The liver forms the crux of the situation. (See Diagram 37.) A vein carrying blood from the intestine and spleen is broken up into fine capillaries to pass through that organ, and the pressure in this vein is extremely low. How is a sufficiently rapid flow of blood to be maintained? The answer to this riddle is best given by Diagram 38, which shows how, by the contraction of the diaphragm at each breath, the large veins entering the heart are subjected to a negative pressure which draws blood out of the liver, while, simultaneously, that organ is squeezed and the blood it contains forced out. Obviously this natural pump influences not only the flow of blood, but also that of the lymph, and what was said about the hepatic vessels also holds good for the thoracic duct, up which the lymph, rich with fat absorbed from the intestine, passes to be emptied into the large veins near the heart. So, though vigour in the action of the diaphragm is more favourable to health than necessary to life, deep breathing is an essential factor in the well-being of the body. [Illustration: DIAGRAM 37.—A DIAGRAMMATIC VIEW OF THE CIRCULATION THROUGH THE ORGANS UPON WHICH THE DIAPHRAGM PRESSES WHEN IT DESCENDS.] [Illustration: DIAGRAM 38.—ILLUSTRATING THE INFLUENCE OF THE DIAPHRAGM UPON THE CIRCULATION THROUGH THE VISCERA.] VII. All the movements as yet described are absolutely necessary to the continuation of life; they are, moreover, independent of the efforts of the will. But there remains yet another kind of movement without which the body, left to itself, would die. This is the movement of the limbs—organs by which the body is able to move from one place to another, to capture its food and convey it, viâ the mouth, to its stomach—in a word, to satisfy its chemical and physical needs. To understand how the limbs work requires a knowledge of their anatomy, for which we have not time or space here; but the principle throughout is that of a system of levers, the bones, worked by the voluntary muscles. Here, as before, a diagram will probably be found to convey more than could ever be expressed in words. (See Diagram 39.) The diagram represents very roughly, but it is hoped very plainly, the main principle of the elbow-joint; but for an exact knowledge of the mechanism of the joint, and the comparative strain upon, and therefore strength of, each muscle, the reader must consult some work on anatomy. He will there find, if he goes on to read the description of the hand, what a wonderful precision, complexity, and amount of movement can be obtained by variations of this simple device. [Illustration: DIAGRAM 39.—DIAGRAM OF THE ARM. 1, Lifting; 2, pressing.] Here, however, we must leave the study of the manner and object of the bodily movements, and proceed to investigate the far more intricate question of how they originate and are controlled. ESSAY IV. THE NERVOUS SYSTEM. I.