444 HUMOURS OF THE EYE. 



brane. The term foramen is misapplied to this spot, for the vascu- 

 lar layer and the membrani Jacobi are continued across it ; the ner- 

 vous substance alone appearing to be deficient. It exists only in 

 animals having the axis of the eyeballs parallel with each other, as 

 man, quadrumana, and some saurian reptiles, and is said to give 

 passage to a small lymphatic vessel. 



The zonula ciliaris (zonula of Zinn)* is a thin vascular layer 

 which connects the anterior margin of the retina with the anterior 

 surface of the lens near to its circumference. It presents upon its 

 surface a number of small folds corresponding with the ciliary pro- 

 cesses, between which they are received. These processes are 

 arranged in the form of rays around the lens, and the spaces be- 

 tween them are stained by the pigmentum nigrum of the ciliary 

 processes. They derive their vessels from the vascular layer of the 

 retina. The under surface of the zonula is in contact with the 

 hyaloid membrane, and around the lens forms the anterior fluted 

 wall of the canal of Petit. 



The connexion between these folds and the ciliary processes may 

 be very easily demonstrated by dividing an eye transversely into 

 two portions, then raising the anterior half, and allowing the vitreous 

 humour to separate from its attachment by its own weight. The 

 folds of the zonula will then be seen to be drawn out from between 

 the folds of the ciliary processes. 



HUMOURS. The Aqueous humour is situated in the anterior and 

 posterior chambers of the eye ; it is a weakly albuminous fluid, hav- 

 ing an alkaline reaction, and a specific gravity very little greater 

 than distilled water. According to Petit, it scarcely exceeds four 

 or five grains in weight. 



The anterior chamber is the space intervening between the cornea 

 in front, and the iris and pupil behind. 



The posterior chamber is the narrow space, less than half a line 

 in depth,f bounded by the posterior surface of the iris and pupil in 

 front, and by the ciliary processes, zonula ciliaris, and lens behind. 



The two chambers are lined by a thin layer, the secreting mem- 

 brane of the aqueous humour. 



The Vitreous humour forms the principal bulk of the globe of the 

 eye. It is an albuminous fluid resembling the aqueous humour en- 

 closed in a delicate membrane, the hyaloid, which sends processes 

 into its interior, forming cells in which the humour is retained. A 

 small artery may sometimes be traced through the centre of the 

 vitreous humour to the capsule of the lens ; it is surrounded by a 

 tubular sheath of the hyaloid membrane. This vessel is easily in- 

 jected in the foetus. 



* John Gottfried Zinn, Professor of Anatomy in Gottingen; his " Descriptio Anato- 

 mica Oculi Humani," was published in 1755 ; with excellent plates. It was re published 

 by Wrisberg in 1780. 



t Winslow and Lieutaud thought the iris to be in contact with the lens ; it frequent- 

 ly adheres to the capsule of the latter in iritis. The depth of the posterior chamber is 

 greater in old than in young persons. 



