STRUCTURE OF THE EYEBALL 



101 



Structure of the Eyeball 



The human eyeball is an irregular sphere which for simplicity is 

 reduced to an idealized schematic form having an equivalent anterior- 

 posterior diameter of 24.15 mm, with transverse diameter 24.13 mm 



R=7.7 



F = -15.707 



Temporal side 

 ! \ 



Center of rotation 



i2 = -12 



HH 



t i 



ON\N' 



VisuaUxis 3^to 5X. AM ' ' /C~ ;\ 



9°uDward \\ 3.6 \ / 7.2 



Fovea 

 centralis 



F= 24.387 mm 



AH= 1.348 mm 

 AH=1.602 



Optical axis \B= 24.00 

 w=1.336 



Thickness >\ \ / 



0.5 ^=1.336' 

 «=1.376 



Scale 

 5 mm 



i_j i i_i i 



Lens mean n'=1.413 

 Nasal side 



Fig. Ill— 1. Gullstrand's schematic eye. Unaccommodated and simplified. 

 Length, A to retina 24.00 mm. All distances on optical axis measured from anterior 

 pole A to B in millimeters. 



and vertical dimension 23.48 mm. In males the dimensions are from 

 0.5 to 0.6 mm greater than in females. Its mass is about 7 grams, and 

 its volume about 6.5 cc; the average specific gravity is therefore 1.08. 

 Five sixths of its external surface is formed by a firm white membrane 

 called the sclera. Its anterior convex protrusion, the cornea, is transpar- 

 ent and has an area equal to one sixth of the surface area. This corneal 

 area has a horizontal diameter of 12.0 mm and a vertical diameter of 

 11.0 mm. Abnormal dimensions may arise as indicated in Fig. III-2. 

 Attached to the eyeball behind and slightly to the nasal side is the optic 

 nerve, the function of which is to convey to the brain the nerve impulses 

 initiated in the retina by the light transmitted by the optical system. 



Cornea 



The protruding anterior transparent convex structure, called the 

 cornea, is taken in the ideal schematic eye as 0.5 mm in thickness, and 

 with an average index of refraction of 1.376. 



The anterior surface of the cornea has a radius of curvature of 7.7 mm. 

 That large departures from this average value may exist is illustrated 

 by the accompanying photographic reproductions, Fig. III-2. The 

 posterior surface of the cornea has a radius of 6.8 mm. The simplified 

 schematic cornea has been taken as having a radius of curvature of 

 7.8 mm, as shown in Table III— 1. 



