902 A MANUAL OF PHYSIOLOGY 



on which the geometrical figure of the eyeball, and therefore its 

 efficiency as an optical instrument, depend. The balance between 

 secretion and absorption is accurately adjusted in health, but in 

 disease it may be upset, as in glaucoma, where the intra-ocular tension 

 is so much increased as to interfere with the circulation, and injuri- 

 ously affect the nutrition and function of the retina. Experimentally, 

 occlusion of all the arteries supplying the head causes a rapid fall of 

 tension, and the cornea becomes wrinkled and slack to the touch. 

 On restoring the circulation after not too long an interval, the tension 

 gradually returns to normal, and then becomes markedly hyper- 

 normal, even when the general arterial pressure is still low. This 

 is probably due to the crippling of the elements which secrete and 

 absorb the intra-ocular fluids, or of the capillary walls, so that a 

 proper adjustment can no longer be attained, as happens in a tissue 

 rendered oedematous by temporary anaemia. Where asphyxia of 

 the eyeball is avoided or is brief the intra-ocular pressure varies 

 directly as the blood-pressure in the ocular vessels within a wide 

 range (Henderson and Starling). 



Refraction in the Eye Formation of the Retinal Image. 

 The amount of refraction which a ray of light undergoes at 

 a curved surface depends upon two factors the radius of cur- 

 vature of the surface, and the difference between the refractive 

 indices of the media from which the ray comes and into which 

 it passes. The smaller the radius of curvature, and the greater 

 the difference of refractive index, the more is the ray bent from 

 its original direction, A ray of light passing into the eye meets 

 first the approximately spherical anterior surface of the cornea, 

 covered with a thin layer of tears. Since the refractive index 

 of the tears is much greater than that of air, the ray is strongly 

 refracted here. The anterior and posterior surfaces of the 

 cornea being practically parallel, and the refractive indices of 

 the tears and aqueous humour being nearly equal, but little 

 refraction takes place in the cornea itself. At the anterior and 

 posterior surfaces of the lens the ray is again refracted, since the 

 refractive index of the aqueous and vitreous humours is less than 

 that of the lens. The following tables show the radii of curva- 

 ture of the refracting surfaces and the refractive indices of the 

 dioptric media, as well as some other data which are of use in 

 studying the problems of refraction in the eye : 



In accommodation for 

 Far Vision. Near Vision. 



rCornea 7'8 mm. 7-8 mm. 



Radius of curvature of \ Anterior surface of lens 10-0 6 - o ,, 

 I Posterior surf ace of lens 6'o 5-5 

 /Anterior surface of cornea and an- 

 terior surface of lens - - 3*6, , 3*2,, 

 Distance J Anterior surface of cornea and pos- 

 between "" terior surface of lens - -7-6,, 7'6,, 

 Anterior and posterior surface of lens 4/0 ,, 4-4 ,, 

 Posterior surface of lens and retina -14-6 ,, 14*6 ,, 

 Anterc-posterior diameter of eye along the axis 22^2 ,, 22-2 ,, 



