976 THE SENSES 



stroma or framework of the iris are two arrangements of smooth mus- 

 cular fibres, which confer on it the power of adjusting the size of the 

 pupil. One of these the sphincter pupillae consists of a well-defined 

 band of concentric fibres surrounding the margin of the pupil. The 

 other the dilator pupillae is less sharply differentiated. It is repre- 

 sented 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 potenial 

 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 ligament of the lens. The attachment of the suspensory 

 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 per- 

 fectly 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) con- 

 stitute much the largest portion. A very small amount of protein 

 (o-oi to 0-04 per cent.) is present, also a little dextrose (0-05 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 0-06 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 

 ibout 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 conse- 

 quent 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 tc 



