain. This was because in the 

 lesions of civil life homonymous hemianopsia rarely affected the center 

 of the visual field in the blind half of the eyes. Holmes and Lister 

 found, however, in cases where the visual area of one occipital lobe 

 was completely destroyed that vision was lost in the central part of the 

 corresponding half of the retina quite as completely as in the more 

 peripheral portions: They conclude consequently that the macula, like 

 the rest of the retina, is not represented bilaterally in the cortex. The 

 discrepancy in the condition, as they observed it, in wounded soldiers, 

 and as it occurs in civil life, depends upon the fact that in the latter 

 case the cause of the lesion is usually a disturbance of the circulation 

 of the cortex as the result of thrombosis, hemorrhage, etc. The macular 

 part of the visual area is supplied with capillaries from two sets of ar- 

 teries, those of the median and of the lateral surface of the occipital 

 lobe. An occlusion of the median supply would destroy the visual area 

 for the peripheral half of the retina, but would leave intact the macular 

 region which would be sufficiently nourished by the blood from the 

 lateral arteries. 



Riddoch 11 has noted that in case of injury to the occipital lobe, disso- 

 ciations in visual sensations occur which are quite comparable to the dis- 

 sociations which may occur in lesions of the sensory cortex for cutaneous 

 and deep sensibility. These manifest themselves in the case of patients 

 who are recovering from a functional disturbance of the visual areas. 

 The first visual perception to appear is the recognition of the movement 

 of an object in the visual field, which occurs long before the object as 



