CHAP. XVII.] STRUCTURE OF THE LENS. 35 



behind. And the rest of the superficial plane are intermediate to 

 these, and as nearly parallel as their curved course will allow. If we 



Fig. 123. Fig . ,24. 



Lens, hardened in spirit and 

 partially divided along the three 

 interior planes, as well as into 

 lamellae. Magnified 3J diame- 

 ters. After Arnold. 



Triple line on the surface of 

 the Lens of the Sheep, with the 

 radiation of the fibres indicated. 

 The interspace between the radi- 

 ating lines at the circumferences 

 of this figure would each include 

 about a hundred fibres. Magni- 

 fied 3 diameters. 



now consider that these lines on the surface are but the edges of 

 planes which dip to the centre, and afford points of divergence and 

 concourse for all the fibres deep as well as superficial, we shall 

 readily comprehend what may at first sight seem an intricate 

 structure. This arrangement was known to Leeuwenhoeck, and has 

 been shown by Sir D. Brewster to present varieties in different 

 classes of animals. In the human lens we find the tripartite division 

 is seen imperfectly, and only in the centre; for the three primary 

 diverging lines bifurcate again and again, and with considerable 

 irregularity, so that the ultimate subdivision is into from twelve to 

 sixteen parts in the adult, but only from four to six in the foetus. 



To the account now given may be added, that as the fibres are 

 shorter in proportion as they are more internal, so do they appear 

 narrower, more cylindrical, solid, and intimately united to each other, 

 as we trace the structure inwards. The superficial fibres are flat- 

 tened according to the surface they answer to; and of all it may be 

 said, that they are narrower towards their extremities, as their 

 arrangement renders necessary. The edges of the fibres in fishes 

 are most beautifully toothed, and dovetailed together, as Sir D. 

 Brewster pointed out (fig. 122, d); and something similar may be 

 detected in the more superficial fibres of the lens of the larger mam- 

 malia, and in man. But the deepest fibres present scarcely any trace 

 of this elegant structure. Near the tripartite division of the lens 

 the fibres are more united than elsewhere, and appear more or less 

 consolidated together. The average thickness of the fibres in man 

 is about -5-3*5-0 of an inch. 



D2 



