VISUAL ORGAN. 



355 



of the myelin sheaths the nerve becomes considerably thinner 

 on entering the eyeball. 



(5) The Lens. 



In the lens we may distinguish the substantia lentis and the 

 lens capsule. The lens is an epithelial structure formed from 

 the ectoderm. It consists in the beginning of cylindrical cells, 



FIG. 266. 



Capsule of len, 



Lens epithelium 



' "l 



I I 



I i 



I 'I 1 ' 1 



'ft. 



m 



! 



'''' .'! 



Lets fibres- 



Part of a meridional section through the border of an ape's lens. X 200. 



which during subsequent development increase in height at 

 the posterior surface of the lens. This increase goes on until 

 exceedingly long cells are formed, the lens fibres. 



In adults the substantia lentis consists of lens fibres, which 



