108 Histology and Embryology. By C. S. Minot. 
injected human lip may be taken. The skin proper is penetrated 
by papillae sent up from the underlying connective tissue, known in 
anatomy as the cutis, and carrying the blood-vessels. There is a 
network of small arteries in the cutis, and from this there pass up 
from three to five fine branches into each papilla, and form by 
division and intercommunication a wide capillary network. One 
or several fine capillaries bend round, and form the veinlet which 
passes down the middle of the papilla, from top to bottom, in a 
nearly straight line, and sometimes taking up fine branches on the 
way until it finally connects with the venous network of the cutis. 
This arrangement of the vessels is very characteristic ; similar 
ones occur elsewhere, where there are well-developed papillae, 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, &c., 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, as in 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 
coloured 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 cutting 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 endeavours 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 knowledge 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 
