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.
