820 THE APPARATUS OF THE SENSES. 



It is composed of a membrane that becomes slightly fibrous at the 

 periphery of the cornea, where it forms, in passing on to the iris, the 

 pectinated ligament. It has also a covering of polygonal epithelial cells, 

 which are provided with a large nucleus. 



(Some authorities give five layers to the cornea, the first being the 

 conjunctival. The second consists of a very elastic tissue, perfectly 

 structureless, and possessing a remarkable tendency to curl up; while 

 boiling, or the action of acids, does not render it opaque as with the other 

 layers ; very fine fibres pass obliquely between it and the next layer the 

 cornea proper consisting of a large number of strata with branched 

 fusiform cells. The fourth layer is also elastic and like the second, though 

 thinner. The fifth layer consists of the epithelial cells already mentioned. 

 Wilson says that the opacity of the cornea produced by pressure on the 

 globe, results from the infiltration of fluid into the areolar tissue connecting 

 its layers, and that this appearance cannot be produced in a sound living 

 eye, although a small quantity of serous fluid (liquor corneca) is said to 

 occupy the spaces in that tissue.) 



Vessels. The cornea has but little vascularity. The vessels form loops 

 around its borders, and in the Sheep they advance to near the middle of its 

 surface. 



Nerves. These were discovered by Schlemm. They penetrate by the 

 periphery of the cornea, and form a network on its surface. According to 

 Kiihne, Hoyer, and Conheim, the ultimate nervous ramifications pass into 

 the epithelium of the anterior face, and arrive between the most superficial 

 cells. 



3. The Choroid Coat. (Fig. 383 c.) 



(Preparation. If the cornea has not yet been removed, it and the sclerotic may now 

 be dissected away from the choroid or second tunic. The connections between them are 

 closest at the circumference of the iris, and at the entrance of the optic and ciliary nerves 

 and arteries. Fine blunt-pointed scissors are necessary. A small portion* of the 

 sclerotic, near its anterior circumference, is pinched up and clipped off, the edge of the 

 incision is raised, the circumference of the sclerotic divided, and that tunic removed 

 piecemeal; a gentle pressure with the edge of the knife will remove it from its 

 attachments around the circumference of the iris. This dissection is best conducted 

 under water. The ciliary nerves and long ciliary arteries will be seen passing forward, 

 between the sclerotic and choroid, to the iris.) 



The choroid is a thin, dark-coloured membrane spread over the inner 

 face of the sclerotic, whose general conformation it repeats. It is divided 

 into two zones by the ora serrata a denticulated line which corresponds to 

 the point where the retina changes its characters. 



Posterior or choroid zone. Throughout the extent of this zone the 

 choroid is uniformly thin, and corresponds, by its external face, to the 

 sclerotic ; by its internal face, it is in contact with the retina, but does not 

 adhere to it. Posteriorly, it shows an opening through which the optic 

 nerve passes. In front, at the anterior opening of the sclerotic, it is 

 continuous with the anterior zone. 



The inner face of the choroid is not uniform in colour, being perfectly 

 black in the lower part of the eye; this is abruptly terminated at a 

 horizontal line that passes about the 8th or 9th part of an inch above the 

 optic papilla. From this line, on the segment of a circle from T 4 to ^ of 

 an inch in height, it shows most brilliant colours : at first blue, then an 

 azure-blue, afterwards a brownish-blue, and after this the remainder of the 

 eye is occupied by an intense black. The bright portion is the tapetum. 



Anterior or ciliary zone. This includes two parts: the " ciliary circle 



