The body is an organization of different kinds of cells; it grows through the growth and reproduction of these cells; and its life as a whole is maintained by providing such conditions as will enable the cells to keep alive. Of chief importance in the work of the body is a nutrient fluid which supplies the cells with food and oxygen and relieves them of waste. A moving portion of this fluid, called the blood, serves as a transporting agent, while another portion, called the lymph, passes the materials between the blood and the cells. Through their effects upon the blood and the lymph, the organs of circulation, respiration, digestion, and excretion minister in different ways to the cells, and aid in the maintenance of life. By their combined action two distinct movements are kept up in the body, as follows: 1. An _inward_ movement which carries materials from the outside of the body toward the cells. 2. An _outward_ movement which carries materials from the cells to the outside of the body. Passing _inward_ are the oxygen and food materials _in a condition to unite with each other_ and thereby change their potential into kinetic energy. Passing _outward_ are the oxygen and the elements that formed the food materials _after having united_ at the cells and liberated their energy. As a final and all-important result, there is kept up a _continuous series of chemical changes_ in the cells. These liberate the energy, provide special substances needed by the cells, and preserve the life of the body (Fig. 92). In the chapters which follow, we are to consider the problem of adjusting the body to and of bringing it into proper relations with its surroundings. PART II: MOTION, COORDINATION, AND SENSATION CHAPTER XIV - THE SKELETON One necessary means of establishing proper relations between the body and its surroundings is _motion_.(77) Not only can the body move itself from place to place, but it is able to move surrounding objects as well. In the production of motion three important systems are employed—the muscular system, the nervous system, and a system of mechanical devices which are found mainly in the skeleton. The muscular system supplies the energy for operating the mechanical devices, while the nervous system controls the movements.(78) Although the skeleton serves other purposes, such as giving shape to the body and protecting certain organs, its main use is that of an aid in the production of motion.