154 CIRCULATORY SYSTEM. 



In many animals, the blood is propelled from a central point, 

 called the heart, to all parts of the body, and then returns again 

 to the heart. The first movement is executed through canals 

 called arteries, and the second through veins. It is the most 

 simple scheme by which a circulation can be carried on through 

 a sanguiferous system, and requires a heart with only two cavi- 

 ties; one for propellingblood into the arteries, or departing tubes, 

 and another as a reservoir for receiving the blood of the return- 

 ing tubes, or the veins. The two cavities must be near each 

 other, and have a valvular opening between them, which will 

 permit the blood to pass from the venous into the arterial reser- 

 voir; but not from the arterial into the venous. A circulation of 

 this simple cast is found in fish, and in animals generally whose 

 respiration is effected on the surface of the body; but in man, 

 and in other warm-blooded animals, where respiration is carried 

 on interiorly by means of the lungs, their circulatory apparatus 

 is double; one part being for the lungs, and the other part for 

 the body generally. 



In man, the heart consists of four cavities: two auricles, or re- 

 servoirs of venous blood, and two ventricles, into which the ve- 

 nous blood is transmitted, and which, in their functions, may be 

 compared to the forcing-pump of a fire-engine. The circulation 

 is effected in the following manner: The blood contained in the 

 right auricle of the heart flows into the right ventricle, and from 

 the latter it is forced through the pulmonary artery into the 

 lungs* It returns from the lungs through the four pulmonary 

 veins, and is received into the left auricle of the heart; from the 

 latter it flows into the left ventricle, and is propelled from it into 

 the aorta. The aorta then distributes it through the whole body 

 by an infinitude of small branches; from the latter it is collected, 

 by corresponding veins, into two trunks, the Ascending and the 

 Descending Cava. The ascending vena cava brings the blood 

 from the lower extremities and from the abdomen; the descend- 

 ing vena cava brings the blood from the head and neck, the up- 

 per extremities, and the parietes of the thorax. These two trunks 

 finally discharge the blood into the cavity from which it started, 

 to wit, the right auricle. The same round is then renewed, and 

 continues to be repeated during the whole course of life. It is 

 customary for anatomists to call the route of blood from the 

 right ventricle, through the lungs, to the left auricle inclusively 



