CHAP, in.] SIGHT. 973 



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 medullary canal, suggests that the former furnish the 

 aqueous humour in some such way as the latter furnish the cere- 

 bro-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 processes 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 indica- 

 tions of secretory activity ; but, as we shall presently have occa- 

 sion 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 strorna 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 rjosterior 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 ( 535) 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 accommo- 

 dation, 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 proi < 

 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. 



