CIRCULATION OF THE BLOOD. 



509 



mentioned, are situated between the two auricles and ventricles, and at 

 the origin of the pulmonary artei-y and aorta. Their mechanical action 

 is different, the two auriculo-ventricular valves operating upon the same 

 principle, and the two valves at the starting point of the large arteries 

 being similar in function and operation. Both the auriculo-ventricular 

 valves at their bases constitute complete cylinders which originate in the 

 auriculo-ventricular ring, which is often cartilaginous, and even, in some 

 animals, as in the bird, furnished with a bone. 



In the heart of the ox are found two bony structures at the origin of the aorta, 

 to the larger of which are fastened the right leaflet of the aortic semi-lunar valve 

 and the central portion of the mitral valve, while the smaller is in connection with 

 the left semi-lunar valve of the aorta (Figs. 203 and 204). 



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IK 



Fig. 205.— Heart of the Horse, Seen froji the Right Side, the Right 

 Auricle and Right Ventricle being Laid Open. {MUller.) 

 HbHb, pericardium slit open and drawn to the sides; rV. right auricle; rK. right ventricle ; IK, left 

 ventricle ; 1. inferior, or posterior, vena cava, with probe inserted in it : 2, superior, or anterior, vena 

 cava: 3, azygos vein; 4, pulmonary veins: 5, posterior aorta; 6. anterior aorta: 7, right auricular 

 appendage : 8, Lower's ridge ; 9, orifice of the coronary vein : 10, oval foramen ; 11, right coronary artery ; 

 12, longitudinal fissure, with 13, its arterv, and 14. its vein; 15. columns carnaj; 16 16, papillary 

 muscles; 17 17, chordse tendlnse ; and 18, a leaflet of the tricuspid valve. 



The cylindrical form of these valves exists only a short distance from 

 this ring, and then the valve divides into a number of different segments, 

 which, in the right ventricle, are three in number, hence the name of 

 tricuspid valves, and in the left are two, hence the name of mitral valves. 



