THE CORNEA. 253 



salt solution. Then whilst the edge of the cut scle- 

 rotic is held by the one pair of forceps, with another 

 pair the iris is seized close to the same spot, and is 

 easily torn away from the sclerotic, bringing the lens 

 with it. So that only the cornea, together with a 

 narrow rim of sclerotic, is now left, and since it is 

 floating in fluid it retains its convexo-concave form, 

 and all crumpling of the tissue is avoided. The salt 

 solution is now poured ofl', leaving, however, just 

 enough to float the cornea in, arid the watch-glass is 

 filled up with one-half per cent, of chloride of gold 

 solution. The cornea is allowed to remain in this a 

 full hour; it is then removed to a beaker of water 

 acidulated with acetic acid, and is placed in a warm 

 place in the sunlight (see p. 96). After two days 

 the fluid in the beaker is renewed, a teaspoonful of 

 methylated spirit being added to prevent the growth 

 of fungi, and in two days more the cornea may be 

 taken out and prepared for the microscope. It is 

 first placed in a flat dish of distilled water, and the 

 epithelium, which is very dark and opaque, is gently 

 scraped off the anterior surface. This done, the 

 sclerotic rirn is cut ofl' with scissors. It is as well 

 to change the w r ater at this stage, so as to get rid of 

 the debris of epithelium. The next process consists 

 in the separation of the corneal substance into two, 

 three, or more thin lamellae. With a little practice 

 it is not at all difficult, thin as the object already 

 seems, by holding it down at one edge with a pair 

 of forceps and working from the same edge with an- 

 other pair, to separate a very thin lamina from the 

 concave posterior surface, consisting of the membrane 

 of Descemet and a delicate layer of the proper sub- 

 stance of the cornea with its corpuscles. This poste- 

 rior lamella is not only the easiest to obtain, but is 

 also, in the frog's cornea, the most important, for it 

 contains the closest and finest plexus of nerves. To 

 mount it, all that is necessary is to float it on to a 

 glass slide, to cover the preparation, and add glyce- 

 rine at the ed;e of the cover-glass. But. since the 

 99 



