age into the ventricle.

The ventricular cavity should now be opened up as far as the aorta, and
the semilunar valves examined. Cut open the aorta, and notice the form
of the _semilunar valves_.

Experiment 202. _To show the circulation in a frog's foot_ (see
Fig. 78, p. 192). In order to see the blood circulating in the membrane
of a frog's foot it is necessary to firmly hold the frog. For this
purpose obtain a piece of soft wood, about six inches long and three
wide, and half an inch thick. At about two inches from one end of this,
cut a hole three-quarters of an inch in diameter and cover it with a
piece of glass, which should be let into the wood, so as to be level
with the surface. Then tie up the frog in a wet cloth, leaving one of
the hind legs outside. Next, fasten a piece of cotton to each of the two
longest toes, but not too tightly, or the circulation will be stopped
and you may hurt the frog.

Tie the frog upon the board in such a way that the foot will just come
over the glass in the aperture. Pull carefully the pieces of cotton tied
to the toes, so as to spread out the membrane between them over the
glass. Fasten the threads by drawing them into notches cut in the sides
of the board. The board should now be fixed by elastic bands, or by any
other convenient means, upon the stage of the microscope, so as to bring
the membrane of the foot under the object glass.

The flow of blood thus shown is indeed a wonderful sight, and never to
be forgotten. The membrane should be occasionally moistened with water.

Care should be taken not to occasion any pain to the frog.

Experiment 203. _To illustrate the mechanics of respiration_[58]
(see Experiment 122, p. 234). "In a large lamp-chimney, the top of which
is closed by a tightly fitting perforated cork (A), is arranged a pair
of rubber bags (C) which are attached to a Y connecting tube (B), to be
had of any dealer in chemical apparatus or which can be made by a
teacher having a bunsen burner and a little practice in the manipulation
of glass (Fig. 171). From the center of the cork is attached a rubber
band by means of a staple driven through the cork, the other end of
which (D) is attached to the center of a disk of rubber (E) such as
dentists use. This disk is held to the edge of the chimney by a wide
elastic band (F). There is a string (G) also attached to the center of
the rubber disk by