EYE. 



[ 255 ] 



EYE. 



that their broad surfaces are parallel with 

 the surface of the lens ; and that they follow 

 a direction from the middle of the anterior to 

 that of the posterior surface, curving laterally 

 in their course ; not, however, exactly from 

 the middle, but from the arms of a star-shaped 

 kind of centre : at which parts (figs. 221, 222) 

 the fibres are replaced by a homogeneous 



Fig. 221. 



Anterior view of human crystalline lens (adult) ; show- 

 ing the stars and the direction of the fibres. 



Magnified 5 diameters. 



or finely granular matter. The arms of the 

 star present upon the surfaces, are the extre- 

 mities of planes extending through the 

 substance of the lens, from which the 

 inner fibres take their origin. The arms 



Fig. 222. 



Posterior view of lens. 

 Magnified 5 diameters. 



of the anterior and posterior stars are 

 not parallel with each other, nor are the 

 fibres arising from any part of the arm 

 of one cross, inserted into the correspon- 

 ding part of the arm of the opposite cross. 

 Great variety exists in different animals in 

 the structure and arrangement 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 inti- 

 mate ; hence the lens separates more readily 

 into parallel laminae in the direction 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. 41, 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. 213, 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 capsule of 

 the lens, somewhat behind its margin; and 

 an anterior (u), connected with the ciliary 

 processes, which becomes attached to the 

 capsule of the lens a little in front of its cir- 

 cumference ; between these two is the canal 

 of Petit ( C.P.). 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 

 made out by dissecting the eye under water, 

 but the more delicate structures should be 

 immersed in the liquid of the anterior 

 chamber ; solution of chromic acid is useful 

 for hardening the parts to allow of sections 

 being made with a Valentin's knife. The lens 

 should be hardened by maceration, either in 

 solution of chromic acid, or by drying. The 

 fibres may be well preserved in the dry state. 



The structure of the eyes of the lower 

 animals is briefly noticed under the classes, 

 &c. In the mammalia generally, it is essen- 

 tially the same as in man, and the eye of the 

 ox or sheep may be selected for examination. 



BIBL. Kolliker, Mikroskop. Anat. bd. ii; 

 Todd and Bowman's Physiol. of Man. 



