ANATOMY OF THE EYE 103 



Another form of cyst is sometimes met with in the ovary and also in 

 tlie testicle. This is known as a dermoid cyst, so called on account of its 



. A, Cysts or Cavities from 



being formed out of the elements of skin and containing one or more skin 

 products, such as hairs, fat, teeth, &c. 



These growths are considered to arise out of some error in the develop- 

 ment of the animal in which they occur, and are essentially congenital. 



There are no symptoms which will rightly guide us in a diagnosis of 

 these affections. They seldom disturl) the general health and rarely 

 occasion death. 



11. THE EYE 



ANATOxMY OF THE EYE 



The eye is an instrument by which light, colour, form, and movement 

 are recognized, and by which, combined with other faculties, we acquire 

 a knowledge of distance, relation, position, and size of objects. 



It occupies the fore or outer part of the cavity named the orbit, the 

 bones of which form a very efKcient means of protection to it against 

 injury. It re,sts on a soft bed of fat, which enables it to yield to direct 

 blows, and it is further protected by a retractor muscle, which with- 

 draws it under cover of the bones, whilst the eyelids and the membrana 

 nictitans (haw) cover it in front. It is consequently only rarely injured. 

 The membrana nictitans, or haw, is a triangular piece of cartilage which 

 gradually becomes thinner from back to front. It is situated in the inner 



