284 



CHORDATE ANATOMY 



ventricular valves. Two large veins, the precava and postcava, enter 

 the right atrium, while four pulmonary veins return blood from the lungs 

 to the left atrium. The pulmonary artery connects with the right, the 

 aorta with the left ventricle. 



The walls of the ventricles are thicker than those of the atria, and 

 the wall of the left ventricle is thicker than that of the right, for the right 



^LEFT SUBCLAVIAN ART. 

 ARCH OF AORTA 



^PULMONARY ARTERY 



:z^, PULMONARY 

 VEINS 



POST CAVA 



TRICUSPID' 

 VALVE 



Fig. 257. — A diagram of the chambers of the mammalian heart and their associated 

 vessels and valves. The walls of the ventricles are shown in black, those of the auricles 

 are stippled. The direction of flow of blood is indicated by arrows. (Redrawn after 

 Jammes.) 



ventricle pumps blood only to the lungs, while the left ventricle forces 

 blood to all other parts of the body and is correspondingly more muscular. 

 The heart wall has three layers, endocardium, myocardium, and 

 epicardium. The endocardium consists of a thin layer of connective 

 tissue and an endothelial layer continuous with that of the blood-vessels. 

 The myocardium is the thick muscular layer. The fibers of cardiac 

 muscle are striped, and are peculiar in having anastomosing connexions 

 with one another. The epicardium is a thin layer of connective tissue 



