972 THE AQUEOUS HUMOUR. [BOOK in. 



The perivascular and other lymph-spaces of the retina are 

 in connection with the lymph-spaces of the optic nerve, which 

 in turn join the subarachnoid space of that nerve, and this is 

 continuous with the corresponding space in the brain. There 

 appear to be also paths uniting these lymphatics of the retina 

 and optic nerves with the perichoroidal spaces and Tenonian 

 cavity, and so with the external lymphatic system. 



606. In the special lymph apparatus the ciliary processes, 

 the iris, the aqueous humour and the vitreous humour are con- 

 cerned. 



The aqueous humour. We have more than once spoken of 

 the anterior chamber as a lymphatic cavity; nevertheless the 

 aqueous humour contained in it differs greatly from ordinary 

 lymph. Not only does it contain much more water, the total 

 solids being not much more than 1 p.c. (1-3 p.c.) but also the 

 relative proportion of the solids between themselves is different 

 from that of lymph, and special substances are present in it. 

 The proteids are particularly scanty, not more than about ! 

 p.c. ; these are serum-albumin, globulin, and apparently fibrino- 

 gen. Inorganic salts are present in about the same proportion 

 as in blood and lymph, viz. -8 p.c. ; and these, chiefly sodium 

 chloride, with an unusual proportion (-4 p.c.) of so-called 

 extractives, furnish nearly all the solid matter. Among these 

 extractives is a substance which reduces cupric solutions but 

 which is not a sugar, though its exact nature is as yet un- 

 known ; urea and sarcolactic acid (in some combination) are 

 also said to be, at least often, present. The reaction is neutral 

 or faintly alkaline. 



Like the ' serous fluid ' in the large serous cavities and the 

 cerebro-spinal fluid in the cavities of the central nervous system, 

 the aqueous humour comes and goes ; the particular fluid 

 which at any given moment is present in the eye has not 

 always been there ; some of the fluid is continually passing 

 away and fresh fluid continually arriving. If fluid be with- 

 drawn from the anterior chamber by puncture of the cornea, 

 the chamber is soon refilled ; indeed, under certain circum- 

 stances, a considerable quantity of fluid may be drained away 

 from the chamber, fresh fluid taking the place of that which 

 escapes. And, though under normal conditions the quantity 

 of aqueous humour is fairly constant, the fluid may be in excess 

 or may be deficient, and the one phase may pass into the other. 

 The question therefore arises, Whence comes the fluid and 

 whither does it go ? 



The characters of aqueous humour just given shew that in 

 many respects it resembles cerebro-spinal fluid though differing 

 in several features. That fluid, we have seen reason to 

 believe, is in part at all events furnished by the choroid plex- 

 uses, by a process which presents some analogies with the act 



