GENERAL FEATURES OF THE CIRCULATION 869 



From the ventricle the blood passes into the aorta, whence it is carried 

 partly by the pulmonary artery to the lungs, partly by arteries to the 

 different organs of the body. The blood which has passed through the 

 lungs and been arterialised flows through the pulmonary veins to the left 

 auricle, whence it passes into the ventricle and mixes with the venous 

 blood which is arriving from the right auricle. The pulmonary circulation 

 is thus merely a branch of the general or systematic circulation. The 

 bulbus aortae in the frog is divided into two parts by means of a spiral 



FIG. 371. Diagram of circulatory system in A, fish ; B, amphibian (frog) ; C, mammal. 



v, ventricle; a, auricle; K, gill capillaries; A, aorta; c, systemic capillaries; 

 L, lung capillaries ; r, I, right and left auricles ; rV, IV, right and left ventricles. 



valve, by which a partial separation of the blood coming from the right and 

 left auricles is effected, and the venous blood from the right auricle directed 

 especially into the pulmonary artery. 



In birds and mammals the heart has become entirely divided into two 

 halves, right and left, which have no communication with one another 

 except by way of the blood-vessels and capillaries. The right auricle 

 receives the venous blood from all parts of the body and sends it on to 

 the right ventricle, whence it is forced into the lungs along the pulmonary 

 artery. In the lungs it takes up oxygen and becomes arterial and is 

 returned by the pulmonary veins to the left auricle and so to the left 

 ventricle. The rhythmic contractions of the left ventricle then force the 

 blood into the aorta, whence by the branching arteries it is carried to all 

 parts of the body. The whole vascular system is distensible and elastic, 

 so that its capacity will increase with the pressure of the blood contained 

 in it. Since the driving force is furnished by the heart the pressure which 



