Chap, hi.] SIGHT. 975 



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 

 and concurrently. We have no satisfactory evidence that any 

 large amount of fluid passes direct from the choroid through the 

 retina, past the internal limiting and hyaloid membranes into 

 the vitreous humour; as far as we know the whole of the lymph 

 of the retina is carried away by the optic nerve in the manner 

 mentioned above ; and we must therefore conclude that the 

 region of the zonule of Zinn serves as the door both for the 

 entrance and exit of fluid, the circulation through the vitreous 

 humour between its indistinct concentric lamellae being secured 

 by diffusion assisted by the movements of the eyeball. 



This important now of what we may call modified lymph like 

 that of the more ordinary lymph in other parts of the body, is 

 determined in the first instance by the blood flow, and we may 

 apply to the eye the remarks which were made when (§ 244) 

 we treated generally of the relations of lymph to blood-supply. 

 Broadly speaking the intraocular pressure rises and falls with 

 the general blood-pressure; the dim cornea and sunk eye that 

 betoken the approaching end are due to the fall of blood-pres- 

 sure which accompanies death. A local fall, preceded by a 

 transient rise, may be brought about by stimulation of the cervi- 

 cal sympathetic, and a local rise by stimulation of the ophthalmic 

 branch of the fifth nerve, stimulation of the third nerve having 

 apparently little effect in either direction. We may add that, 

 tempting as the view may seem that the lymph arrangements 

 of the eye are under the direct control of the nervous system, 

 we have no evidence that such is the case. 



Concerning the influence of the nervous system on the gen- 

 eral nutrition of the eye, and the disorders which follow upon 

 section or injury to the fifth nerve we have already, in an 

 earlier part of the work (§ 439), said all that at present we 

 have to say. 



