132 PHYSIOLOGY AND TEMPERANCE. 



8. The Crystalline Lens. As may be seen in a section 

 of the eye-ball, the lens is placed in the front part of the 

 globe. It is a flattened, double-curved body, with the flatter 

 side in front, resembling two watch crystals of different 

 curves placed back to back. It consists of layer upon layer 

 of a soft jelly-like substance, the inner layers being more 

 dense than the outer. It is surrounded by a delicate cover- 

 ing, which fixes it to the inside of the eye all round. Both 

 the lens and its covering are transparent. 



The lens not only allows the rays of light to pass through, 

 but it draws them nearer together, that when they reach the 

 retina they may be brought to a point, or focus. In this way 

 all the rays of light coming from the different parts of an 

 object, or all the objects in the field of our observation when 

 we look out, are brought to a focus on different parts of the 

 retina, and clearly picture there a miniature of all that is 

 before the eye. The optic nerve carries this impression to 

 the brain and we see. 



9. The Vitreous Humor. The greater part of the 

 interior of the eye-ball is filled with the vitreous humor. 

 It is a thin, jelly-like substance, made up mainly of water, 

 with a small quantity of albumen and salts. This liquid is 

 contained in compartments formed by the walls of a delicate 

 membrane. (Fig. 40.) 



10. The Aqueous Humor. The space between the 

 cornea and the lens is filled with the aqueous humor. This 

 space is partly divided into two compartments by the iris. 

 The aqueous humor is more watery than the vitreous. 



11. The Iris. Floating in the aqueous humor is a 

 circular curtain or disc, called the iris, from its resembling 

 a rainbow in its many colors. It has an opening in its 

 centre, the pupil. The iris is made up chiefly of muscular 

 fibres, and has a background of coloring matter. The mus- 

 cular fibres are arranged in two layers. In one the fibres 



