Organs of Respiration and Circulation. 



579 



driven into the arteries, that arise from two of its cavities, to be 

 conveyed throughout the body. The heart is a double organ, 

 and usually described as having two sides, a right and left. It is 

 also divided into four cavities ; the right auricle and ventricle, 

 and the left auricle and ventricle. The two auricles, and also the 

 ventricles, are separated from each other by a muscular partition, 

 so that the right side has no direct communication with the left. 

 In the accompanying sketch, fig. 4, the cavities of the right side 

 are laid open to illustrate the course of the blood. The two venae 

 cavae, marked d and e, receive the blood from the veins of the 

 system which unite to form these vessels, and they empty them- 

 selves into a, the auricle. From this cavity, by the contraction 

 of its muscular sides, the blood is driven into b, the ventricle. 

 The filling of this second cavity leads likewise to the contraction 

 of its walls, by which the blood is propelled intojf, the pulmonary 

 artery ; as the rising of the valve c prevents the blood passing 

 back into the auricle by closing the auriculo-ventricular opening. 



Fig. 4. 



a. The auricle. b, b. The ventricle. c. The valves which prevent the return of the blood from 

 the ventricle to the auricle. d. The anterior vena cava. e. The posterior cava. /.The 

 pulmonary artery. g. The anterior aorta. h. The posterior aorta. i. The pulmonarv 

 veins, 



