OF THE BLOOD AND CIRCULATION. 201 



animal, as determined by the quality of the blood distributed 

 in those organs. 



361. In mammals and birds the heart is divided, by a 

 vertical partition, into two cavities, each of which is again di- 

 vided into two compartments, one above the other (fig. 217). 

 The two upper cavities are called auricles, and the lower ones 

 are called ventricles. Reptiles have two auricles and one 

 ventricle (fig. 219) ; fishes have one auricle and one ventricle 

 only (fig. 220). The plan (fig. 217) represents the course of 

 the blood in mammals and birds, in which we have a double 

 circulation ; a lesser one through the lungs, and a greater one 

 through the body. 



362. The auricles do not communicate with each other, 

 in adult animals, nor do the ventricles. The former receive 

 the blood from the body and the respiratory organs through 

 veins, and each auricle sends it into the ventricle beneath, 

 through an opening, guarded by valves to prevent its reflux ; 

 while the ventricles, by their contractions, force the blood 

 through arteries into the lungs, and through the body generally. 



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

 contract simultaneously ; so, also, do the ventricles. These 

 successive contractions and dilatations constitute the pulsations 

 of the heart. The contraction is called systole, and the dilata- 

 tion is called diastole. Each pulsation consists of two move- 

 ments, 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 its age, sex, and the degree of health : in adult 

 man, they are commonly about seventy beats per minute. 



364. The course of the blood, in those animals which 

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

 the left ventricle (fig. 218, /, v). By the contraction of this 

 ventricle, the blood is driven through the main arterial trunk, 

 called the aorta (#), and is distributed by its branches through- 

 out the body ; it is then collected by veins, carried back to 

 the heart, and poured into the right auricle (r, a], which sends 

 it into the right ventricle (r, v}. The right ventricle propels 

 it through another set of arteries, the pulmonary arteries ( p], 

 to the lungs ; it is there collected by the pulmonary veins, and 



