THE MECHANISM OF THE HEART PUMP 893 



band of muscular fibres, ' the bundle of His,' which rises in the wall of the 

 right auricle and passes beneath the foramen ovate and across the auriculo- 

 ventricular junction into the inter- ventricular septum. The exact course 

 of these fibres and their significance will be considered later. 



The normal direction of the blood-flow through the heart is determined 

 mainly by the valves which guard the auriculo-ventricular orifices and the 

 openings of the aorta and pulmonary artery. The auriculo-ventricular 



3.. 



FIG. 393. Left auricle and ventricle, with outer side cut away to show chief points 



in anatomy of heart. (TESTUT.) 



1, aorta ; 2, pulmonary artery ; 3, ant. coronary vessels ; 5, 5', pulmonary veins ; 

 6, left auricle ; 7, auricular appendage ; 10, cavity of left ventricle ; 11, 12, mitral 

 valves ; 13, 14, papillary muscles ; 16, arrow pointing to aortic orifice. 



valves are tubular membranes attached round the entire circumference of 

 the auriculo-ventricular ring. They are composed of fibrous and elastic 

 tissue, covered on each side with endocardium, and project downwards into 

 the cavities of the ventricles. On each side the membrane is divided by 

 deep incisions into large flaps, three in number on the right side (the tricuspid 

 valves) and two in number on the left side (the mitral valves). The sail-like 

 margins of these valves are connected by thin tendinous cords to the papil- 

 lary muscles, which are nipple- shaped projections of the muscular walls of 

 the ventricles. By this means the edges of the valves are kept close together 



