THE SENSES. 265 



nuation of that of the conjunctiva (this is the firmly applied conjunctiva cor- 

 nea) ; the posterior concave surface forms the anterior wall of the anterior 

 chamber of the eye : its posterior border, bevelled obliquely on the external 

 surface, is extremely firmly united with the Sclerotica. (According to Va- 

 lentin, both membranes grasp one another by indentations of looped and re- 

 flected fibres.) 

 The cornea consists of four layers : 



a. Epithelium on the anterior surface, forms after death a slimy covering, 

 which makes the cornea appear opaque (conjunctiva cornea). 



b. The proper cornea is laminated, and consists of flat cell fibres which in- 

 terlace in all directions, becoming in boiling water a white jelly and dissolved. 



c. The membrana Demoursii s. Descemetii, a firm cartilaginous, colourless, 

 and transparent thin membrane, which remains unchanged in boiling water ; 

 does not pass over upon the Iris, but passes over to its external border and 

 terminates between sclerotica and lig. ciliare with a sharp border. 



d. The pavement Epithelium on the posterior surface of the Demoursian 

 membrane reaches only as far as the external border of the Iris, and looks 

 into the anterior chamber of the eye (this is the so-called, membr. humoris 

 aquei). The cornea is nourished mediately only, by the aqueous humour with 

 which it is saturated. In the fetus a capillary rete is found under the ante- 

 rior epithelium, the branches of which come from the vessels of the conjunc- 

 tiva sclerotica; in the adult the 



Canalis Schlemmii (see before, 497), near to the border of the cornea, is 

 said to be a venous sinus, but it receives no branches from the cornea. 

 The layers, a, c, and d, possess no vessels ; in the inflammatory condition 

 of the eye such may be regarded as but only newly formed. Nerves : (disco- 

 vered by Schlemm in the eyes of animals) arise from the ciliary nerves on the 

 sclerotica, and are lost at the border of the cornea; Pappenheim has followed 

 them between the lamella. 



The cornea is more convex and thicker in the embryo and the newly born 

 than in the adult. Impinging rays of light passing through it become refracted 

 in a converging direction. 



Weber has classed the cornea with the Epidermic formations. According 

 to Arnold its substance passes immediately into that of the (fibrous) Sclero- 

 tica, and it bears all the characters of a serous membrane, as: transparency, 

 secretion of serum, vascular canals, formation of bloodvessels, regeneration; 

 the laminated structure is only an artificial production. 



Arnold admits of a second layer between sclerotica and choroidea ; this is 

 the 



Arachnoidea oculi, which is said to form a closed serous sac, and has 

 been already observed by Zinn, Wardrop, and others, the external por- 

 tion of which has been designated lamina fusca sclerotica, the internal 

 membr. suprachoroidea. Cases of a morbid collection of fluid (by Riolan, 

 Weller, Scarpa) are said to support this acceptation. According to 



