852 



THE SENSE OF SIGHT 



other meridians until the boundaries of the entire visual field have been accurately 

 mapped out.'^ 



CUnically this instrument is employed for determining the seat of lesions of the 

 retina or of the optic tract and visual center. Obviously, any defect of the optic 

 path must give rise to a retinal area of indifference and hence, to a dark zone within 

 the visual field. Thus, it will be remembered that unilateral lesions of the occipital 

 cortex give rise to the condition of hemianopia or half-blindness of the retinae on 

 the corresponding side. If their right halves have in this way been rendered 

 functionally useless, the left halves of the visual fields are blotted out. Direct 

 vision, however, is retained, because each fovea centrahs is connected with both 



oooo 



Fig. 452. — The Perimeter. 



occipital centers. Very similar defects in the visual field follow injuries to the opti- 

 cal tract or to the retina itself. Thus, the occlusion or rupture of a terminal branch 

 of the retinal artery most generally leads to a uselessness of a circumscribed patch 

 of the retina with a corresponding defect in the visual field of this eye. This defect, 

 however, cannot become apparent unless the corresponding area of the opposite 

 retina has also been injured. Admittedly, the two retinae act in unison and com- 

 pensate for minor defects so long as the mjury remains confined to one of them. 

 This functional reciprocity has already been fully discussed in the paragraphs deal- 

 ing with the blind spot. It was then found that while a certain number of the rays 



1 Peter, Principles and Practice of Perimetry, New York, 1916. 



