STRUCTURE OF THE EYE 



289 



aqueous humour, and the posterior by the semi-gelatinous 

 vitreous humour. 



The greater part of the aqueous humour, after being 

 secreted by the cihary glands, passes into the anterior 

 chamber between the lens and the free margin of the iris. 

 It leaves the anterior chamber by the sinuses of Fontana, 

 situated near the attached border of the iris, and enters 

 the canal of Schlemm (Fig. 47). The aqueous humour 



FILTRATION ANOLE 



CANAL OF SCmlIMM 



CtLIAI^r 

 OLAHD3 



Fig. 47. — Diagram showing origin and fate of aqueous humour 

 (Hartridgc, from Starling's Princifles of Physiology). 



exerts a pressure of from 25-40 mm. of mercury (intra- 

 ocular pressure). 



The retina is composed essentially of the rods and cones 

 and their nervous connections, these being supported by 

 a scaffolding of connective tissue. It should be realised 

 that the rods and cones are directed into the substance 

 of the eyeball — that is to say, away from the source of 

 light, not towards it, as might be supposed. The layers 

 of the retina from without inwards are shown in the 

 accompanying figure. It will be seen that the rods and 

 cones abut distally against a layer of pigm^ted epithelium, 

 19 



