226 BUREAU OF ANIMAL INDUSTRY. 



auricles into the A-entricles, but preventing- a reverse course. The 

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

 of the heart toward the apex. They are separated bv a partition, and 

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

 aorta — b}- which they send blood to all parts of the bod3\ At the 

 mouth of the aorta and at the mouth of the pulmonary artery is an 

 ari-angement of valves in each case which prevents the reflux of blood 

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

 are composed of two ilaps, hence it is called the hicus])id 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 providecl with 

 three-flapped semilunar-shaped valves, to prevent the regurgitation 

 of blood when the ventricles contract. The veins emptying into the 

 auricles arc not capable of closure, but the posterior vena cava has an 

 imperfect valve at its aperture. 



The inner surface of the heart is lined b}" 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 vense cavie. 

 It then passes through the right auriculo-ventricular opening into the 

 right ventricle, thence through the pvdmonary 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 pulmonar}^, or lesser, being performed Iw the right side, 

 and the sj^stemic, 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 

 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 j)lace in regular rhythmical 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 



