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 

 pulmonarjr circulation, and bringing us 

 bnck to the point from which we started 

 (Fig. 194). 



2. The Action or 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 Dugong, showing the Sep- 

 akation of the right and left, or pul- 

 MONARY and Systemic Hearts. (Perrier.) 



Fig. 194.— Scheme of the Circu- 

 lation in Mammals. ILandois.) 



a, right auricle ; A, right ventricle ; h, left 

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

 2, aorta, with semilunar valves ; L, area of pul- 

 monary circulation ; K, area of systemic circu- 

 lation in region supplying the sujierior vena 

 cava, o; G. area supplying the inferior vena 

 cava, u ; d d, intestine : m, dlesenteric artery ; 

 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-qurrent in these different parts of the circulatory apparatus 

 will be studied in turn. 



The heart is a muscular reservoir, divided, as alread}' indicated, in 



