CENTRES FOR VISION 703 



CENTRES FOR VISION 



Experiments have been made on the lower animals by Ferrier, 

 Munk, Exner and many others, with the object of locating in the 

 cerebrum a centre for vision. It is important, however, to compare 

 the results of such experiments with cases of cerebral lesions in the 

 human subject. As the general result of experiments, both on dogs 

 and monkeys, and of pathological observations, the present opinion is 

 that the centres for vision are in the occipital lobes. The convolutions 

 immediately above and below the calcarine fissure, on the mesial surface 

 of the cerebrum (Figs. 134 and 140, pages 566 and 570), seem to be the 

 cerebral terminations of fibres that are continuous with the optic tracts. 

 These fibres are not crossed in the cerebrum, but the conductors decus- 

 sate at the optic chiasm as they pass to the eyes. Cases have been 

 observed in the human subject in which lesion of these parts on one 

 side has been followed by loss of vision in one lateral half of the retina 

 in either eye. This condition is called hemianopsia. In these instances 

 the blindness is confined to the temporal side of the retina correspond- 

 ing to the lesion and the nasal side of the retina of the opposite eye. 

 This is called lateral homonymous hemianopsia, and this is the form 

 that occurs in unilateral cerebral lesion. In dogs and in monkeys, de- 

 struction of both occipital lobes and both angular convolutions produces 

 total and permanent blindness of both eyes. 



The complete and perfect perception of visual impressions involves 

 intellectual action connected with the simple visual sense. An individ- 

 ual may see objects and yet not be able to appreciate their significance. 

 In the condition known as word-blindness, words are seen, but they con- 

 vey no idea. A dog with part of the occipital lobes removed may see 

 objects such as food, but does not recognize their character. There are, 

 indeed, psychical centres, which elaborate the impressions received by 

 the simple visual centres. 



What seems at present to be the most rational view to take in regard 

 to the location and action of the visual centres is the following : 



1. The centre for simple visual impressions is on the inner surface 

 of the cerebrum, on either side of the calcarine fissure, between the 

 cuneus and the lobulus lingualis. This part is connected with homony- 

 mous halves of the retina of either eye the temporal half of the retina 

 of the same side and the nasal half of the retina of the opposite side. 

 The part above the calcarine fissure is connected with the upper portion 

 of the retina, and the part below, with the lower portion of the retina. 



2. The action of the cortex of the convex surface of the temporal 

 lobe perhaps only on the left side is necessary for full visual per- 



