VITREOUS HUMOR. 345 



and its capsule are frequently the seat of a morbid opacity 

 called cataract. Hence we have the lenticular and capsular 

 cataracts, which destroy the transparency of the lens or of 

 its capsule, prevent the rays of light from passing to the 

 retina, and consequently produce blindness. 



The vitreous humor (Fig. 98) forms the last of the re- 

 fracting media. It is also called the hyaloid ~body, (voxoj, 

 glass,) from its resemblance to melted glass. It occupies 

 the posterior three-fourths of the globe of the eye. Is a per- 

 fectly transparent body, spheroidal in its shape, except in 

 its anterior portion, where there is a depression for the re- 

 ception of the lens, and lies in contact with the whole inner 

 surface of the retina, which is expanded over it. 



This humor is of a gelatinous or semi-fluid consistence, 

 holding a medium refractive power between the lens and 

 aqueous humor. It consists of a very delicate, transparent 

 membrane, called the hyaloid membrane, and a fluid sub- 

 stance not unlike water. 



This membrane, besides enclosing the vitreous humor, 

 sends into its interior numerous processes, which, inter- 

 lacing and uniting, form a very fine cellular tissue, con- 

 taining the humor in its various cells thus differing from 

 the aqueous which is received in a single capsule. It has 

 sufficient strength to support the humor when suspended, 

 and on being punctured, the fluid is seen to escape drop by 

 drop, till the whole is discharged. If inflated and dried, 

 or immersed in alcohol, the interior septa and cellular ar- 

 rangement will become still more evident. 



The vitreous fluid, chemically, is found to diifer very little 

 from the aqueous, being formed principally of water, and 

 about two per cent, of animal and saline matter. The 

 vitreous humor is supplied with blood by a small artery 

 from the arteria centralis, which is seen to take a "tortu- 

 ous" course from behind, and lose itself on the capsule of 

 the lens. This vessel, it is supposed, also supplies the 

 hyaloid membrane. The vascular layer of the retina like- 

 wise sends vessels into the vitreous humor as well as the 

 ciliary processes in front. 



