The tongue of the frog is also sometimes used for the purpose of showing the circulation of the blood, which is done in the following manner:--The body is wrapped with the calico, and made fast to the plate as before, only the _mouth_ of the frog is brought to the opening. The tongue is then gently drawn out of the mouth and pinned down over the aperture, when the circulation will be well shown. But, as Dr. Carpenter observes, the cruelty of this mode of treatment is so repulsive that it is unjustifiable.
Tadpoles of the frog (which, of course, are only obtainable in their season) are good subjects for showing the circulation of the blood. They are best suited for the microscope when about one inch long. The tadpoles of the newt and toad also are equally suitable. They may be placed in a very shallow glass trough with a little water, and a narrow band of linen bound lightly round in some part not required for examination, to keep them from moving; or they may be laid upon a glass plate with a drop or two of water, and a thin glass covering lightly bound upon it. Dr. Carpenter, however, places them first in cold water, gradually adding warm until the whole becomes about 100°, when the tadpole becomes rigid, whilst the circulation is still maintained. I have not, however, found this necessary, the thin glass accomplishing all that is desired. The tail is generally the most transparent, and shows the circulation best; but in some of the newt larvæ the blood may be traced down to the very extremities if they are not _too old_. Mr. Whitney places the tadpole upon its back, by which means the heart and other internal arrangements may be seen.
Amongst fishes also may be found subjects for the same purpose, but they seldom furnish as good examples as those before mentioned, because the blood-vessels are not nearly so abundant, as in the foot of the frog, &c. The stickleback is, however, procurable almost in any place during the summer months, and may be laid in a shallow trough, loosely bound down as the tadpole. The tail may be covered with a piece of thin glass to prevent him curling it to the object-glass. The power needed with this will be about the same as with the other subjects--viz., half to quarter inch object-glass.