_Of Injecting Different Systems of Vessels with Different Colours._--It is often desirable to inject different systems of vessels distributed to a part with different colours, in order to ascertain the arrangement of each set of vessels and their relation to each other. A portion of the gall-bladder in which the veins have been injected with white lead, and the arteries with vermilion, forms an attractive preparation. Each artery, even to its smallest branches, is seen to be accompanied by two small veins, one lying on either side of it. By this method four different sets of tubes have been injected--the artery with vermilion, the portal vein with white lead, the duct with Prussian blue, and the hepatic vein with lake. There are also opaque colouring matters which may be employed for double injections. _Injecting the Lower Animals._--The vessels of fishes are exceedingly tender, and require great caution in filling them. It is often difficult or quite impossible to tie the pipe in the vessel of a fish, and it will generally be found a much easier process to cut off the tail of the fish, and put the pipe into the divided vessel which lies immediately beneath the spinal column. In this simple manner beautiful injections of fish may be made. _Mollusca_ (slug, snail, oyster, &c.).--The tenuity of the vessels of the mollusc often renders it impossible to tie the pipe in the usual manner. The capillaries are, however, usually very large, so that the injection runs very readily. In different parts of the bodies of these animals are numerous lacunæ or spaces, which communicate directly with the vessels. Now, if an opening be made through the integument of the muscular foot of the animal, a pipe may be inserted, and thus the vessels may be injected from these lacunæ with comparative facility. _Insects._--Injections of insects may be made by forcing the injection into the general abdominal cavity, when it passes into the dorsal vessel and is afterwards distributed throughout the system. The superfluous injection is then washed away, and such parts of the body as may be required removed for examination.