*Kinds of Solutions in the Body.*—The substances in solution in the body liquids are of two general kinds known as _colloids_ and _crystalloids_. The crystalloids are able to pass through membranous partitions, while the colloids are not. An example of a colloid is found in the albumin of an egg, which is unable to penetrate the membrane which surrounds it. Examples of crystalloids are found in solutions of salt and sugar in water. The inability of a colloid to penetrate a membrane is due to the fact that it does not form a true solution. Its particles (molecules), instead of being completely separated, still cling together, forming little masses that are too large to penetrate the membrane. Since, however, it has the appearance, on being mixed with water, of being dissolved, it is called a _colloidal solution_. The crystalloid substance, on the other hand, completely separates in the water and forms a _true solution_—one which is able to penetrate the partition or membrane. *Osmosis not a Sufficient Cause.*—The passage of materials through animal membranes, according to the principle of osmosis, is limited to crystalloid substances. But colloid substances are also known to pass through the various partitions of the body. An example of such is found in the proteids of the blood which, as a colloidal solution, pass through the capillary walls to become a part of the lymph. Perhaps the best explanation offered as yet for this passage is that the colloidal substances are changed by the cells lining the capillaries into substances that form true solutions and that after the passage they are changed back again to the colloidal condition. *Summary.*—Between the cells and the capillaries is a liquid, known as the lymph, which is similar in composition and physical properties to the blood. It consists chiefly of escaped plasma. The vessels 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.