452 



MAMMALIA— PHYSIOLOGY 



Vessels conveying blood to tissues and intervening between arteries 

 and veins — capillaries.^ 



The heart is a thick, muscular, hollow organ with great blood 

 vessels originating from the broad anterior part. It is enclosed in a 

 sac of fibrous tissue lined with epithelium called the pericardium. 

 It consists of 4 chambers, 2 of which are called auricles and 2 

 ventricles. 



The auricle and ventricle of the right side are completely sepa- 

 rated from those of the left side. The auricles open into the ven- 

 tricles by valved apertures and valves guard the openings of the 

 great vessels. Between the right auricle and right ventricle is the 

 tricuspid valve. Between the left auricle and the left ventricle is the 

 mitral or bicuspid valve. Between the auricles is a partition (auricu- 

 lar septum). Between the ventricles is the ventricular septum. 



X i6oo X 750 



Fig. 245^. Red and White Corpuscles of Human (Left) and Frog (Right) Blood. 



* (Drawn by Norris Jones.) 



There are two large veins which empty into the right auricle. 

 The superior vena cava descends, while the inferior vena cava ascends. 

 The coronary veins come from the heart substance itself. Into the 

 left auricle, four pulmonary veins enter, two coming from the right 

 and two from the left lung. 



From the right ventricle arises the large pulmonary artery 

 which divides into a branch for each lung. From the left ventricle 



^ In the portal system (page 453) the capillaries in the liver intervene between the 

 hepatic portal vein and the hepatic veins. 



