390 VETERINARY PHYSIOLOGY 



attached to the great vessels above and to the diaphragm 

 below. 



4. Endocardium. — This is a thin layer of fibrous tissue 

 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 — the ventricles. 

 There is thus on each side of the heart a valve between the 

 auricles and the ventricles, and a valve between the ventricles 

 and the great arteries. 



(1) Auriculo-ventricular Valves, — On each side of the 

 heart the auriculo-ventricular 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 attached to the apices of the papillary muscles 

 by the chordae tendineas (figs. 165 and 167). 



(i.) On the left side of the heart there are two main 

 cusps, forming the mitral valve (fig. 167) — 



(a) 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. This cusp is very strong, and 

 the chordae are inserted chiefly along its edges. 



(b) The posterior or left cusp is smaller. It takes origin 

 from the back part of the auriculo-ventricular ring. The 

 chordce tendinese are not only inserted into its edge, but run 

 up along its posterior aspect to be inserted into the auriculo- 

 ventricular rino-. 



When the papillary muscles contract, the cusps are 

 drawn together. The edge of each cusp thins out to form a 

 delicate border, which, when the cusps are approximated, 

 completely seals the aperture. 



(ii.) On the right side of the heart the auriculo- 

 ventricular orifice is separate from the pulmonary opening, 

 and the three cusps of the tricuspid valve are developed in 

 connection with the crescentic opening from the auricle 



