VI.
Comparisons are rightly regarded as objectionable, so it would hardly be safe to say that the group of movements whose primary object is filling the lungs, and which we must study next, is the most important in the body, especially when we have just been speaking of the circulation, which, however, would be of but little use if the blood could not be oxidized; but we can at least say that its importance cannot be overrated, so far-reaching are its effects.
The lungs are, as we have described them above, a pair of delicate membranous sacs connected by a tube, the trachea, with the alimentary canal, from which they originally budded out. They are subdivided, though how we need not describe in detail, into a vast number of small compartments, so as to give the maximum surface in the space accorded them, and the whole somewhat resembles a cluster of grapes, the stalks being the branches of the trachea. The membranous parts are pervaded by an elastic network, enveloping the compartments in such a way that it would reduce them permanently to the resemblance of a bunch of raisins rather than grapes, were it not that they are enclosed in an airtight box—the thorax—from the walls of which they cannot shrink without causing a vacuum. Owing, however, to the latter arrangement and the trachea being open to the external world, they are always more or less distended with air.
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.
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].
Diagram 35.—Showing how the Capacity of the Thorax is increased by raising the Ribs and depressing the Diaphragm.
Diagram 36.—Model for showing Effect of Movements of the Thorax on the Pulmonary Circulation.
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.
Diagram 37.—A Diagrammatic View of the Circulation through the Organs upon which the Diaphragm presses when it descends.
Diagram 38.—Illustrating the Influence of the Diaphragm upon the Circulation through the Viscera.