1877] The Study of Zoölogy in Germany. 405 
round and form the veinlet which passes down the middle of the 
papilla, from top to bottom, in a nearly straight line, and some- 
times taking up fine branches on the way until it finally con- 
nects with the venous net-work of the cutis. 
This arrangement of the vessels is very characteristic ; similar 
ones occur elsewhere, where there are well-developed papillæ, as, 
for instance, on the tongue or in the intestine. But each organ 
presents characteristic peculiarities in the distribution of its blood- 
vessels, and to an experienced histologist the veins, capillaries, 
and arteries of the liver and kidney, ete., are as distinctive of 
each organ as is its general shape and appearance. 
As the presence of the valves does not permit us to inject the 
lymphatics from a large stem in the finer branches, asin the blood- 
vessels, a different method of forcing in the fluid has to be adopted. 
A small syringe with a very fine sharp point, such as is known 
among instrument-makers as a hypodermic syringe, must be used. 
The point is made to penetrate in the connective tissue, and the 
colored liquid— the best is a solution of Prussian blue — is forced 
out slowly and gently, and fills at first the cavities of the tissue 
and then the small lymphatics. These injections are difficult to 
make and by no means always succeed well. Perhaps the best 
place to try first is the interdigital web of the hind-foot of a frog, 
or the outer half, that is, the muscular part of the walls of the small 
intestine ; but the easiest of all to fill are the lymphatics of 
the dog’s testicles. When the injection has been once made in 
the way indicated, the tissue or organ may be hardened for cut- . 
ting either in chromic acid or in alcohol. 
Such, then, are some of the principal means employed to in- 
vestigate the microscopical structure of animals. They all have 
this much in common, that they are endeavors to render certain 
characters more visible than they are naturally. This we do 
whether we stain the nucleus, or inject the blood-vessels, or 
isolate single cells. It may well be added that a good knowl- 
edge of optics is necessary to a good histologist. 
The worker should also remember that American instruments 
are usually much less convenient and practical than the German 
and French microscopes, while the lenses are no better, though 
enormously more expensive. The writer personally likes Zeiss’s 
instruments very much. As this optician manufactures his ob- 
Jectives upon mathematical principles, he is able to make them 
all nearly alike ; but it must be understood that there are many 
others whose objectives are also of the best quality. At present 
