630 MANUAL OF HISTOLOGY. 



possesses no form elements, but is simply water containing 1-1-5 per 

 cent, of solid matter in solution , which is secreted, probably, by the 

 vessels of the ciliary processes and iris. The solids dissolved in the 

 humour aqueous are albumen in combination with soda, urea according 

 to Millon (p. 42), extractives, and mineral substances. Among the latter 

 chloride of sodium is the chief. 



We avail ourselves here of an analysis of Lohmeyer's, in which he gives 

 the following average composition of the fluid, as found in the eye of the 

 calf: 



Water, 986-870 



Albuminate of sodium, .... . 1-223 



Extractives, 4*210 



Chloride of sodium, 6 '890 



Chloride of potassium, . . ; . . 0'113 



Sulphate of potassium, ..... 0-221 



Earthy phosphates, 0-214 



Lime earths, . . . . . . . 0'259 



The refracting index of the human aqueous humour stands, according 

 to Krause, at 1*3349. The exponents of refraction for the vitreous 

 humour, lens, and cornea, are all mentioned with their respective tissues. 



The fact that puncture of the vitreous humour causes the escape of some 

 fluid, but not of all contained in it, seems to indicate (apart from its finer 

 texture), the presence of some structure within it of membranous or septal 

 nature. But this is still a matter of great obscurity. The existence of 

 a system of concentric lamellae, or of vertical partitions arranged like those 

 of an orange, radiating from a centre, has been supposed by many from 

 what they have observed in artificially hardened eyes. Neither of these 

 views, however, has been confirmed. 



But the presence of an external envelope, of the membrana hyaloidea, 

 was for years admitted by all. This is an extremely delicate, structure- 

 less pellicle lying loose upon the retina, and only attached at the entrance 

 of the optic nerve to the latter, and anteriorly to the ciliary body. 



In the neighbourhood of the ora serrata it was formerly supposed that 

 the membrana hyaloidea split into an interior delicate and posterior 

 thicker lamina. These, increasing gradually in distance from one another, 

 were held eventually to be inserted into the capsule of the lens and fused 

 into it. The posterior leaf was known as the true hyaloidea, and the anterior 

 the zonula Zinnii, while the passages enclosed within them, which encircles 

 the lens in its equator, received the name of the canal of Petit. The latter 

 is occupied either by a small amount of fluid during life (Koelliker), or 

 the two lamina come into contact (Herile). 



This, however, has been denied, within the last few years, by the most 

 competent authorities (Merkel, Iwanoff). 



According to their views there is no such thing as a special membrana 

 hyaloidea, but merely a limitans of the retina, while anteriorly the zonula 

 ciliaris becomes well marked as a special layer. Schwalbe, however, 

 opposes this view in favour of the old idea. 



The zone of Zinn, closely united to the ciliary processes, is indented by 

 them, presenting somewhat the appearance of a ruff, inserted with its 

 wavy border into the capsule of the lens. Although to the naked eye it 

 appears formed of a strong transparent membrane, it presents under the 

 microscope a system of very pale, stiff fibres, holding a meridional dircx. 



