306 



PHYSIOLOGY 



CHAP. 



less marked differences of curvature than the peripheral zone, 

 where a rapid flattening is shown in the curves, with a rapid fall 

 in refractive power. Moreover, the curves of the horizontal and 

 vertical meridians in the optic zone are not symmetrical in 

 relation to the optic axis, and on comparing the two curves it is 

 seen that refraction is more uniform in the horizontal than in the 

 vertical meridian. Gullstrand concludes that the visual zone of 

 the corneal surface has a transverse-oval shape. As shown by 

 the two curves it extends from the visual axis nasalwards for 

 about 20, outwards for about 25, upwards 15, downwards 20. 

 He believes that the rapid flattening of the vertical meridian is 

 in relation to the pressure exerted by the eyelids on the cornea. 



Astigmatism depends on the different curvatures of the 

 cornea in the different meridians, and means that the rays that 

 fall on one meridian are more highly refracted and reunite earlier 



FIG. 139. Diagram to show the curvature of the cornea (A) in the horizontal, (B) in the vertical 

 meridian. (After Gullstrand.) The figures to the right of A and B show the angle formed by 

 single points of the corneal surface with the visual axis ; the figures in the middle of A and B 

 give the amount of refraction at the corresponding points of the two corneal meridia in 

 dioptres. The irregularities of both curves are due to the unequal distribution of the 

 lachrymal secretion over the surface of the cornea. 



than those that fall on other meridians, i.e. have a different focal 

 point. When the two meridians in which the focal distance 

 reaches its maximum and minimum lie one vertically over the 

 other, the astigmatism is regular. This form is very often present 

 in so slight a degree that it produces no striking deformation of 

 the retinal images. 



The degree of astigmatism is expressed in dioptres, and is 

 calculated as the difference in the static refraction of the two 

 meridians in which refractive power is maximal and minimal. 



According to the measurements made- by Nordenson on young 

 students of 7 to 20 years of age, .in 452 eyes only 42, i.e. 

 9 per cent, showed no form of astigmatism. Astigmatism of more 

 than 1 D. was present in sixty-four students; of more than 1-5 D. 

 in four. In the astigmatic eyes the most refractive meridian 

 was in 85 per cent the vertical, only in 1-5 per cent the hori- 



