NERVE 139 



stimulated by the light coming from the object. If a large 

 surface is acted upon, the object seems large ; if a small 

 surface, the object seems small. But the extent of eye-cells 

 acted on depends not merely upon the size of the object, but 

 also upon its distance from the eye, since the further the 

 object is from the eye the smaller is the image formed. 

 Hence, ideas of size are judgments based upon the size of 

 the picture in the eye, and the estimation of the distance 

 of the object and past experience plays an important part. 



The idea of thickness or contour of an object is also largely 

 a judgment based upon colour and shading. AVhen a cube 

 is looked at, it is judged to be a cube because of the 

 different deo-rees of illumination of the different sides — 

 degrees of illumination which may be reproduced in a flat 

 picture of such a cube. 



B. Anatomy of the Eye. 



Before attempting to study the i^hysiology of the eye, the 

 student inust dissect an ox's or a pig^s eye, and then make 

 himself faviiliar with the microscopic structure of the 

 various parts. 



The eye may be described as a hollow sphere of fibrous 

 tissue (fig. 62), the posterior part, the sclerotic (ScL), being 

 opaque ; the anterior part, the cornea (Cor.), being trans- 

 parent and forming in most animals part of a sphere of 

 smaller diameter than the sclerotic. In the horse the curva- 

 ture of the cornea is less in the horizontal plane than in 

 the vertical. Inside the sclerotic coat is a loose fibrous 

 layer, the choroid {Chor,), the connective tissue cells of 

 which are loaded with melanin, a black pigment. This is 

 the vascular coat of the eye — the larger vessels running in 

 its outer part, and the capillaries in its inner layer. 

 Anteriorly, just behind the junction of the cornea and 

 sclerotic, it is thickened and raised in a number of ridges, 

 the ciliary processes (Cil.M.), running from behind forward 

 and terminating abruptly in front. The ciliary muscle is 

 situated in these. It consists of two sets of non-striped 

 muscular fibres — first, radiating fibres which take origin 



