CHAP, in.] SIGHT. 171 



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 flow 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 

 ( 302) 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- 

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

 transient rise, may be brought about by stimulation of the cervical 

 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 general 

 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 ( 549), said all that at present we have to say. 



