DO OF THE BLOOD 



239. The two auricles dilate at the same instant, and also 

 contract simultaneously ; so also do the ventricles. These 

 successive contractions and dilatations constitute the pul- 

 sations of the heart. The contraction is called systole, 

 and the dilatation is called diastole. Each pulsation con- 

 sists of two movements, the diastole or dilatation of the 

 ventricles, during which the auricles contract, and the systole 

 or contraction of the ventricles, while the auricles dilate. 

 The frequency of the pulse varies in different animals, and 

 even in the same animal, according to age, sex, and the de- 

 gree of health. In adult man, they are commonly about 

 seventy beats per minute. 



240. The course of the blood in those animals which 

 have four cavities to the heart is as follows, beginning with 

 the left ventricle (Fig. 85, lv}. By the contraction of this 

 ventricle, the blood is driven through the main arterial 

 trunk, called the aorta (Fig. 90, a), and is distributed by its 

 branches throughout the body ; it is then collected by the 

 veins, carried back to the heart, and poured into the right 

 auricle (Fig. 85, ra), which sends it into the right ventricle 

 (rv). The right ventricle propels it through another set of 

 arteries, the pulmonary arteries (Fig. 90, p), to the lungs (Z) ; 

 it is there collected by the pulmonary veins, and conveyed 

 to the left auricle (Fig. 85, la), by which it is returned to the 

 left ventricle, thus completing the circuit. 



241. Hence the blood in performing its whole circuit 

 passes twice through the heart. The first part of this cir- 

 cuit, the passage of the blood through the body, is called 

 the great circulation ; and the second part, the passage 

 of the blood through the lungs, is the lesser or pulmonary 

 circulation : this double circuit is said to be a complete 

 circulation. In this case the heart may be justly re- 

 garded as two hearts conjoined, and in fact the whole of 

 the lesser circulation intervenes in the passage of the blood 



