that contain it
are connected with the system for the circulation of the blood. By adding
new material to the lymph and withdrawing waste material from it, the
blood keeps this liquid in a suitable condition for supplying the needs of
the cells. Supplementing each other in all respects, the blood and the
lymph together form the nutrient cell fluid of the body. The interchange
of material between the blood and the lymph, and the lymph and the cells,
takes place in part according to the principle of osmosis.

*Exercises.*--1. Explain the necessity for the lymph in the body.

2. Compare lymph and water with reference to density, color, and
complexity of composition.

3. Compare lymph and blood with reference to color, composition, and
movement through the body.

4. Show how blood pressure in the capillaries causes a flow of the lymph.

5. Show how contracting muscles cause the lymph to move. Compare with the
effect of muscular contraction upon the blood in the veins.

6. Trace the lymph in its flow from the right hand to where it enters the
blood; from the feet to where it enters the blood.

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 tu