ORGAN OF SIGHT IN MAMMALIA. 



'257 



A 



Crystalline lens, human, at different 

 ages ; nat. size. cv". 



204 



it becomes rather flatter and also firmer in texture. The density 

 of the lens is not the same throughout, the surface being nearly 

 fluid, while the centre scarcely yields to the pressure of the 

 finger and thumb, especially in advanced 

 life. The eye is thus rendered achro- 

 matic. The specific gravity of the lens 

 to water is as 10024 to 10000 : the re- 

 fractive power of the centre of the lens 

 is to that of water as 18 to 7. Brewster 

 found the following; to be the refractive 



o 



powers of the different humours of the human eye, the ray of 

 light being incident upon them from the eye : ( aqueous humour, 

 1-336; crystalline, surface 1'3767, centre 1-3990, mean 1-3839 ; 

 vitreous humour, 1'3394. But as the rays refracted by the aque- 

 ous humour pass into the crystalline, and those from the crys- 

 talline into the vitreous humour, the indices of refraction of the 

 separating surface of these humours will be, from the aqueous 

 humour to the outer coat of the crystalline, 1*046(5; from the 

 aqueous humour to the crystalline, using the mean index, 1*0353 ; 

 from the vitreous to the outer coat of the crystalline, 1*0445 ; 

 from the vitreous to the crystal- 

 line, using the mean index, 

 1-0332.' If the lens with the 

 capsule attached to the hyaloid 

 membrane be placed in water, the 

 following day it is found slightly 

 opaque or opaline, and split into 

 several portions by fissures ex- 

 tending from the centre to the 

 circumference, as in fig. 204, B. 

 If allowed to remain some days 

 in water, it continues to expand 

 and unfold itself; and if then transferred to spirit and hardened, 

 it may be unravelled by dissection, fig. 204, c, and its fibrous 

 structure demonstrated. 



In Man and Mammals generally three septa diverge from each 

 pole of the lens at angles of 120, the septa of the posterior sur- 

 face bisecting the angles formed by the septa of the anterior sur- 

 face : the fibres diverge from these septa as shown in fig. 205. 

 The denticulated structure by which the fibres are laterally united, 

 or interlock, is shown in vol. i. p. 333, fig. 217, in the crystalline 

 lens of a cod. The human lens is inclosed in a transparent, 

 firm, elastic capsule. A branch of the ' arteria centralis retinae ' 



VOL. III. 8 



A, Crystalline lens, natural state; B, peripheral 

 softer portion fissured by action of water; 

 c, resolution of nucleus into fibres, magni- 

 fied, cv". 



