MECHANISM OF THE CIECULATION. 53 



pulmonary veins terminate, the blood passes into the left ven- 

 tricle, and by its powerful action is driven through the aorta 

 and its branches into all parts of the body, to return once 

 more by the veins to the right auricle of the heart. Thus is 

 completed the double circulation in man, mammals, and birds. 



Mechanism of the Circulation. 



98. Movements of the Heart. The cavities of the 

 heart being muscular, contract and dilate : the dilatation is 

 called the diastole,* the contraction, sytole.^ As the auricles 

 of the heart dilate, they receive the blood, the right that of 

 the body, the left, that which is returning from the lungs ; 

 when full, they contract on their contents, forcing the blood 

 into the ventricles ; these in their turn suffer dilatation as 

 they fill, and, contracting suddenly, force the blood, the right 

 into the pulmonary artery, the left into the aorta. These 

 movements of the heart continue whilst life endures, and are 

 much influenced by various circumstances, such as exercise, 

 disease, &c. The number of these contractions, usually felt 

 in the radial artery at the wrist, varies with years ; they are 

 most frequent in the young, and average about seventy in 

 the adult, at noon. They are affected by every change in the 

 position of the body. 



99. Passage of the Blood into the Cavities of the 

 Heart. This has been already de- 

 scribed, but may be thus again briefly 

 adverted to. The blood having an 

 arterial character, returns from the 

 lungs into the left auricle. This au- 

 ricle communicates with the left ven- 

 tricle, by an opening called the left 

 auriculo-ventricular aperture, through 

 which the blood flows from the auricle 

 into the ventricle. In this passage 

 is placed a valve, called mitral, permit- . 



, * , i n1 j , tig. 33. Section of the 



ting the blood to pass, but not to re- Heart.j 



turn. The arterial blood now collected 



in the ventricle is acted on by this fleshy cavity, and forced 



* From Siourre'AAu), I dilate. t From <n/<TTeAAw, I enclose. 



t Theoretic section of the heart, to show the mechanism of the play of the 

 valves : a, auricle receiving the veins e e; b, ventricle separated from the 

 auricle by the valvules c;d, fleshy bridles or stays of these valvules ; 

 -f, artery springing from the ventricle ; g, valvules situated at the entrance 

 of the artery. 



