398 CHAPTER 57. 



822. The Ketina or inner (ininy. 



The Jk'tiiui or iunor lining is mostly composed oi" a soft pulj)}' expansion 

 of the optic nerve, covering the interior siu-face of the globe. It is the 

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

 the inner and posterior i)art of the eye. The principal arteiy, which 

 sujiplies the internal jiarts of the eye, also enters through the same 

 foramen. 



823. The Conjunctival Memhrane. 



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 i>oi-tion of the sclerotic coat, the inner 

 surface of the eyelids, and the nuirgins of the cornea. Its e])ithelial layer 

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

 connecting the exterior \nnU of the eye and the eyelids. 



824. Interior Str2ictures of the Eye. 



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

 Aqueous, Vitreous, and Crystalline. 



825. The AqneoKS Humor. 



The Aqueous humor consists of clear slightly saline watery iluid which 

 fills u}) the space between the cornea and the crj'stalline 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 ever}^where lines the 

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

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

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

 ency it reachly permits the jmssage of the rays of light to the retina. 



826. The Vitreous humor. 



The Vitreous humor is a transparent fluid contained iu innuuicraMe 

 cells, formed by the hyaloid memlH-ane. In ai>j>carance it resembles jelly. 

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

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



827. The Cnjstalline 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 aijueous and vitreous 

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



