244 CHAPTER XVIII. 



the epithelium and soaked into the fibrous tissue, on the 

 surface of which it is reduced by the action of light. The 

 cells of the tissues will be found unstained. 



Silver nitrate is generally employed in solution in the 

 following manner : A 1 per cent, solution is taken, to which 

 two, three, or four volumes of water are added. The mode 

 of employment varies in its details according to circum- 

 stances, a point which is very important to observe. 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 with distilled water, in order to remove the 

 albuminates and white blood-corpuscles that are found on 

 its surface ; it is then washed with the solution of silver 

 nitrate. In order to obtain a powerful stain it is necessary 

 that this part of the operation be performed in direct sun- 

 light, or at least in a very brilliant light. As soon as the 

 tissue has become white, and 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, and the 

 forms of the cells would be disguised. 



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 

 \vas present, and these deposits might easily be mistaken, for 



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

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

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

 them. They will be found described and figured in Journ. Roy.Mic. Soc., 

 ii. 1879, i). 357, and in ROBIN'S Journ. dc VAnat., 1879, p. 54. They may 

 he obtained, in sots of various sizes (that of seven eighths of an inch being 

 the most convenient for 3x1 slides), of Burge & Warren, 42, Kirby 

 Street, Hatton Garden, London, E.G., price ten shillings the dozen pairs. 



This useful little apparatus has been re-invented by ETEENOD (Zeit. f. 

 irinx. j\lilc., iv, 1, 1887, p. 39), and is made according to his designs by 

 DiMnaurex, Bandagiste, Fusterie, Geneva (Switzerland). 



