388 



COMPARATWE ANATOMY 



A similar serous epithelium is reflected on the outer side of the pericardial 

 cavity. The cavity between contains a small amount of fluid. 



The muscles of the myocardium are wound circularly around the 

 heart and arranged in layers. The fibers of the outer layers run at 

 approximately right angles to those of the inner layers, thus insuring a 

 maximum contraction of the heart cavities during contraction or systole. 



ijt-LEFT SUBCLAVIAN ART. 

 ARCH OF AORTA 



""^-'PULMONARY ARTERY 



PULMONARY 

 VEINS 



'post cava 



tricuspid' 



VALVE 



Fig. 325. — 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.) 



The muscle of the atria is mostly independent of that of the ventricles 

 and the two are separated by a connective tissue septum. There is, 

 however, an atrioventricular bundle of specialized muscle fibers which 

 extends from the atrial septum to the ventricular septum and serves to 

 convey to the ventricles the rhythm of contraction of the atria. 



The atrio-ventricular valves are attached by chordae tendineae to 

 the walls of the ventricles in such a manner as to open freely into the 

 ventricles but to prevent the return of blood when the ventricles contract. 

 The attachment of the chordae tendineae to the heart-wall is by means of 



