it is to be remembered that it also must be air-tight, and in order to resemble the human diaphragm, it must have a conical appearance when at rest.  In order to avoid making any holes in the rubber, the two attachments (one of the rubber band, and the other of the string) were made in this wise:  the rubber was stretched over a button having an eye, then under the button was placed a smaller ring from an old umbrella; to this ring was attached the rubber band, and to the eye of the button was fastened the operating string.  When not in use the diaphragm should be taken off to relieve the strain on the rubber band.”

Experiment 204. To illustrate the action of the intercostal muscles (see sec. 210).  The action of the intercostal muscles is not at first easy to understand; but it will be readily comprehended by reference to a model such as that represented in Fig. 172.  This maybe easily made by the student himself with four laths of wood, fastened together at the corners, A, B, C, D, with pins or small screws, so as to be movable.  At the points E, F, G, H, pins are placed, to which elastic bands may be attached (A).  B D represents the vertebral column; A C, the sternum; and A B and C D, the ribs.  The elastic band F G represents the external intercostal muscles, and E H, the internal intercostals.

  [Illustration:  Fig. 172.]

If now the elastic band E H be removed, the remaining band, F G, will tend to bring the two points to which it is attached, nearer together, and the result will be that the bars A B and C D will be drawn upwards (B), that is, in the same direction as the ribs in the act of inspiration.  When the elastic band E H is allowed to exert its force, the opposite effect will be produced (C); in this case representing the position of the ribs in an act of expiration.

Experiment 205.  Pin a round piece of bright red paper (large as a dinner-plate) to a white wall, with a single pin.  Fasten a long piece of thread to it, so it can be pulled down in a moment.  Gaze steadily at the red paper.  Have it removed while looking at it intently, and a greenish spot takes its place.

Experiment 206.  Lay on different parts of the skin a small, square piece of paper with a small central hole in it.  Let the person close his eyes, while another person gently touches the uncovered piece of skin with cotton wool, or brings near it a hot body.  In each case ask the observed person to distinguish between them.  He will always succeed on the volar side of the hand, but occasionally fail on the dorsal surface of the hand, the extensor surface of the arm, and very frequently on the skin of the back.

Experiment 207. Wheatstone’s fluttering hearts.  Make a drawing of a red heart on a bright blue ground.  In a dark room lighted by a candle hold the picture below the level of the eyes and give it a gentle to-and-fro motion.