314 THE SENSES 



retinae, which enters the optic nerve just before it expands, 

 and, running in its substance, is distributed as far as the 

 ciliary processes anteriorly. 



The Crystalline Lens is a biconvex transparent body situ- 

 ated just behind the iris. Its function is to refract the rays 

 of light, and its action in this respect is similar to such 

 lenses in optical instruments. It is held in place by the sus- 

 pensory ligament. Its anterior convexity is more marked 

 than its posterior. It is enveloped by a thin transparent cap- 

 sule. 



The Suspensory Ligament is a continuation of the an- 

 terior layer of the hyaloid membrane of the vitreous humor. 

 When this layer reaches the edge of the lens (coming for- 

 ward) it divides into two parts, one passing in front of and 

 the other behind that body ; the divisions are continuous re- 

 spectively with the anterior and posterior portions of the cap- 

 sule of the lens. The ligament supports the lens. 



The Aqueous Humor is behind the cornea and in front of 

 the lens and suspensory ligament. The iris has been said to 

 separate this cavity into anterior and posterior chambers 

 communicating through the pupillary opening. The aqueous 

 humor is colorless and perfectly transparent. It serves to 

 refract the rays of light, having for that purpose the same 

 index as the cornea. 



The Vitreous Humor occupies about the posterior two- 

 thirds of the globe, and is back of the lens and suspensory 

 ligament surrounded by the delicate hyaloid membrane. It 

 is of a gelatinous consistence, and is divided into numerous 

 compartments by very delicate membranes radiating from 

 the point of entrance of the optic nerve. It is a transparent 

 refracting medium. 



Ocular Refraction. In order for the image of an object 

 to be distinct the rays passing from it must fall on a single 

 portion of the retina, viz., the fovea centralis. The sensi- 

 bility of the retina to light decreases in passing away from 

 the fovea. All rays would not meet on the retina unless they 



