of digestion, the superior and inferior mesenteric veins from the intestines, the splenic vein from the spleen, and the gastric from the stomach. These join together to form the portal vein, the only vein that breaks up into capillaries. This divides and ramifies through the liver, whence it emerges as the hepatic veins. The whole is known as the portal circulation.
Pulmonary Circulation.—Of the pulmonary circulation and its vessels a few words might also be said. The pulmonary artery, which carries the blood from the right ventricle to the lungs, is only about two inches long and divides into a right and a left pulmonary artery, which pierce the pericardium and go to their respective lungs. The right one is the larger and longer, for it has farther to go and gives off a branch to supply the third lobe of the right lung. The vessels finally divide and subdivide, terminating in the pulmonary capillaries. The venous capillaries then gather together to form a main vein in each lobule, these veins uniting into two trunks for each lung, the pulmonary veins, which empty into the left auricle.
Nerves of Heart.—The muscular fibers of the heart have the power of rhythmical contraction. Independent nerve centers or ganglia are also found in the muscular walls and influence the mechanism of the heart, especially the acceleratory mechanism. Thus, in some of the lower animals the heart can be removed from the body, and if placed in normal salt solution will go on beating for some time. The heart is controlled, however, by two nerves, the vagus or pneumogastric and the sympathetic. Of these the vagus is the inhibitory mechanism. It acts as a check and makes the heart's action regular and rhythmic. If it is cut, the action of the heart becomes very rapid and irregular. The sympathetic is the acceleratory mechanism. When the vagus alone is stimulated, it first slows, then stops the heart, for it weakens the systole and prolongs diastole. Acceleration follows stimulation of the sympathetic, both the rapidity and the force of the
