214 CHAPTEI? XVII. 



preservative reagents. With metallic impregnations the case 

 is different ; perfectly fresh tissues that is, such as are either 

 Hying, or at all events have not been treated by any reagent 

 whatever will also impregnate with the greatest ease and 

 precision. Indeed, some impregnations will not succeed at 

 all with tissues that are not fresh in the sense above 

 explained. 



Silver. 



351. Silver Nitrate: Generalities. The principles of its 

 employment are given by RANVIER (Traitc, p. 105) as 

 follows : 



Silver nitrate may be employed either in solution or in 

 the solid state. The latter method is useful for the study 

 of the cornea and of fibrous tissues, but is not suitable for 

 epithelia. For the cornea, 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 



* 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. Roy. Mir. 

 Soc., ii, 1879, p. 357, and in ROBIN'S Journ. de I'Anat., 3879, p. 54. 

 They may be obtained of Burge & Warren, 42, Kirby Street, H;ii<< n 

 Garden, London, E.G. 



