CHAP. HI.] SIGHT. 1307 



medullary canal, suggests that the former furnish the aqueous 

 humour in some such way as the latter furnish the cerebro-spinal 

 fluid. There is a certain amount of experimental evidence in 

 favour of this view, for when such a substance as fluorescin, which 

 can be detected by the greenish tinge which it gives to the fluids 

 and tissues, is injected into the body, into the subcutaneous 

 connective tissue or peritoneal cavity for instance, not only does 

 it speedily appear in the aqueous humour, but the ciliary pro- 

 cesses are said to be the parts of the eye in which its presence 

 may be first detected. It may be urged that, unlike the epithelium 

 covering the choroid plexuses, the pars ciliaris retinae bears no 

 distinctive histological indications of secretory activity ; but, as 

 we shall presently have occasion to point out, a wholly analogous 

 layer of epithelium, that lining the cavities of the internal ear, 

 though possessing no marked secretory features, certainly furnishes, 

 by an act very similar to secretion, a more or less lymph-like fluid, 

 the so-called endo-lymph. The phrase ' secretion ' however must 

 not be strained. The somewhat specialized loose stroma of both 

 the ciliary processes and iris undoubtedly contains in its meshes 

 a large quantity of what we may suppose to be ordinary lymph ; 

 and what is intended by the above view is that while some of this 

 lymph may pass by the perichoroidal spaces and so away as 

 ordinary lymph, a much larger proportion passes on to the free 

 surfaces abutting on the posterior and anterior chambers, and 

 in so passing becomes modified in nature. 



The fluid thus furnished by the ciliary processes makes its 

 way, in the first place, into the posterior chamber; but though the 

 iris, as we have seen ( 722) lies close on the lens, there is 

 undoubtedly a communication between the two chambers suffi- 

 ciently free to allow fluid to pass readily from one to the other 

 and so to fill the anterior chamber from the posterior. It is 

 difficult to suppose that some of the lymph with which the sponge- 

 like stroma of the iris is laden, does not find its way direct through 

 the anterior surface of the iris into the anterior chamber ; and 

 such a transit would probably be assisted by the continual changes 

 of the pupil. On the other hand the extent of surface furnished 

 by the ciliary processes, which moreover also have the advantages of 

 movement in each act of accommodation, is very large compared 

 with that of the iris ; hence we may probably with confidence 

 conclude that the greater part of the aqueous humour is furnished 

 by the ciliary processes, though the iris may contribute. We may 

 add that probably the iridic contribution differs in nature from 

 the rest, since the epithelium which the fluid has to traverse is a 

 thin layer of flat epithelioid plates. 



The answer to the question, How does the aqueous humour 

 leave the anterior chamber ? presents perhaps less difficulties. As 

 we have seen ( 717), the anterior chamber at the ' iridic angle' 

 communicates freely with the spaces of Fontana, and these with the 



