140 



HUMAN PHYSIOLOGY. 



The Crystalline lens is situated immediately behind the pupil, in the 

 concavity of the vitreous humor. It is inclosed in a highly elastic, trans- 

 parent membrane, the capsule. The lens is a transparent, double- convex 

 body, y of an inch transversely, * of an inch antero-posteriorly ; it is 

 held in position by the suspensory ligament, formed by a splitting of the 

 hyaloid tunic, the external layer of which passes in front of the lens, the 

 internal layer behind it. Its function is to aid in refracting the rays of 

 light and bring them to a focus upon the retina. 



FIG. 16. 



DIAGRAM OF A VERTICAL SECTION OF THE EYE. 



i. Anterior chamber filled with aqueous humor. 2. Posterior Chamber. 3. Canal of 



Petit. 

 a. Hyaloid membrane. b. Retina (dotted line). c, Choroid coat (black line). 



d. Sclerotic coat. e. Cornea, f. Iris. g. Ciliary processes, h. Canal of Schlemm 



or Fontana. z. Ciliary muscle. {From Holden' s Anatomy.) 



Vision. The eye may be regarded as a camera obscura, in which 

 images of external objects are thrown upon a screen, the retina, by means 

 of a double convex lens. 



The Essential Conditions for proper vision are : I. Certain refract- 

 ing media, <?. g., cornea, aqueous humor, and crystalline lens, by which 

 the rays of light are so disposed as to form an image. 2. A diaphragm^ 

 the iris, which, by alternately contracting and dilating, increases or dimin- 

 ishes the amount of light entering the eye. 3. A sensitive surface, to 



