CIRCULATION OF THE BLOOD. 



499 



pleting the circuit of the greater or systemic circulation. From the right 

 auricle the blood is emptied into the right ventricle, from which it is 

 forced by the heart's contractions into a single large trunk, the pul- 

 monary artery. This artery, which, however, contains venous blood, 

 soon divides into two trunks, one going to each lung, each of which 

 divides and subdivides in the lung-tissue to form a second capillary net- 

 work, in which the dark venous blood is subjected to the influence of 

 the air contained in the air-cells, gives up its carbon dioxide and 

 certain organic impurities, and absorbs 

 oxygen, becoming again bright-red arterial 

 blood. It is then collected by the pul- 

 monary veins and carried to the left 

 auricle, thus completing the lesser or 

 pulmonary circulation, and bringing us 

 back to the point from which we started 

 (Fig. 194). 



2. THE ACTION OF THE HEART. The 

 circulation of the blood consists in the con- 

 tinuous movement of the blood in a series 

 of ramifying tubules or blood-vessels, and 



FIG. 193.-HEART OF DTTGONG, SHOWING THE SKP- 



A RATION OF THE RIGHT AND LEFT, OR PUL- 

 MONARY AND SYSTEMIC HEARTS. (Pei'rier.) 



FIG. 194. SCHEME OF THE CIRCU- 

 LATION IN MAMMALS. (Landois.) 



a. right auricle ; A, right ventricle ; b, left 

 auricle : B. left ventricle : 1, pulmonary artery ; 

 2, aorta, with semi lunar valves ; L, area of pul- 

 monarv circulation ; K, area of systemic circu- 



* ; < Id, i minnteric arr 



q, portal vein; L, liver; h, hepatic vein. 



owes its maintenance largely to the motor power derived from the 

 contractions of the heart. The circulatory apparatus consists, there- 

 fore, of the heart, the arteries, the veins, and midway between the 

 latter the capillaries. The conditions which govern the movement of 

 the blood-current in these different parts of the circulatory apparatus 

 will be studied in turn. 



The heart is a muscular reservoir, divided, as already indicated, in 



