CIRCULATORY SYSTEM. 49 



the left half, by a vertical partition. Each auricle, how- 

 ever, communicates by little openings, regulated by valves, 

 with the ventricle below it. 



The heart is a very powerful muscle, and its cavities 

 dilate and contract with remarkable regularity while life 

 lasts. 



As the auricles dilate they receive the blood ; the right, 

 that which is brought to it from the body by the veins, and 

 the left, that which flows into it from the lungs : when 



7 O 7 



full the auricles contract, forcino- their contents into the 



* o 



ventricles ; these dilate as they fill, and then suddenly con- 

 tract upon their contents, the right forcing the blood into 

 the pulmonary artery, and the left into the aorta, the great 

 artery which gives rise to all the arteries of the body, ex- 

 cepting only the pulmonary arteries. 



The whole course of the blood in Mammals, and in Birds, may now 

 be briefly stated. By the contraction of the right auricle, the blood 

 which it contains is forced into the right ventricle, little valves pre- 

 venting its return ; by the contraction of the right ventricle, the blood 

 is forced thence into the pulmonary artery, through which it enters 

 the lungs, where it is purified and changed from a bluish to a bright 

 red color ; from the lungs the pulmonary veins conduct it to the left 

 auricle ; the left auricle forces it into the left ventricle, by whose con- 

 traction it is forced through the aorta and its various branches to all 

 parts of the body. Through the veins, as before stated, it conies back 

 to the right auricle of the heart having lost the amount required for 

 nourishing and building up the system, and having been increased 

 by the amount added by the chyle, in the manner already described on 

 page 44. 



It must be added here, however, as a matter of fact, that in point 

 of time the ventricles are the first, and the auricles the second to con- 

 tract in each systole. 



The circulation above described is what is called a com- 

 plete and double circulation. It is called complete because 

 all of the blood circulates through the lungs before going 

 .3 



