10 



forward as the ciliary body, terminating in a ragged edge — 

 called the oi^a serraia. The retina is formed by the expan- 

 sion of the optic nerve ; the nervous elements are imbedded 

 in and spread over a fibrous frame-work. At the point of 

 entrance of the optic nerve is found, on the retina, a small 

 oval elevation, known as optic puinlla. From its centre 

 and its border emerge and radiate the blood vessels of the 

 retina. This disc or elevation is the only portion of the 

 retina where the sense of vision is wanting, and is, in con- 

 sequence, called the hlind spot. In the exact centre of the 

 retina posteriorly corresponding to the axis of the eye, is a 

 triangular yellow space called the macula liitea — the spot 

 where vision is most distinct and perfect. The extreme 

 complexity in the arrangement of the nervous elements of 

 the retina may be partially comprehended by the fact that 

 they are divided into ten different microscopic layers. These 

 various nervous elements receive the impression of the in- 

 verted image or picture of the object or objects in the field 

 of vision and the optic nerve conveys this impression or per- 

 ception to the brain. 



The humors or semiliquids of the eye are the 

 Agueous Humor, the Vitreous Humor and the Crystalline 

 Lens. 



The agueous humor is a watery liquid that is found in 

 the small chambers in front and behind the iris. It is 

 secreted by Descemet's membrane, which lines the chambers 

 containing the humor. This humor maintains the convexity 

 of the cornea, facilitates movements of the iris and the lens, 

 and, to some extent, assists in the refraction of the light 

 passing through it to the lens and the retina. 



If by surgical operation, accident or disease, this humor 

 is permitted to escape from the agueous chambers, it is rapidly 

 regenerated. 



The crystalline lens is a double convex, clear, semi-solid 

 body, and lies behind the pupil with its anterior surface 



