346 HUMAN ANATOMY. 



Behind, and a little to the nasal side, where the optic nerve 

 enters, it presents a perforated appearance the lamina cribrosa 

 j the larger opening of which the porus opticus transmits the 

 arteria centralis retina?; the others transmit the ciliary vessels 

 and nerves. It is continuous in front with the cornea, over- 

 lapping it a little on its outer margin. 



In structure it is made up of white, fibrous tissue, with a 

 small quantity of elastic fibres, and connective-tissue corpuscles. 

 It probably contains no nerves. 



THE CORNEA is the convex, transparent, nearly circular 

 tissue forming the anterior one-sixth of the globe. It is from 

 one twenty-second to one thirty-second of an inch in thickness. 

 Its thickness at the periphery is 1.12 millimetres, hence its pos- 

 terior surface is more curved than the anterior. Its transverse 

 diameter is a little greater than the vertical, owing to the over- 

 lapping of the sclerotic above and below. It is composed of four 

 layers : 



1. Con junctival epithelium ; 



2. Cornea proper, { 



4! 



The conjunctival epithelium consists of several layers of 

 cells (columnar, polyhedral, and squamous) covering the ante- 

 rior surface of the cornea, continuous with the conjunctiva. 



The anterior elastic lamina is the name given to the outer 

 epithelial layer of the cornea proper. 



The cornea proper is made up of a transparent fibrous struc- 

 ture, identical with the sclerotic, and consisting of about sixty 

 layers or lamina, connected by a cement substance inclosing cor- 

 neal spaces, each of which contains a corneal corpuscle. 



The posterior elastic lamina consists of an elastic homo- 

 geneous membrane, internal to the proper structure of the cornea, 

 and constituting, with the epithelial lining, the membrane of Des- 

 cemet, or Demours. 



The structure of the cornea is non-vascular, being nourished 

 by channels representing lymphatic vessels, and continuous with 

 the corneal spaces. The nerves derived from the ciliary nerves 

 are numerous, and form between the outer surface of the cornea 

 proper and the epithelial covering the subepithelial plexus, from 

 which is given off the intraepitlielial plexus. 



The second tunic consists of the choroid, lining the sclerotic 

 coat throughout; the iris, the circular curtain suspended in the 



