WARM-BLOODED CIRCULATION. 253 



and left cavities are kept perfectly distinct from 

 one another, and are separated by thick partitions, 

 allowing of no direct transmission of fluid from the 

 one side to the other. These two hearts may there- 

 fore be compared to two sets of chambers under the 

 same roof; having each their respective entrances 

 and exits, with a party-wall of separation between 

 them. This junction of the two hearts is conducive 

 to their mutual strength ; for the fibres of each in- 

 termix and even co-operate in their actions, and 

 both circulations are carried on at the same time ; 

 that is, both ventricles contract or close at the same 

 instant; and the same applies to the auricles. The 

 blood which has just returned from the body, and 

 that from the lungs, the former by the venae cavae, 

 the latter by the pulmonary veins, fill their respec- 

 tive auricles at the same instant ; and both auricles, 

 contracting at the same moment, discharge their 

 contents simultaneously into their respective ven- 

 tricles. In like manner, at the moment when the 

 left ventricle is propelling its aerated blood into 

 the aorta, for the purposes of general nutrition, the 

 right ventricle is likewise driving the vitiated blood 

 into the pulmonary artery, in order that it may be 

 purified by the influence of the air. Thus the same 

 blood which, during the interval of one pulsation, 

 was circulating through the lungs, is, in the next, 

 circulating through the body ; and thus do the 

 contractions of the veins, auricles, ventricles, and 

 arteries all concur in the same general end, and 

 establish the most beautiful and perfect harmony of 

 action . 



Evidence is afforded of the human conformation 

 being expressly adapted to the erect position of the 

 body by the position of the heart, as compared 



