GENERAL PLAN OF THE CIRCULATION. 



275 



to appear to the casual observer to be one single organ. In com- 

 mon language, the heart of the mammalia is said to have two sides, 

 each of which is a forciug'-pump ; but the blood, before it passes 

 from one side to the other, has to circulate through one or other 

 of the sets of vessels found in the general organs of the body, and 

 in the lungs, as the case may- 

 be. This is shown at fig. 11, 

 whef3 the blood, commencing 

 with the capillaries on the gene- 

 ral surface at (A), passes through 

 the veins which finally end in 

 the vena cava (B), and enters 

 the right auricle (C). From 

 this it is pumped into (D) the 

 right ventricle, which, contract- 

 ing in its turn, forces it on into 

 the pulmonary artery (E), spread- 

 ing out upon the lining mem- 

 brane of the lungs, to form the 

 capillaries of that organ at F, 

 from which it is returned to the 

 left auricle (G) through the pul- 

 monary veins. From the left 

 auricle it is driven on to the left 

 ventricle; and this, by its power- 

 ful contractions, forces the blood 

 through the aorta (I), and the 

 arteries of the whole body, to 

 the capillaries (A), from which 

 the description commenced. But 

 though this organ is thus made 

 up of two pumps, yet they are 



united into one organ, and the two auricles and two ventricles 

 each contract at the same moment, causing only a double sound to 

 be heard, instead of a quadruple one, when the ear is applied to 

 the chest. In the diagram it will be seen that one-half of the 

 cavities and vessels is shaded, indicating that it contains dark 

 blood, while the other contains blood of a bright red color. But 

 though we commonly call the one venous, and the other arterial, 

 the distinction only applies to the general circulation ; for that of 

 the lungs is exactly the reverse, the pulmonary artery (E) contain- 

 ing dark blood, and the pulmonary veins bringing it back to the 

 heart after it is purified, and has again received oxygen sufiicient 

 to develop the scarlet color again. Between the auricles and ven- 

 tricles, and again at the openings of the latter cavities into their 

 respective arteries, valves of a ibrm pt ciliar to each are placed, so 



Fig. 11.— plan of the circulation. 



A. Capillaries on the general surface. 

 S. Vena cava. 



C. Right auricle. 



D. Eight ventricle. 



E. Pu'lmonary artery. 



r. Capillaries of the lungs, uniting to form 

 the pulmonary veins, which enter 



G. The left auricle. 



H. The left ventricle. 



I. The aorta posterior, dividing into small- 

 er arteries, and united with the capil- 

 laries at A. 



J. The trunk of the aorta anterior. 



