OEGANS OF SENSE. 173 



It is invested by a single layer of pavement cells, 

 which is reflected upon the anterior surface of the 

 iris, and ceases at the margin of the ptfpil (membrane 

 of Demoins, of Descemet, fig. III. 4, 5). 



There is no distinct line of demarcation, as inspec- 

 tion by the unassisted eye would lead us to suppose, 

 between the cornea and sclerotica. The two mem- 

 branes blend insensibly with each other (fig. I. 3). 

 The fibres of the sclerotica become rarified as they 

 approximate the corneal margin, and they can be 

 clearly seen to be continuous with the branches of its 

 plasmatic cells (fig. II. 1, 2). 



The sclerotica has but few vessels,- and hardly any vessels and 



' J J nerves. 



nerves. There are no blood-vessels in the cornea; 

 its network of plasmatic cells affords ample circulation 

 for the nutritive fluid. Its nerves are very numerous, 

 and form a rich web of filaments which are found 

 chiefly near its anterior surface (Kolliker) ; according 

 to some authorities they terminate by free extre- 

 mities. 



The second tunic of the eye-ball is formed poste- 

 riorly by the choroid, and in front by the iris. The 

 choroid coat lines the internal surface of the sclero- 

 tica, very accurately, and is continuous in part with 

 the iris, without any line of demarcation (PL XXVIII. 

 fig. I. 11, 12). It is loosely .connected by its external 

 surface to the sclerotica, by means of the ciliary ves- 

 sels and nerves, and an occasional very delicate fasci- 

 culus of fibrous tissue ; its brownish-black color is 

 explained by the presence of a layer of irregularly 

 branching cells filled with pigment granules (PL II. 

 fig. II. 1, 2, 3). Its internal surface is likewise covered 



