THE CIRCULATION OF THE BLOOD. 



265 



veins, there is an exception in the case of the arteries and veins of some of the 

 abdominal viscera. Thus the veins emerging from the capillaries of the 

 stomach, intestines, pancreas, and spleen, instead of passing directly to the 

 inferior vena cava, unite to form a large vein the portal vein which enters 

 the liver. In this organ the portal vein divides to form a second capillary 

 system which is in close relation to the liver cells and from which arise the 

 veins which unite to form the hepatic veins. These latter vessels empty 

 and discharge the blood into the inferior vena cava just below the diaphragm. 

 From the foregoing facts physiologists frequently divide the general 

 circulation into: 

 i. The pulmonary circulation, which includes the course of the blood from 



the right side of the heart through the lungs to the left side of the 



heart. 



FIG. 118. RIGHT CAVITIES OF THE FIG. 119. RIGHT CAVITIES OF THE HF.ART. 

 HEART. Auriculo-venlricular valve open, Auriculo-ventricular valve closed, semilunar 

 semi-lunar valves closed. (.Da//0.) valves open. (Dalton.} 



2. The systemic circulation, which includes the course of the blood from 



the left side of the heart through the aorta and its branches, through 

 the capillaries and veins, to the right side of the heart. 



3. The portal circulation, which includes the course of the blood from the 



capillaries of the stomach, intestines, pancreas, and spleen through 



the portal vein, to the liver. 



Orifices and Valves. The movement of the blood along the path of 

 the circle above outlined is accomplished by the alternate contraction and 

 relaxation of the muscle walls of the heart. That the movement may be a 

 progressive one, that there shall be no regurgitation during either the con- 

 traction or the relaxation, it is essential that some of the orifices of the 

 heart be closed during each of these periods. This is accomplished by the 

 heart valves. 



The right auriculo-ventricular opening is surrounded and strengthened 

 by a ring of fibrous tissue to which is attached a membrane partially sub- 



