The thorax, which they thus must always exactly fill, is a conical-shaped box, its walls being the ribs, and its floor a sheet of muscle known as the diaphragm. It contains, besides the lungs, only the heart and large bloodvessels. The problem, therefore, of drawing air into the lungs and (after the gaseous interchange described in Essay II., Section IV., has taken place) of expelling it again, becomes solely a matter of increasing and decreasing the capacity of the thorax. (See Diagram 34.) This can be done in two ways: the diameter through the ribs can be increased, or the diaphragm can be pulled down, increasing its depth. Actually, both these methods come into play together. Diagram 35 will probably give a better idea of how this is done than could easily be conveyed by a verbal description. An attempt is here made to show the action of the ribs and the diaphragm—first, of each separately, then of the two combined. The elasticity of the lungs themselves is sufficient to drive out the tidal air if the diaphragm and the muscles of the ribs are relaxed, though in hard breathing a muscular movement may depress the ribs and a contraction of the abdominal muscles force up the diaphragm. [Illustration: DIAGRAM 34.—MODEL (ADAPTED FROM RUTHERFORD) FOR SHOWING HOW THE LUNGS ARE FILLED WITH AIR BY ALTERING THE SIZE OF THE THORAX.] But though the primary object of raising the ribs and depressing the diaphragm may be to fill the lungs, its secondary influence upon the trunk as a whole is hardly less important. The effect upon the circulation is profound. The compartments of the lungs are enveloped in innumerable capillary bloodvessels, and, as these lie around and between them in the cavity of the thorax, they must, when breath is drawn in, be subjected to a negative pressure before the lung itself, and be the first to experience a positive pressure when the air is expelled. Here, again, a diagram is the best explanation. (See Diagram 36.) The pulmonary vessels, moreover, are not the only ones influenced. The reader who attentively examined Diagram 13 must have been struck by the peculiarities of the circulation through the spleen, intestine and liver, and the obstacles which this repeated breaking up into fine vessels must offer to the flow of blood, as described in Section V. of this essay. [Illustration: DIAGRAM 35.—SHOWING HOW THE CAPACITY OF THE THORAX IS INCREASED BY RAISING THE RIBS AND DEPRESSING THE DIAPHRAGM.] [Illustration: DIAGRAM 36.—MODEL FOR SHOWING EFFECT OF MOVEMENTS OF THE THORAX ON THE PULMONARY CIRCULATION.]