THORAX. 143 



valves and the auriculo-ventricular, witli the mitral 

 valve. 



The right auricle covers the auriculo-ventricular 

 opening, and on its superior face has the openings 

 of the pulmonary veins, from four to eight in number. 



Structure : Annular fibrous frame, made of four 

 bands, one for each orifice at the base of the ventricu- 

 lar mass. Muscular tissue composed of uniting and 

 proper fibres of the ventricles and auricles. The fibres 

 proper of the ventricles form a cone open at both ends. 

 The uniting fibres cover the external surface of the 

 ventricles, twist around them and penetrate into and 

 line the inside of the ventricular cavities. In the auri- 

 cles the uniting fibres form two thin bands goin^from 

 one to the other ; tlie proper fibres divide into fasci- 

 culi disposed in rings, round the auriculo-ventricular 

 openings, or in sphincters round the openings of the 

 veins. Blood vessels are represented by the coronary 

 arteries and veins. The nerves come from the great sym- 

 pathetic and pneumogastric. It has two serous mem- 

 branes, an internal or endocardium, lining the cavities 

 and extending into the cavity of the blood vessels ; an 

 external or pericardium, having a fibrous cone for a 

 frame, on the inside of which is spread the serous mem- 

 brane proper or two layers, parietal covering the 

 fibrous cone, and a visceral resting on the lieart itself. 



