VI. 



THE LENS. 

 BY PROFESSOR BABUCHIN. 



THE lens, an organ which in form and in transparency closely 

 resembles an ordinary glass bi-convex lens, but which varies 

 in shape in different animals, is one of the most important 

 constituents of the dioptric apparatus of the eye. In Man 

 its antero-posterior axis is one-third less than the transverse 

 diameter, whilst in many animals the lens is almost spherical. 

 To whatever extent the form and dimensions of the lens 

 may vary, its elementary structure and plan are alike in all 

 Vertebrata. It is everywhere composed of two constituents, 

 the cellular elements, which form the body of the lens (paren- 

 chyma lentis, proper substance of the lens), and the invest- 

 ing membrane, which, exhibiting no further differentiation 

 of structure, completely invests it, and is termed the capsule 

 of the lens. 



The substance of the lens may be regarded as composed of 

 two layers. 'One of them, the anterior, is very thin, and begins 

 to increase in thickness near the sequator, either very gradu- 

 ally, as in Man or Mammals, or rather rapidly, as in Birds and 

 the scaly Amphibia. The posterior layer, on the other hand, 

 is very thick, and becomes gradually thinner from the axis of 

 the lens, where it is thickest, towards the sequator. Near the 

 ^equator the two layers coalesce, a little more forward in some 

 animals, in others a little further backwards, or, to describe it 

 more accurately, they are continuous with one another by A 

 rounded border. Except at this border, the two above-described 

 layers may be everywhere easily separated from one another, 



