152 PRACTICAL ORxMTIIOLOGY. 



We have still to examine the fluids or humours of the eye, 

 of which there are three. 



The vitreous Jmmour, which fills the space behind the lens and 

 ciliary zone, is a transparent, somewhat gelatinous watery fluid, 

 enclosed in a membrane, named the hyaloid, and intersected by 

 filmy laminae or cellules, so that an incision into the mem- 

 brane does not cause the whole of the fluid to escape. The 

 pecten projects into the midst of this humour, generally termi- 

 nating somewhat behind the lens, but sometimes reaching it. 



The chry stall ine humour is that which, with its capsule, con- 

 stitutes the lens. This body is of a round, somewhat flattened 

 form, its posterior surface more convex than the anterior. 

 Fig. 10, a, represents it as viewed laterally ; h, as seen from 

 before. Its capsule, or coat, is much denser than that of the 

 other humours. Although the contents of this capsule are 

 fluid, the central parts are much denser than those toward the 

 exterior. 



The aqueous humour^ or that which fills the part anterior to 

 the lens, is perfectly limpid, and, like the vitreous, enclosed in 

 a delicate capsule or membrane. 



Omitting here any account of the blood-vessels and nerves 

 with which the eye is supplied, I may briefly explain the man- 

 ner in which vision is effected. The retina, or expansion of 

 the optic nerve, at the bottom of the eye, is the part which 

 gives the sensation of light ; and the other parts of the eye are 

 intended for collecting and modifying the rays emanating from 

 objects, so as to produce, through the retina, an image of these 

 objects. Kays of light being deflected from their course in 

 passing from a rarer into a denser medium, those proceeding 

 from an object, and passing through the cornea, are made to 

 converge in a small degree. If the rays are too numerous or 

 intense, they are diminished by the contraction of the pupil, 

 which, on the other hand, enlarges when the rays are scanty 

 and the light feeble. The rays to which the pupil gives ad- 

 mittance now penetrate the chrystalline lens, which being a 

 dense body with two convex surfaces, refracts them so as to 

 cause them rapidly to converge as they traverse the vitreous 

 humour. The parts of the organ are so adjusted that the focus 



