306 



PHYSIOLOGY 



CHAP. 



less Diarked 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 optie 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 fur 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. 



Astit/iinitix/ii depends on the different curvatures of the 

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

 fall on one meridian are mure highly refracted and reunite earlier 



-. 



Km. 13'.'. I'iaxiam tu show tin- curvature of the <>] nea (A) in tin- lien i/cmtal. (B) in tin- vei t ical 

 meridian. (Aftei (iullstiancl.) Tlie li.uiires tu tin- il-lit ..t A and 11 show the an-le tm nn-d 1>\ 

 single points of tin- coi neal sin face with tin- visual axis ; the li^nres in tin- middle of A ami I! 

 give tin- amount of refract ion at tin- corresponding points of tin- two rorn>>al nierklia in 

 dioptres. Tin- trregolaritlea of l.olh runes an- due to the- unc-'iual clistiiliution of the' 

 lachrymal secietion over the >m face 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 13. 

 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- 



