192 



EYE. 



laining its own weight and preserving its form 

 when placed in water, and in air presents the 

 appearance of a gelatinous mass, scarcely de- 

 serving the name of solid. The cellular struc- 

 ture, in which the watery fluid is lodged, has 

 been called the hyaloid membrane, and the 

 whole mass denominated the vitreous humour. 

 The fluid of the vitreous humour, according 

 to Berzelius, is composed of water, containing 

 about one and a half per cent, of animal and 

 saline ingredients; it has a saline taste, and 

 acquires a slight opaline tint by being boiled. It 

 consists of water 98.40, chloruret of soda with 

 a little extractive matter 1.42, a substance solu- 

 ble in water 0.02, and albumen 0.16. Its 

 specific gravity is 1.059. When the hyaloid 

 membrane is examined in its natural state, its 

 cellular organization can scarcely be ascertained 

 on account of its transparency ; but if it be 

 suspended on the point of a pin until the fluid 

 is allowed to drop out, it may be inflated with 

 a fine blowpipe and dried, or if the whole be 

 placed in strong spirit or weak acid, the mem- 

 brane becomes opaque, and its organization 

 obvious. It has been supposed that the cells 

 in which the fluid is lodged present a determi- 

 nate form, and attempts have been made to 

 prove this by freezing the eye and examining 

 the frozen fragments; but any one who has 

 seen the hyaloid membrane rendered opaque 

 by acid must allow that the cells are too minute 

 to admit of such investigation, and that the 

 frozen masses, supposed to be the contents of 

 cells, are merely fragments of the hyaloid 

 membrane with their contained fluid. Although 

 the hyaloid membrane is perfectly transparent, 

 and the red particles of the blood do not circu- 

 late in its vessels, there can be little doubt 

 that its growth and nutrition are effected by 

 the circulation of a transparent fluid in vessels 

 continuous with those conveying red blood. 

 It is an established fact that transparent tex- 

 tures which in a natural state do not exhibit 

 a trace of coloured fluid, when excited or 

 inflamed, become filled with red vessels, as 

 may be seen in the conjunctiva. It is there- 

 fore reasonable to admit that the hyaloid mem- 

 brane does not present a deviation from this 

 general law. The fluid of the vitreous humour, 

 it is to be presumed from analogy, is secreted 

 by the vessels of the hyaloid membrane, and 

 if no red vessels can be detected, the secretion 

 must be accomplished by transparent ones. It 

 has already been stated that the vascular layer 

 of the retina adheres to the surface of the 

 vitreous humour, and that the points of adhe- 

 sion are stronger along the course of the vessels 

 than in the intermediate spaces ; it is therefore 

 most probable that the more superficial part of 

 the sphere is supplied with transparent blood 

 from the arteries of the retina, while a branch 

 directly from the central artery, as it penetrates 

 the porus opticus, enters behind, ana extends 

 to the back of the lens : such a branch can be 

 injected in the foetus, and is found to ramify 

 on the back of the capsule of the lens ; and in 

 the eyes of large quadrupeds a transparent 

 production, probably vascular, has been ob- 

 served proceeding from the entrance of the 



optic nerve into the mass of the vitreous 

 humour. It is also probable that the ciliary 

 processes of the choroid, which are buried in 

 the hyaloid membrane anteriorly, supply blood 

 to that part of the sphere. That the vitreous 

 humour undergoes changes analogous to those 

 which take place in textures supplied with red 

 blood, is proved by its hyaloid membrane 

 being found opaque and thickened in eyes 

 which have been destroyed by internal inflam- 

 mation. A total disorganization of the vitreous 

 humour is a frequent occurrence, the hyaloid 

 membrane losing its cohesion to such a degree 

 that the fluid escapes from the eye as freely as 

 the aqueous humour when the cornea is divided 

 in the operation of extraction ; and after the 

 lens and its capsule have been removed by 

 operations with the needle, opacity of the 

 hyaloid membrane is occasionally, although 

 rarely, observed. Allusion has frequently been 

 made in books to an appearance in the 

 eye denominated glaucoma, attributed, rather 

 vaguely, to opacity of the vitreous humour; it 

 appears, however, to be nothing more than the 

 usual opacity of the lens which occurs in 

 advanced life, seen through a dilated pupil. 

 As an additional proof of the vascularity of 

 the vitreous humour may be adduced the fact, 

 that in the eyes of sheep, injured by blows in 

 driving' to the shambles, the vitreous humour 

 is deeply tinged with red blood. 



The spherical mass of vitreous humour, it 

 has already been stated, is exactly fitted into 

 and adheres to the inner surface of the retina. 

 From the anterior termination of the retina to 

 the posterior chamber of the aqueous humour, 

 it is in contact with, and adhering to, the 

 ciliary processes of the choroid. Where it is 

 truncated or compressed on its anterior part to 

 form the posterior chamber of the aqueous 

 humour, it has the crystalline lens fitted into a 

 depression in its centre, while a narrow circle 

 of it appears between the circumference of the 

 lens and the anterior extremities of the ciliary 

 processes of the choroid, forming part of the 

 boundaries of this chamber of aqueous humour. 



If the eye be allowed to remain for a day or 

 two in water in order to destroy by maceration 

 the delicate connexions between the hyaloid 

 membrane and the choroid, and then the 

 vitreous humour with the lens attached care- 

 fully separated, the point of a fine blowpipe 

 may be introduced under the surface of the 

 hyaloid membrane at the circumference of the 

 lens, and a series of cells encircling the lens 

 inflated. This is the canal of Petit, or canal 

 godronne. It is thus described by the dis- 

 coverer in the Histoire de 1'Academie des 

 Sciences for 1726. " I have discovered a small 

 canal surrounding the crystalline, which I call 

 the circular canal godronne; it can be seen 

 only by inflating it, and when filled with air it 

 forms itself into folds similar to the ornaments 

 on silver plate, called for this reason Vuiselle 

 godronne. It is formed by the doubling of the 

 hyaloid membrane, which is contracted into 

 cells at equal distances by little canals which 

 traverse it, and which do not admit of the 

 same degree of extension as the membrane, 



