SENSE OF SIGHT. 



425 



The transparent cornea is formed of a basement membrane 

 of collagenous tissue (see Fig. 23, p. 96), covered, both in front 

 and behind, with an epithelial layer; that of the posterior sur- 

 face is simple (membrane of Demours or of Descemet) ; that 

 of the anterior surface resembles the epithelium of the con- 

 junctival mucous layer, which is itself continuous with the skin 

 and the epidermis; thus superficial diseases of the cornea are 

 closely connected with diseases of the skin or epidermis. The 

 aqueous or watery humor is contained in the anterior cham- 

 ber (where we shall presently study an appendage of the 

 choroid, called the iris), between the posterior surface of the 

 cornea, and the anterior surface of the crystalline lens ; this 

 fluid closely resembles water, containing in solution a small 

 quantity of albumen and salts, and is secreted by the mem- 

 brane of Demours (membrane of the aqueous humor). 



The crystalline lens consists of an exterior membrane, 

 called the capsule of the crystalline lens, and an inner sub- 

 stance, called the body of the crystalline. The capsule is an 

 amorphous and very elastic tissue ; if an incision is made into 

 its substance, it retracts and expels its contents (as in the 

 operation for cataract) ; its whole inner surface is cov- 

 ered with cells, from which the con- 

 tents, or body of the crystalline lens 

 may be reproduced. This body, in- 

 deed, is formed of prismatic elements, 

 arranged regularly in concentric 

 layers (Fig. 112), and which are pro- 

 duced by the metamorphosis of cells; 

 the study of embryology shows the 

 germ from which the crystalline pro- 

 ceeds to be an offshoot from the epi- 

 dermis (Fig. 113), which, finally, 

 becomes detached and remains alone 

 in the centre of the eyeball or ocular 

 globe. The layer of cells lining the inner surface of the cap- 

 sule thus corresponds to the layer of Malpighi in the skin : 

 the crystalline lens is developed by means of this layer, in 

 which are constantly found the zones of young cells in the 

 course of transformation into prisms. 



* This figure shows the regular disposition of the prisms of the crystalline 

 lens, the extremities of which are joined together on both surfaces, the "point at 

 which they meet forming a sort of three-cornered star ; thus a lens which has 

 been hardened, either by the effect of heat or of chemical reagents, generally 

 cracks in this stellate form, corresponding with the lines shown above. 



Fig. 112. Disposition of 

 the nbres of the lens.* 



