428 NORMAI, HISTOLOGY AND ORGANOGRAPHY. 



disappear first. Then the outer molecular layer is 

 lost, so that the nuclear layers become confluent. 

 Ultimately but two rows of cells remain, which are 

 continued over the ciliary body as the pars ciliaris 

 retince, already mentioned in the description of the 

 ciliary body. The ora serrata forms a zigzag line 

 which marks the posterior border of the ciliary folds. 

 The iridica retina has already been described in 

 connection with the description of the iris. 



REFRACTING MEDIA. 



The refracting media of the eye are the cornea, 

 aqueous humor, lens, and the vitreous humor. The 

 cornea is described on another page. 



i. The aqueous humor is a structureless fluid re- 

 sembling lymph that fills the chamber of the eye in 

 front of the lens. The iris is suspended in this fluid 

 and divides the chamber into an anterior and a 

 posterior compartment. The fluid is largely a se- 

 cretion from epithelial cells, or, according to some, 

 from epithelial glands said to be located in the region 

 of the ciliary body. The aqueous humor is re- 

 placed if accidentally lost. 



2. The Lens. The origin of the lens has already 

 been described as an ectodermal invagination in the 

 form of a vesicle. The cells of the posterior wall of 

 this vesicle form the bulk of the lens. These cells 

 become long and slender and are known as the lens 

 fibers, while the cells of the anterior wall remain low 

 and cubical and form the anterior epithelium of the 

 lens. Surrounding the lens on all sides is the lens 

 capsule. This capsule is a homogeneous membrane, 



