110 THE HEART AND CIRCULATION. 



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 pul- 

 monary 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, especi- 

 ally 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 



