Perhaps the most interesting of these objects is the circulation of the blood through the finer vessels of various parts of the animals made use of for these purposes, which parts, it is evident, must be very transparent to afford a perfect view of this phenomenon. The web of the frog’s foot is very frequently made use of, but requires a certain arrangement, which we will now describe. A piece of thin wood (Dr. Carpenter recommends _cork_) is taken, about eight inches long and three wide; about an inch from one end is cut a hole, half or three-quarters of an inch in diameter. The body of the frog is then placed in a wet bag, or wrapped in wet calico, whilst the hind-foot projects; the whole is then laid upon the piece of wood so that the foot, which is left free, may be extended over the hole. The web must then be spread out, and secured either by threads to small pins on the wood, or the pins must be driven through the web into the wood, and so kept in position. A few bands of tape must be passed round the body, the leg, and the wood, to prevent any disarrangement arising from the animal starting, &c. Care must be taken that the tape is not too tight, else the circulation will be very slow or altogether stopped. The wood must now be fixed upon the stage, with the aperture under the object-glass: this is sometimes done by simply binding it, or a spring is fixed so as to accomplish the same object without so much trouble. With a half-inch power the blood may now be seen to flow very distinctly. The frog may be used for hours if care is taken to prevent the web from becoming dry, by wetting it with a little water from time to time. The piece of wood or cork upon which the frog is laid is often made to give place to the “frog-plates,” supplied by opticians. These are made of brass, somewhat resembling the piece of wood above recommended, but each maker’s pattern differs according to his own taste. 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.