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 means of which the diaphragm may be lowered.

  [Illustration:  Fig. 171.]

Such is a description of the essentials of the model.  The difficulties encountered in its construction are few and easily overcome.  In the first place, the cork must be air-tight, and it is best made so by pouring a little melted paraffin over it, care being taken not to close the tube.  The rubber bags were taken from toy balloon-whistles.

In the construction of the diaphragm,