56 Physiology. 



unite to form the pre-caval vein, which runs downward and 

 enters the right auricle. The post-caval vein begins in the 

 lower part of the abdomen, by the union of the two large 

 veins from the lower limbs. As it runs upward, it receives 

 branches from the kidneys and from the liver; it passes 

 through the diaphragm and enters the right auricle. (See 

 Figs. 29, 30, and 32.) 



The Distribution of the Arteries and Veins. The organs 

 of the body receive a supply of blood in proportion to 

 their size and activity. The artery supplying the blood to 

 any organ and the vein which returns it usually lie side 

 by side (see Fig. 32). The larger arteries are usually 

 deep-seated and in protected places. 



The Action of the Heart. The heart consists of muscle 

 fibers so arranged that they form a thick-walled bag, which 

 stands expanded when the muscles relax. But when the 

 fibers shorten, the heart contracts and the blood is forced 

 out. 



The complete action of the heart consists of three parts, 

 the contraction of the auricles, the contraction of the 

 ventricles, and the pause. 



The Pause. During the pause the blood is steadily 

 pouring into the auricles ; into the right auricle from the 

 caval veins, into the left auricle from the pulmonary veins. 

 At this time the aur-vent are open, and their flaps hang 

 loosely beside the walls of the ventricles. The blood, 

 therefore, instead of stopping in the auricles, passes on 

 into the ventricles. As the ventricle fills, the aur-vent 

 valves float up, as seen in the experiment of pouring water 

 into the ventricle. (See right-hand part of Fig. 33.) 



The Contraction of the Auricle. When the ventricle is 

 full, but not stretched, and the auricle partly full, the auricle 



