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meehanisiu of Aalves, allowiiig tlie transmission of l)lood from the 

 auricles into the ventricles, l3nt pi-eventing a reverse course. The 

 ventricles are thiek-walled cavities, forming the more massive portion 

 of the heart tov.ards tlie apex. Thej' are separated by a iiartition, 

 and are connected with the great arteries, the pulmonary artery and 

 the aorta, b}' which they send blood to all parts of the body. At the 

 mouth of tlie aorta and at the mouth of the j)iilMionary artery is an 

 arrangement of valves in each case which prevents the reflux of blood 

 into the ventricles. The auriculo-ventricular valves in the left side 

 are comi)osed 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 tendi- 

 nous 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 short- 

 ening of muscular pillars with which the cords are connected. The 

 arterial openings, both on the right and on the left side, are i^ro- 

 A'ided with three-flapped semi-lunar shaped valves, to prevent the 

 regurgitation of blood when the ventricles contract. The veins emp- 

 tying into tlie auricles are not capable of closure, liut the iiosterior 

 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 ven?B 

 cavpp. It then passes through the right auriculo-ventricular opening 

 into the right ventricle, thence through the iiulmonary 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 b]*anclies into the system, 

 the veins returning it again to the heart. The circulation, therefore, 

 is double, the pulmonary or lesser being performed by the right, and 

 the systemic or greater by the left side. 



As the lilood is forced through the heart by forcible contractions 

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

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

 arteries throughout the system. The contraction of the auricles is 

 quickly followed by that of the ventricles, and then a slight pause 

 occurs; this takes place in regular rythmical order during health. 



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

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

 of the heart, and regulates, slows, and governs its action. When the 



