NEUKO-MUSCULAK MECHANISM 119 



distant. Since the estimation of the size of an object depends 

 upon the judgment of its distance, the estimate we form of 

 the size of such objects as a range of hills is often erroneous. 

 When objects are near the eye, a special mechanism comes 

 into play to enable their distance to be determined (see p. 144). 

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

 a judgment based upon colour and shading. When a cube is 

 looked at, it appears solid because of the degrees of illumination 

 of the different sides degrees of illumination which may be 

 reproduced in a flat picture of such a cube. When the 

 object is near the eyes, by using the two eyes together a 

 means of determining solidity comes into action in such 

 animals as monkeys (see p. 144). 



B. Anatomy of the Eye 



Before attempting to study the physiology of the eye, the 

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

 himself familiar with the microscopic structure of the various 

 parts. 



The eye may be described as a hollow sphere of fibrous 

 tissue (fig. 52), the posterior part, the sclerotic (Scl.), being 

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

 and forming part of a sphere of smaller diameter than the 

 sclerotic. 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 (Oil. M.}, 

 running from behind forward and terminating abruptly in 

 front. In these the ciliary muscle is situated. It consists of 

 two sets of non-striped muscular fibres first, radiating fibres, 

 which take origin from the sclerotic just behind the corneo- 

 sclerotic junction, and run backwards and outwards to be 

 inserted with the bases of the ciliary processes ; second, cir- 

 cular fibres which run round the processes just inside the 

 radiating fibres. The choroid is continued forward in front of 

 the ciliary processes to the pupil as the iris, and in it are also 



