914 



PHYSIOLOGY 



by means of a spiral 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. 



A B C 



Fia. 382. Diagram of circulatory system in A, fish; B, amphibian (frog); C, mammal. 



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

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



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 causes the flow 

 of blood through the system must decline as we pass from the arterial to the 

 venous side. The chief function of the large arteries is to serve as elastic 

 conduits, whereas the small arteries or arterioles leading from the arteries 

 to the capillaries have in addition the function of regulating the amount of 

 blood flowing through the capillary area of the organs which they supply. 

 The veins have the function of conducting blood at a low pressure from 

 capillaries to heart and of storing up any excess of blood which is not 

 in i mediately taken up by the heart. 



Corresponding to this difference in function we find varial ions in the structure of the 

 blood vessels according to their situation in the circuit. The vessels which carry the 

 blood from the heart to the tissues, the arteries, are thick-walled, and contain an 

 abundance of muscular and elastic element^ in their walls. The typical medium-sized 



