NOURISHMENT AND PROTECTION OF THE EYE 517 



(3) Between the suspensory ligaments of the lens, to the anterior surface of the 

 vitreous, then down the hyaloid canal to the papilla of the optic nerve, and 

 thus out via the lymphatics of the nerve sheath or the retinal vessels. But 

 whatever the fate of the liquid may be, it is clear that the amount secreted 

 must be the same as that which leaves, because otherwise there would be 

 a variation in the intraocular pressure. Insufficient pressure will tend to 

 disturb the correct relationship between the internal structures of the eye, 

 and at the same time will prevent the proper action of the ciliary muscle in 

 causing accommodation, because the suspensory ligaments of the lens will 

 already be relaxed. Too great a pressure on the other hand will interfere 

 with the proper blood supply to eye, and will prevent accommodation 

 because the tension in the choroid will be too great for the ciliary muscles 



1 



FKJ. 258. Diagram showing origin and fate of aqueous humour. 



to overcome (see page 526). It is therefore important that there should 

 be a proper control of the intraocular pressure. Experiments by Starling 

 and Henderson in which the intraocular pressure was determined by a 

 null method showed that such a mechanism exists, because as the arterial 

 pressure increased, so also did that in the eyeball. 



Whereas the arterial pressure varied between 70 and 180 mm. (by a 

 difference of 110mm.) the intraocular pressure was found to vary between 

 23 and 40 mm. (that is by 17 mm. only). The change in intraocular pressure 

 is therefore less than one-sixth that taking place in the blood ; the control 

 mechanism would therefore appear to have very considerable efficiency. 



GLAUCOMA. The normal intraocular pressure in man is found to be 

 between 25 and 30 mm. of mercury. The tension thus set up in the walls 

 of the eyeball is principally borne by the sclera ; to some extent however 

 assistance is rendered by the choroid owing to its elasticity, and by Tenon's 

 capsule owing to the tonic contraction of its smooth muscle fibres (innervated 

 by the sympathetic). 



In abnormal conditions the efferent channels may become closed, either 

 from pressure of the lens on the iris (as in hyperinetropia),or from the presence 



