THE SENSES 901 



differentiated. It is represented by radial bundles of elongated, 

 spindle-shaped cells running in from the ciliary border of the iris 

 towards the pupil. Between the iris and the posterior surface of 

 the cornea is the anterior chamber of the eye, filled with the aqueous 

 humour. Between the iris and the anterior surface of the lens lies 

 the posterior chamber, which is rather a potential than an actual 

 cavity. The space between the lens and the retina is accurately 

 occupied by an almost structureless semi-fluid mass, the vitreous 

 humour, enclosed by the delicate hyaloid membrane, which in front 

 is reflected over the folds of the ciliary processes, and blends with 

 the suspensory ligamsnt of the lens. The attachment of the sus- 

 pensory ligament is rendered firmer by the connection of this part 

 of the hyaloid membrane to a circular fibrous portion of the 

 vitreous. Around the edge of the lens is left a space, the canal of 

 Petit. 



Chemistry of the Refractive Media. The aqueous humour is a 

 perfectly colourless, watery liquid, of slightly alkaline reaction to 

 litmus. The specific gravity is about 1008, and the total solids about 

 i per cent. Of the solids the inorganic salts (mainly sodium 

 chloride) constitute much the largest portion. A very small amount 

 of protein (o'oi to 0-04 per cent.) is present, also a little dextrose 

 (o'05 per cent.), and minute traces of urea and other substances. 

 The liquid of the vitreous humour has a very similar composition, 

 except that it contains a mucin-like body, hyalomucoid, to the 

 amount of o'o6 to o'i per cent. A similar mucin-like substance is 

 present in the cornea. The freezing-point of both liquids is a little 

 lower than that of blood-serum, A being about o'6. 



The lens is far richer in solids than the aqueous and vitreous 

 humours with which it is in contact (30 to 35 per cent, of solids, 

 60 to 65 per cent, of water). The salts, with small quantities of 

 lecithin and cholesterin, make up about i per cent. ; the balance of 

 the solids consists of proteins. The physical alterations, with 

 production of turbidity, which occur in the lens, and presumably in 

 its proteins, when water enters or leaves it in too great amount 

 through imbibition or osmosis, are of importance in connection 

 with the etiology of cataract. The anatomical and physiological 

 integrity of its capsule is a prime factor in the maintenance of that 

 high degree of transparency which is necessary for the function of 

 the lens. Cataract can be experimentally induced by injuring the 

 capsule. In like manner the cornea is protected against injurious 

 changes in its water-content (normally about 80 per cent.) and 

 consequent turbidity by the epithelium, which separates it from 

 the tears, and the endothelium, which separates it from the aqueous 

 humour. 



Secretion of the Intra-ocular Liquids. The aqueous humour is 

 secreted by the uveal epithelium covering the ciliary processes, and 

 to some extent by that covering the iris. As it is continually secreted, 

 so it is continually absorbed, the absorbed constituents finding their 

 way eventually into the vein or venous sinus called the canal of 

 Schlemm and the bloodvessels of the iris and ciliary processes. The 

 source of the liquid of the vitreous body is also the uvea. While the 

 intra-ocular liquids differ from ordinary lymph, there is no reason 

 to doubt that they are secretions which contribute to the nutrition 

 of those transparent structures of the eye which are not, and, on 

 account of their function, cannot be supplied with bloodvessels. 

 Their most obvious use is to maintain the proper intra-ocular pressure 



