CIRCULATION OF THE BLOOD. 119 



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

 the right side through the pulmonary artery, through the capillaries of 

 the lungs and pulmonary veins, to the left side of the heart. 



3. The portal circulation, which includes the portal vein. This vein is 

 formed by the union of the radicles of the gastric, mesenteric, and splenic 

 veins, and carries the blood directly into the liver, where the vein divides 

 into a fine capillary plexus, from which the hepatic veins arise; these empty 

 into the ascending vena cava. 



The Mechanism of the Heart. The immediate cause of the movement 

 of the blood through the blood-vessels is the alternate contraction and relaxa- 

 tion of the muscular walls of the heart, and more especially the walls of the 

 ventricles, each of which plays alternately the part of a force pump and to 

 a slight extent of a suction pump. The motive power is furnished by the 

 heart itself. The contraction of any part of the heart is termed the systole, 

 the relaxation, the diastole; as each side of the heart has two cavities, the walls 

 of which contract and relax in succession, it is customary to speak of an 

 auricular systole and diastole and a ventricular systole and diastole; as the 

 two sides are in the same physiologic relation they contract and relax in the 

 same periods of time. 



Movements of the Heart. At each beat, during the systole, the heart 

 hardens and becomes shortened in its long diameter, its apex is raised up, 

 rotated on its axis from left to right, and pushed forcibly against the walls 

 of the chest. The impulse of the heart, observed about two inches below 

 the nipple and one inch to the sternal side, between the fifth and sixth ribs, 

 is caused mainly by the apex of the heart being pressed more energetically 

 against the chest walls. 



The Cardiac Cycle. The entire period of the heart's pulsation may be 

 divided into three stages, viz.: 



1. The auricular contraction and relaxation. 



2. The ventricular contraction and relaxation. 



3. The pause or period of repose during which both auricles and ventricles 

 are at rest. These three stages constitute collectively a cardiac cycle or a 

 cardiac revolution. 



The duration of a cycle, as well as the duration of its three stages, varies 

 in different animals in accordance with the number of cycles which recur in 

 a minute. In human beings in adult life there are about 72 cycles to the 

 minute; the average duration is, therefore, 0.80 sec. From this it follows 

 that the time occupied by any one of the three stages must be extremely short 



