cotton through the other part to represent the spinal cord.  Push backwards with the forefinger with just enough force to break the toothpick and drive its fragments on to the cotton.

Experiment 200. To illustrate how the pulse-wave is transmitted along an artery. Use the same apparatus as in Experiment 106, p. 201.  Take several thin, narrow strips of pine wood.  Make little flags by fastening a small piece of tissue paper on one end of a wooden toothpick.  Wedge the other end of the toothpick into one end of the strips of pine wood.  Use these strips like levers by placing them across the long rubber tube at different points.  Let each lever compress the tube a little by weighting one end of it with a blackboard eraser or book of convenient size.

  As the pulse-wave passes along under the levers they will be
  successively raised, causing a slight movement of the tissue-paper
  flags.

Experiment 201. The dissection of a sheep’s heart. Get a sheep’s heart with the lungs attached, as the position of the heart will be better understood.  Let the lungs be laid upon a dish so that the heart is uppermost, with its apex turned toward the observer.

  The line of fat which extends from the upper and left side of the heart
  downwards and across towards the right side, indicates the division
  between the right and left ventricles.

Examine the large vessels, and, by reference to the text and illustrations, make quite certain which are the aorta, the pulmonary artery, the superior and inferior venae cavae, and the pulmonary veins.

  Tie variously colored yarns to the vessels, so that they may be
  distinguished when separated from the surrounding parts.

Having separated the heart from the lungs, cut out a portion of the wall of the right ventricle towards its lower part, so as to lay the cavity open.  Gradually enlarge the opening until the chordae tendineae and the flaps of the tricuspid valve are seen.  Continue to lay open the ventricle towards the pulmonary artery until the semilunar valves come into view.

  The pulmonary artery may now be opened from above so as to display the
  upper surfaces of the semilunar valves.  Remove part of the wall of the
  right auricle, and examine the right auriculo-ventricular opening.

The heart may now be turned over, and the left ventricle laid open in a similar manner.  Notice that the mitral valve has only two flaps.  The form of the valves is better seen if they are placed under water, and allowed to float out.  Observe that the walls of the left ventricle are much thicker than those of the right.

  Open the left auricle, and notice the entrance of the pulmonary veins,
  and the passage into the ventricle.