AND CIRCULATION. 115 



aurichs, and the two lower ventricles. Reptiles have two 



Fig. 86. Fig. 87. 



auricles and one ventricle, (Fig. 86.) Fishes have one auri- 

 cle and one ventricle only, (Fig. 87.) 



238. 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 a valve, to" prevent its reflux ; 

 while the ventricles, by their contractions, force the blood 

 through arteries into the lungs, and through the body gen- 

 erally. 



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 pulsa- 

 tions of the heart. The contraction is called systole., and the 

 dilatation is called diastole. Each pulsation consists of two 

 movements, the diastole or dilatation of the ventricles, 

 during which the auricles contract, and the systole or con- 

 traction 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. 



240. The course of the blood in those animals which have 

 four cavities t:> the heart is as follows, beginning with the 

 Wft ventricle (Fig. 85, I. v.) By the contraction of this 



