974 LYMPHATICS OF THE EYE. [BOOK in. 



The answer to the question, How does the aqueous humour 

 leave the anterior chamber? presents perhaps less difficulties. 

 The anterior chamber at the 'iridic angle' communicates freely 

 with the spaces of Fontana, and these with the canal of 

 Schlemm, which in turn is in direct connection with the radi- 

 cles of the anterior ciliary veins. Since the ciliary muscle pulls 

 on the tissue surrounding the canal of Schlemm it is possible, 

 or even probable that the movements of accommodation help 

 alternately to close and open the canal, and thus to pump 

 its contents into the veins; by this means the exit of fluid 

 from the anterior chamber is rendered less dependent on the 

 relative pressures of the blood in the vein and of the fluid in 

 the anterior chamber. By this channel the aqueous humour 

 gains a ready, relatively direct, and short access to the blood- 

 stream. And clinical experience shews that if this way be 

 blocked an accumulation of aqueous humour results. 



We may conclude then that the aqueous humour is a reser- 

 voir intercalated in a stream of a peculiar fluid which is passing 

 from the ciliary processes through the small posterior and larger 

 anterior chamber, the spaces of Fontana and the canal of 

 Schlemm into the venous system. This reservoir on the one 

 hand serves a mechanical purpose in preserving the natural 

 form of the eye and in affording an adequate fluid bed for the 

 movements of the iris, and on the other hand, by bringing new 

 food material and carrying away waste products, enables the 

 lens to carry out the slow and scanty metabolism necessary for 

 its life. 



607. For mechanical purposes the due condition of the 

 vitreous humour is perhaps even more important than that 

 of the aqueous humour. We have already called attention 

 to the fact that the vitreous humour in spite of its being 

 originally a plug of mesoblastic tissue, in adult life closely 

 resembles the aqueous humour in its chemical features j and 

 indeed it is practically an attenuated mesoblastic sponge through 

 which is continually streaming, though at a low rate, a fluid 

 identical with or exceeding like to the aqueous humour. 

 Through the optic disc the fluid of the vitreous humour has 

 access to the lymph-spaces of the optic nerve ; material injected 

 into the pial sheath of the optic nerve finds its way through the 

 optic disc into the vitreous humour passing along a ' central 

 canal,' 'hyaloid canal,' which remains after the disappearance of 

 the prolongation of the arteria cen trails retinae ( 525). And 

 probably some of the fluid of the vitreous humour finds its way 

 by this path into the subarachnoid space. 



But the greater part of the fluid of the vitreous humour 

 seems to belong to the same system as the aqueous humour. 

 Fluids pass readily in some way or other through the* suspensory 

 ligament ; fluid injected into the vitreous humour finds its way 



