46 NORMAL HISTOLOGY. 



is now held for an instant over a jet of steam, when the 

 epithelium will become white or milky in appearance, 

 and can be readily scraped off by passing the blade of a 

 scalpel lightly over the surface. It is necessary to remove 

 the epithelium in order that the silver solution may have 

 ready access to the connective-tissue substance of the 

 cornea ; and the advantage of steaming is, that it can be 

 scraped off without the use of much force, which would 

 disturb the relations of the parts beneath. The epithe- 

 lium being thus removed, a five-per-cent. solution of 

 silver nitrate is allowed to flow over the cornea and re- 

 main for two or three minutes in contact with it. By 

 this treatment the whole cornea becomes opaque and 

 stiff. The silver is now neutralized, and its further action 

 prevented, by washing the cornea with a one-per-cent. 

 salt solution. It is now carefully excised and placed in 

 a dish containing a mixture of alcohol and water, one to 

 two, and exposed to direct sunlight, or bright daylight, 

 for from a few minutes to half an hour, depending upon 

 the intensity of the light. When it has become brown it 

 is to be laid in glycerin and stripped into thin layers, as 

 directed above, for the gold cornea ; the layers are 

 mounted in glycerin. 



If the preparation is successful, clear, branching, com- 

 municating spaces are seen on a yellowish or brown 

 ground. These spaces, although larger, evidently corre- 

 spond in position and in a general way, in shape, to the 

 cells in the cornea, as seen after treatment with chloride 

 of gold ; and if a specimen thus prepared be stained with 

 haematoxylin, the corneal cells will be seen lying within 

 them. 



