170 LYMPHATICS OF THE EYE. [BOOK in. 



the canal of Schlemm, which in turn is in direct connection with 

 the radicles 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 pres- 

 sures 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 accumu- 

 lation of aqueous humour results. 



We may conclude then that the aqueous humour is a reservoir 

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

 the ciliary processes through the small posterior and larger ante- 

 rior 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 carry- 

 ing away waste products, enables the lens to carry out the slow 

 and scanty metabolism necessary for its life. 



806. For mechanical purposes the due condition of the 

 vitreous humour is perhaps even more important than that of the 

 aqueous humour. We have already ( 720) 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 ; and indeed it is practically 

 an attenuated mesoblastic sponge through which is continually 

 streaming, though at a low rate, a fluid identical with or exceed- 

 ing 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 centralis 

 retinae ( 703). 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 into the 

 anterior chamber, and a block at the iridic angle leads to undue 

 distension, not of the anterior and posterior chambers only, but of 

 the whole globe of the eye ; the pressures of the aqueous and 

 vitreous humour are the same and vary similarly andiconcurrently. 

 We have no satisfactory evidence that any large amount of 

 fluid passes direct from the choroid through the retina, past 



