398 CHAPTER 57. 



822. The Retina or inner lining. 



The Retina or inner lining is mostly composed of a soft pulpy expansion 

 of the optic nerve, covering the interior surface of the globe. It is the 

 essential part of the organ of vision. The nerve enters by a foramen at 

 the inner and posterior part of the eye. The principal artery, which 

 supplies the internal parts of the eye, also enters through the same 

 foramen. 



823. The Conjundival Membrane. 



To the above three coats may be added a fourth, called the Conjunc- 

 tiva, which has been alluded to above. It is a thin delicate mucous 

 membrane covering the anterior portion of the sclerotic coat, the inner 

 surface of the eyelids, and the margins of the cornea. Its epithelial layer 

 is continued over the rest of the cornea. Its name is derived from its 

 connecting the exterior parts of the eye and the eyelids. 



824. Interior Structures of the Eye. 



The interior structures of the eye consist of three humors, namely the 

 Aqueous, Vitreous, and Crystalline. 



825. The Aqueous Humor. 



The Aqueous humor consists of clear slightly saline watery fluid which 

 fills up the space between the cornea and the crystalline lens. In this 

 humor the free border of the Iris floats. The space occupied by the 

 aqueous humor in front of the iris is termed the anterior, while that in 

 the rear of it is termed the posterior chamber of the eye. This humor 

 is secreted by an extremely fine membrane, which everywhere lines the 

 wall of these chambers. It forms a liquid medium in which the iris can 

 float and carry on its peculiar duties, hereafter to be detailed. It also 

 assists in maintaining the convexity of the globe, whilst by its transpar- 

 ency it readily permits the passage of the rays of light to the retina. 



826. The Vitreous humor. 



The Vitreous humor is a transparent fluid contained in innumerable 

 cells, formed by the hyaloid membrane. In appearance it resembles jelly. 

 It lies posterior to the aqueous humor and crystalline lens, and fills up 

 the remainder or about four fifths of the bulk of the ocular globe. 



827. The Crystalline Lens. 



The Crystalline humor or Lens, so called from its form and high re- 

 fracting power is a circular biconvex body, having its greater convexity 

 on the posterior side, highly transparent and of firm consistence, im- 

 bedded behind the iris and immediately between the aqueous and vitreous 

 humors. It is enclosed within a capsule of its own, and is connected with 



