516 



PHYSIOLOGY 



vision ; (2) to reduce the light intensity when flying at high altitudes or when travelling 

 towards the sun ; (3) to reduce the irritation caused by ultra-violet orinfra-red rays 

 which aft present in excessive amount at high altitudes. 



NUTRITION OF THE EYE 



The eyeball is richly supplied with blood vessels, which form numerous 

 anastomoses. Among these may be mentioned the arteries of the optic 



nerve sheath, the long and 

 short posterior ciliary arteries, 

 the anterior ciliary arteries 

 which are branches from the 

 muscular vessels, and the con- 

 j unctival arteries. These pierce 

 the sclera to ramify freely in 

 the choroid and the ciliary 

 bodies. The iris is supplied 

 by two concentric vessels, the 

 circulus major arid the circu- 

 lus minor. Between the two 

 pass a number of radial fibres. 

 The retina, as will be shown 

 later, has a separate blood 

 supply through the central 

 artery of the optic nerve. 

 Other structures, notably the 

 transparent optical media of 

 the eye, have no direct blood 

 supply and therefore depend 

 on the flow of lymph from 

 neighbouring structures for 

 their nutrition. This fluid is 

 formed principally by the cili- 

 ary bodies, and is called aque- 

 ous humour. 



AQUEOUS HUMOUR. The 

 chemical composition of this 

 fluid is water containing salts, 

 traces of albumin and glob- 

 ulin, and a reducing sugar; 

 it is probably freely oxy- 

 genated. This fluid after 

 secretion leaves the eye in one of three ways. (1) By travelling 

 through the pupil into the anterior chamber of the eye and then through 

 the spaces of Fontana at the edges of the iris (the so -called filtration angle) 

 into the canal of Schlemm and thus into the ciliary veins. (2) Through the 

 crypts in the anterior surface of the iris into the yews of that structure, 



I i' . 257. Diagram to show the blood supply 

 of the eyeball. Arteries ' lined,' veins ' black.' 



