364 VISION. LECT. XX. 



which is circular, while the other radiates from the centre 

 to the circumference. In its centre, the iris is pierced by 

 a circular opening called the pupil, whose diameter is vari- 

 able according to the intensity of the light. Behind the 

 iris is placed the crystalline humour or lens, a solid, lenti- 

 cular, transparent body, invested by its own proper mem- 

 brane. On the internal surface of the sclerotic, is a dark 

 membrane called the choroid: this is lined by another, termed 

 the retina, which is semi-transparent and thin, and is formed 

 by the expansion of the medullary portion of the optic nerve 

 placed at the bottom of the orbit. The eye is divided by 

 the crystalline lens into two distinct portions. The anterior 

 one, in which the iris floats, is filled with a liquid called 

 aqueous humour, which is very similar to water, and con- 

 tains traces of common salt. The posterior part contains 

 a much denser liquid termed the vitreous humour. When 

 carefully examined, the crystalline is found to consist of 

 concentric layers, whose consistence and refracting power 

 increase from the circumference to the centre. Lastly, the 

 line, according to which the axis of the figure of the eye is 

 directed, in the act of distinct vision, is called the optic axis- 

 The following are the mean dimensions of the different 

 parts of the human eye: 



Millimetres. 



Radius of curvature of the sclerotica - . 10 to 11 



Radius of curvature of the transparent cornea - 7 8 

 Diameter of the iris . . . . 11 12 



Diameter of the pupil - ;* - ,3 7 



Thickness of the transparent cornea 1 



Distance of the pupil from the cornea - 2 



Anterior radius of the crystalline lens - 7 8 



Posterior radius of the crystalline lens - 5 6 



Diameter of the crystalline lens - - 10 



Thickness of the crystalline lens - .5 



Length of the optic axis . - 22 94 



