168 THE AQUEOUS HUMOUK. [BOOK 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. 



804. 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. (l'3p.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 '1 p.c. ; these are serum-albumin, 

 globulin, and apparently fibrinogen. 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 unknown; 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 If fluid be withdrawn from the anterior 

 chamber by puncture of the cornea, the chamber is soon refilled , 

 indeed, under certain circumstances, 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 con- 

 ditions 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 

 ( 694), is in part at all events furnished by the choroid plexuses, 

 by a process which presents some analogies with the act of 

 secretion. And the resemblance between the ciliary processes and 

 the choroid plexuses, for both are vascular folds of pia mater 

 covered with epithelium derived from the lining of the primitive 



