3. Examine a finger nail. Is the free edge or the root the thickest? Trim closely the thumb nail and the nail of the middle finger of one hand and try to pick up a pin, or other minute object, from a smooth, hard surface. The result indicates what use of the nails? Suggest other uses. 4. Examine with a microscope under a low power hairs from a variety of animals, as the horse, dog, cat, etc., noting peculiarities of form and surface. *To illustrate Cooling Effects of Evaporation.*—1. Wet the back of the hand and move it through the air to hasten evaporation. Observe that, as the hand dries, a sensation of cold is felt. Repeat the experiment, using ether, alcohol, or gasolene instead of the water, noting the differences in results. These liquids evaporate faster than water. 2. Wet the bulb of a thermometer with alcohol or water. Move it through the air to hasten evaporation. Note and account for the fall of the mercury. CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM *Coördination and Adjustment.*—If we consider for a moment the movements of the body, we cannot fail to note the coöperation of organs, one with another. In the simple act of whittling a stick one hand holds the stick and the other the knife, while the movements of each hand are such as to aid in the whittling process. Examples of coöperation are also found in the taking of food, in walking, and in the performance of different kinds of work. Not only is coöperation found among the external organs, but our study of the vital processes has shown that the principle of coöperation is carried out by the internal organs as well. The fact that all the activities of the body are directed toward a common purpose makes the coöperation of its parts a necessity. The term "coördination" is employed to express this coöperation, or working together, of the different parts of the body. A further study of the movements of the body shows that many of them have particular reference to things outside of it. In going about one naturally avoids obstructions, and if anything is in the way he walks around or steps over it. Somewhat as a delicate instrument (the microscope for example) is altered or adjusted, in order to adapt it to its work, the parts of the body, and the body as a whole, have to be _adjusted_ to their surroundings. This is seen in the attitude assumed in sitting and in standing, in the position of the hands for different kinds of work, in the variations of the circulation of the blood in the skin, and in the movements for protecting the body.(95) *Work of the Nervous System.*—How are the different activities of the body controlled and coördinated? How is the body adjusted to its surroundings? The answer is found in the study of the nervous system. Briefly speaking, the nervous system controls, coördinates, and adjusts the different parts of the body by fulfilling two conditions: