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valves, allowing the transmission of blood from the auricles into the 

 ventricles, but preventing a reverse course. The ventricles are thick- 

 walled cavities, forming the more massive portion of the heart towards 

 the apex. They are separated by a partition, and are connected with 

 the great arteries, the pulmonary artery and the aorta, by which they 

 send blood to all parts of the body. At the mouth of the aorta and at 

 the mouth of the pulmonary artery is au arrangement of valves in each 

 case which prevents the reflux of blood into the ventricles. The auri- 

 culo-ventricular valves in the left side are composed of two flaps, hence 

 it is called the bicuspid valve. In the right side this valve has three 

 flaps, and is called the tricuspid valve. The flaps which form these 

 valves are connected with a tendinous ring between the auricles and 

 ventricles ; and each flap of the auriculo-ventricular valves is supplied 

 with tendinous cords, which are attached to the free margin and under 

 surface, so as to keep the valves tense when closed ; a condition which 

 is produced by the shortening of muscular pillars with which the 

 cords are connected. The arterial openings, both on the right and 

 on the left side, are provided with three-flapped semi-lunar shaped 

 valves, to prevent the regurgitation of blood when the ventricles con- 

 tract. The veins emptying into the auricles are not capable of closure, 

 but the posterior vena cava has an imperfect valve at its aperture. 



The inner surface of the heart is lined by a serous membrane, the 

 endocardium, which is smooth and firmly adherent to the muscular 

 structure of the heart. This membrane is continuous with the lining 

 membrane of the blood vessels, and it enters into the formation of the 

 valves. 



The circulation through the heart is as follows : The venous blood is 

 carried into the right auricle by the anterior and posterior venae cavae. 

 It then passes through the right auriculo ventricular opening into the 

 right ventricle, thence through the pulmonary artery to the lungs. It 

 returns by the pulmonary veins to the left auricle, then is forced through 

 the auriculo- ventricular opening into the left ventricle, which propels it 

 through the aorta and its branches into the system, the veins returning 

 it again to the heart. The circulation, therefore, is double, the pulmon- 

 ary or lesser being performed by the right, and the systemic or greater 

 by the left side. 



As the blood is forced through the heart by forcible contractions of 

 its muscular walls it has the action of a force pump, and gives the im- 

 pulse at each beat, which we call the pulse — the dilatation of the arteries 

 throughout the system. The contraction of the auricles is quickly fol- 

 lowed by that of the ventricles, and then a slight pause occurs; this 

 takes i)lace in regular rythmical order during health. 



The action of the heart is governed and maintained by the pneumo-gas- 

 tric nerve (tenth pair of cranial nerves) ; it is the inhibitory nerve of the 

 heart, and regulates, slows, and governs its action. When the nerve is 

 cut the heart-beats increase rajjidly, and in fact the organ works without 



