7. What conditions prevail at the cells to cause a movement of food and oxygen in one direction and of waste materials in the opposite direction? 8. What part does water play in the exchanges at the cells? 9. Show that the blood and the lymph together fulfill all the requirements of a nutrient cell fluid in the body. PRACTICAL WORK *To illustrate the Effect of Breathing upon the Flow of Lymph.*—Tightly holding one end of a glass tube between the lips, let the other end extend into water in a tumbler on a table. In this position quickly inhale air through the nostrils, noting that with each inhalation there is a slight movement of the water up the tube. (No sucking action should be exerted by the mouth.) Apply to the movements in the large blood and lymph vessels entering the thoracic cavity. *To illustrate Osmosis.*—1. Separate the shell from the lining membrane at one end of an egg, over an area about one inch in diameter. To do this without injuring the membrane, the shell must first be broken into small pieces and then picked off with a pair of forceps, or a small knife blade. Fit a small glass tube, eight or ten inches long, into the other end so that it will penetrate the membrane and pass down into the yolk. Securely fasten the tube to the shell by melting beeswax around it, and set the egg in a small tumbler partly filled with water. Examine in the course of half an hour. What evidence now exists that the water has passed through the membrane? 2. Tie over the large end of a "thistle tube" (used by chemists) a thin animal membrane, such as a piece of the pericardium or a strip of the membrane from around a sausage. Then fill the bulb and the lower end of the tube with a concentrated solution of some solid, such as sugar, salt, or copper sulphate. Suspend in a vessel of water so that the liquid which it contains is just on a level with the water in the vessel. Examine from time to time, looking for evidence of a movement in each direction through the membrane. Why should the movement of the water into the tube be greater than the movement in the opposite direction? (If the thistle tube has a very slender stem, it is better to fill the bulb before tying on the membrane. The opening in the stem may be plugged during the process of filling.) [Fig. 32] Fig. 32—An osmosometer. NOTE.—With a special piece of apparatus, known as an _osmosometer_, the principle of osmosis may be more easily illustrated than by the method in either of the above experiments (Fig. 32). This apparatus may be obtained from supply houses. CHAPTER VII - RESPIRATION