PARTS FOR THE PROTECTION OF THE EYEBALL. 723 



see objects and yet not be able to appreciate their significance. In the con- 

 dition known as word-blindness, words are seen, but they convey 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, apparently, psychical 

 centres, which elaborate the impressions received by the visual centres. 



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

 to the location and action of the visual centres is the following, which has 

 been adopted and formulated by Hun : 



1. In the lower half of the cuneus and the adjacent part of the median 

 occipito-temporal convolution, is the centre for simple, visual sensation. This 

 part is connected with fibres from homonymous halves of the retina of each 

 eye, the temporal half of the retina of the same side and the nasal half of the 

 retina of the opposite side. 



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 perception and 

 recognition, and for the production of visual memories." This may be called 

 the psychical, visual centre. Psychical blindness may exist, indeed, without 

 loss of visual sensation. 



3. The angular convolution is not a visual centre, as was claimed by Fer- 

 rier. It is related to visual perception only in so far as it affects "the 

 memories of the appearance of written or printed words." In cases of word- 

 blindness lesions have been found in this situation (Stirling). 



The situation of the visual centres, as indicated above, is in parts supplied 

 by the third branch of the posterior cerebral artery. 



Perception of Colors. Physical researches have shown that different colors 

 have different wave-lengths. It is evident that they are appreciated by the 

 visual centres, as distinct impressions for each color and shade of color, al- 

 though, under what may be called normal conditions, the delicacy of color- 

 perception varies in different individuals. Color-blindness is an abnormal 

 condition, in which the power of discrimination between different colors is 

 impaired or lost. Some persons are entirely insensible to colors ; and cases 

 have been reported in which one eye was color-blind, while the other eye was 

 normal (Becker and Hippel). The latter is called unilateral color-blindness. 



Before the cerebral visual centres had been described, various theories 

 were proposed to account for the perception of colors. Some physiologists 

 assumed the existence of separate and distinct elements in the retina for the 

 reception of impressions made by different colors ; but this and other theories 

 have been far from satisfactory. Cases of disease of the brain, in which ordi- 

 nary visual sensations remain but the sense of color is destroyed, seem to 

 show that a part of the visual centre is specially connected with the apprecia- 

 tion of colors. Beyond this, nothing is known of the mechanism of color- 

 perception. 



PARTS FOR THE PROTECTION OF THE EYEBALL. 



The orbit, formed by the union of certain of the bones of the face, re- 

 ceives the eyeball, the ocular muscles, the muscle of the upper lid, blood- ves- 



