252 Life and Health 



The portion behind the iris forms the posterior chamber 

 and contains the crystalline lens and the vitreous humor. 



The vitreous humor fills about four-fifths of the eyeball, 

 and serves to hold the choroid and the retina in position 

 and to maintain the proper relations of the inner structures 

 of the eye. 



The iris consists of a framework of connective tissue, the 

 surface of which is lined by cells containing pigment, which 

 gives color to the eye. Bundles of involuntary muscular 

 fibers are found in the substance of the iris, which are 

 arranged to allow the pupil to contract or dilate more or 

 less widely. 



* 394. The Crystalline Lens. Just behind the pupil and 

 close to the iris is a semisolid, doubly convex body, called 

 the crystalline lens. It is shaped like a magnifying glass, 

 convex on each side, but with the posterior surface more 

 convex than the anterior. In health it is perfectly clear and 

 transparent, and highly elastic. When the lens becomes 

 opaque, from change in old age, or from ulcers or wounds, 

 we have the disease known as cataract. 



The crystalline lens is not placed loosely in the eyeball, 

 but is enclosed in a transparent and elastic capsule sus- 

 pended throughout its circumference by a ligament called 

 the suspensory ligament. This ligament not only retains the 

 lens in place, but is capable of altering its shape. 

 V 395. The Ciliary Muscle. All around the edge, where 

 the cornea, sclerotic, and choroid meet, is a ring of invol- 

 untary muscular fibers, forming the ciliary muscle. When 

 these fibers contract, they draw forwards the attachment 

 of the suspensory ligament of the lens, the pressure of 

 which on the lens is consequently diminished. The elas- 

 ticity of the lens causes it at once to bulge forwards, and 

 it becomes more convex. 



