212 Histology of Minute Blood-vessels. [Wins Oct 1 isto, 
part of the picture; a, a, are the nuclei of the vascular epithelium ; 
b, b, those of the muscle ; ¢, ¢, those of the connective tissue. 
The foregoing description of individual photographs will serve 
to give a correct idea of the epithelium lining the small arteries, 
veins, and capillaries, as shown in a considerable number of pre- 
parations preserved in the Museum, and as observed by me many 
times in tissues extemporaneously prepared. Both Balogh and 
Feltz would seem to have been singularly unfortunate in their silver 
stainings, for they describe the appearances produced as irregular 
and contradictory. Balogh explains the black lines he occasionally 
saw in the vessels after silver injections as due to the precipitate of 
silver occurring preferably on folds in the ining membrane, caused 
by the irregular shrinkage of the vessel produced by the silver injec- 
tion, an error readily corrected by combining gelatine with the 
solution of silver injected ; the vessels are thus equably and smoothly 
distended, yet the epithelium appears mapped out as usual. 
Feltz asserts that if a solution of silver be allowed to dry in the 
light on a collodium film, irregular black lines are produced, quite 
like those observed after its action on organic membranes. I myself 
have examined the irregular figures produced by this experiment, 
and cannot conceive how anyone accustomed to the precise study 
of organic forms can see any similarity between them and the defi- 
nite outlines produced by the action of silver solution on epithelial 
surfaces. 
Besides the preparations exhibiting the vascular epithelium which 
have been described, the Museum possesses, as I have already men- 
tioned, a number in which the epithelium of the skin, of the lymph- 
sacs of frogs, of the peritoneum and of other surfaces, are mapped 
out by silver staining, and the reagent is continually employed by 
myself and my assistants in the investigation of such surfaces. It 
is impossible for anyone who has had such opportunities for observa- 
tion, to avoid being struck by the fact that the outlines obtained 
have a definite form and character for each tissue. It is true that 
the silver staining does not succeed as frequently as carmine stain- 
ing does, that its use requires more skill and that failures are more 
frequent. Sometimes too much action takes place and everything 
is obscured by the black precipitate produced; sometimes, either 
because the tissues are not fresh, or the light not sufficient, or from 
some unexplained reason, the solution does not act at all; but the 
forms above described as characteristic of the arteries are never 
observed in the veins, never do the outlines produced on the surface 
of the skin resemble those seen on the peritoneum, in the lymph- 
sacs, or in the vessels ; each membrane permits only the formation 
of its own characteristic outlines, never of those belonging to another 
tissue; moreover, in all cases where it is possible to observe the 
shape of the epithelial cells without the use of reagents, or to isolate 
