EYE. 



[ 314 ] 



EYE. 



transparent, polygonal, epithelial cells (fig. 

 207, ), from 1-2000 to 1-1200" in diameter. 

 The lens itself consists of long, transpa- 

 rent, six-sided, flattened fibres (fig. 213), 

 from 1-4800 to 1-2400" in breadth and 

 1-8500 to 1-13,000" in thickness ; these are 

 tubular, at least in the outer portions of the 

 lens, and contain a tenacious sarcodic sub- 

 stance, which escapes from the ends of the 

 broken fibres in irregular globules. The 

 form of the fibres is best seen in a transverse 

 section (fig. 214). 



Fig. 214. 



Transverse section of the fibres or tubes of the hu- 

 man lens. 



Magnified 350 diameters. 



The fibres are firmer, narrower, and more 

 highly refractive towards the centre of the 

 lens. Their general arrangement is such 

 that their broad surfaces are parallel with 

 the surface of the lens, and that they fol- 

 low a direction from the middle of the 

 anterior to that of the posterior surface, 

 curving laterally in their course not, how- 

 ever, exactly from the middle, but from 

 the arms of a star-shaped kind of centre, 

 at which parts (fig. 215) the fibres are 



Fig. 215. 



Anterior view of human crystalline lens (adult), 

 showing the stars and the direction of the fibres. 

 Magnified 5 diameters. 



replaced by a homogeneous or finely gra- 

 nular matter. The arms of the star present 

 upon the surfaces are the extremities of 

 planes extending through the substance of 

 the lens, from which the inner fibres take 

 their origin. The arms of the anterior and 

 posterior stars are not parallel with each 

 other j nor are the fibres arising from any 

 part of the arm of one cross inserted into 

 the corresponding part of the arm of the 

 opposite cross. Great variety exists in dif- 

 ferent animals in the structure and arrange- 

 ment of these stars and planes. Thus in 

 the human foetus the star has three arms or 

 planes, whilst in the adult there are from 

 nine to sixteen, of which three are frequently 

 more distinct than the others. In some 

 animals they are replaced by a pole, from 

 which the fibres radiate like meridians, as 

 in the cod, the Triton, and Salamandra ; in 

 others, there is a single plane, as in some 

 fishes, the frog, the hare, the rabbit, and the 

 dolphin ; whilst in most of the mammalia 

 there are three, and in the whale, the bear 

 and the elephant there are four. 



The edges and marginal surfaces of the 

 fibres of the lens are uneven or toothed, so 

 that their lateral connexion becomes more 

 intimate ; hence the lens separates more 

 readily into parallel laminae in the direc- 

 tion of the surface than in the opposite 

 direction. 



In many animals, especially fishes, as the 

 cod, the roach, &c., the irregularities of the 

 fibres of the lens are replaced by beautiful 

 teeth (PI. 50. fig. 6). 



Vitreous humour, or body, is enclosed in 

 a membrane, the hyaloid membrane, which 

 behind the dentate margin of the retina is 

 extremely thin and delicate ; anterior to this 

 it becomes firmer (fig. 207 t) and passes, 

 forming the zonule of Zinn, to fuse with the 

 capsule of the lens. In thus doing, it sepa- 

 rates into two layers: a posterior (v\ 

 which becomes consolidated with the cap- 

 sule of the lens somewhat behind its mar- 

 gin ; and an anterior (u), connected with the 

 ciliary processes, which becomes attached 

 to the capsule of the lens, a little in front 

 of its circumference : between these two is 

 the canal of Petit (C.P.). The posterior 

 layer is sometimes considered as arising 

 from a condensation of the tissue of the 

 vitreous humour. The structure of the 

 vitreous body is still obscure. 



The structure of the eye is very difficult 

 of examination, the parts being so delicate 

 and easily injured. Many of them can be 



