CHAPTER XVII. 199 



for instance, proceed as follows : The eye having been removed, a 

 piece of silver nitrate is quickly rubbed over the anterior surface of 

 the cornea, which is then detached and placed in distilled water ; 

 it is then brushed with a camel's hair brush in order to remove the 

 epithelium. The cornea is then exposed to the action of light. It 

 will be found that the nitrate has traversed the epithelium and soaked 

 into the fibrous tissue, on the surface of which it is reduced by the 

 light. The cells of the tissues will be found unstained. 



It is generally employed in solution, in the following manner : ' 

 In the case of a membrane, such as the epiploon, the membrane must 

 be stretched like a drum-head over a porcelain dish, * and washed first 

 with distilled water, and then washed with a solution of silver nitrate. 

 In order to obtain a powerful stain it is necessary that this part of 

 the operation be performed in direct sunlight, or at least in a very 

 brilliant light. As soon as the tissue has begun to turn of a blackish 

 grey the membrane is removed, washed in distilled water, and 

 mounted on a slide in some suitable examination medium. 



If the membrane were left in the water the cells would become 

 detached, and would not be found in the finished preparation. 



If the membrane had not been stretched as directed the silver 

 would be precipitated not only in the intercellular spaces, but in all 

 the small folds of the surface. 



If the membrane had not been washed with distilled water before 

 impregnation there would have been formed a deposit of silver on 

 every spot on which a portion of an albuminate was present, and 

 these deposits might easily be mistaken for a normal structure of the 

 tissue. It is thus that impurities in the specimen have been described 

 as stomata of the tissue. 



If the solution be taken too weak for instance, 1 : 500 or 1 : 1000, 

 or if the light be not brilliant a general instead of an interstitial 

 stain will result ; nuclei will be most stained, then protoplasm, and 

 the intercellular substance will contain but very little silver. In 

 general in a good " impregnation " the contents of the cells, and 

 especially nuclei, are quite invisible. 



The tissues should be constantly agitated in the silver-bath in 



* The Hoggans Histological Rings will be found much more con- 

 venient. They are vulcanite rings made in pairs, in which one ring just 

 fits into the other, so as to clip and stretch pieces of membrane between 

 them. They will be found described and figured in Journ. Eoy. Mic. 

 Soc., ii, 1879, p. 357, and in ROBIN'S Journ. de VAnat., 1879, p. 54. 

 They may be obtained of Burge & Warren, 42, Kirby Street, Hatton 

 Garden, London, E.G. 



