CIRCULATION 



231 



The endocardium forms a continuous fibrous layer, lined 

 by endothelium, extending from the vessels over the inner 

 aspect of auricles and ventricles. At certain points flaps of 

 this endocardium are developed to form the valves of the 

 heart. 



In the heart, valves are situated at the entrance to and at 

 the exit from the expelling cavities. There is thus on each 

 side of the heart a valve 

 between the auricles and 

 ventricles, and a valve be- 

 tween the ventricles and the 

 great arteries. 



Aurieulo - ventricular 

 Valves. On each side of 

 the heart the auriculo-ven- 

 tricular valve is formed by 

 flaps of endocardium, which 

 hang downwards from the 

 auriculo- ventricular ring like 

 a funnel into the ventricular 

 cavity, and which are attach- 

 ed to the apices of the papil- 

 lary muscles by the chordae 

 tendineae (figs. 109 and 110). 



On the left side of the 

 heart there are two main 

 cusps, forming the mitral 

 valves (fig. 110) 



1st. An anterior or right 

 cusp, which takes origin 

 from, and is continuous with, 

 the right posterior wall of 

 the aorta. It hangs down into the ventricle between the 

 aortic and auriculo-ventricular orifices, thus dividing the 

 ventricle into two parts, an aortic and an auricular part. 

 This cusp is very strong, and in many animals bone is 

 developed in it towards its base. It is composed of dense 

 fibrous tissue, is smooth on both sides, and the chordae are 

 inserted chiefly along its edges. 



2nd. The posterior or left cusp takes origin from the back 



FIG. 110. Vertical Mesial Section through 

 Heart to show Aortic and Mitral Valves. 

 R. V. , right ventricle ; L. V. , left ventricle 

 with papillary muscle ; L. A. , left auricle ; 

 Ao. , aorta with anterior cusp on top of 

 septum. 



