514 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



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the systole of the ventricle are forced back by the escaping current 

 of blood against the walls of the pulmonary artery and aorta; they, 

 therefore, offer no obstacle to the escape of blood from the cavities of the 

 ventricles. At the end of the contraction of the ventricles relaxation 

 commences, the ventricles dilate, and there is, therefore, a tendency for 

 the blood to return into their cavities from the aorta and pulmonary 

 arteiy. This backward current carries the blood behind the free margins 

 of these valves and, distending the pockets behind them, flattens out the 

 valves, causing them to come into complete contact and thus completely 

 obstruct the orifice of these arteries into the ventricles (Fig. 211). This 

 closure of the' semi-lunar valves is rendered more perfect by the fact that 

 the three leaflets do not originate from the walls of the artery on the 

 same plane, but their origins form a spiral line around the base of 

 y these arteries ; consequently, the 



£ _L_ leaflets descend one over . the 



other, and, to a certain extent, 

 overlap each other, and thus 

 completely prevent regurgitation 

 of the arterial blood into the 

 ventricles. 



The Sounds of the Heart. — 

 When the ear is applied to the 

 walls of the thorax over the 

 heart, two sounds are recog- 

 nized, which differ in intensity, 

 in pitch, and in duration. These 

 sounds are described as the 

 first and second sounds of the 

 heart. The first of these sounds 

 is dull and prolonged, and coincides with the systole of the ventricle ; the 

 second sound is shorter, sharper, and of higher pitch, and occurs at the 

 end of the systole, or at the commencement of ventricular relaxation. 

 The musical interval between these two sounds corresponds about to a 

 fourth; the pitch of both sounds varies, but this interval is usually 

 preserved. The first sound is heard with greatest distinctness at the 

 spot where the impulse is felt, and is not dependent upon the cardiac 

 impulse, from the fact that it exists after the removal of the chest-walls. 

 As it coincides with the contraction of the ventricle, it also, of course, 

 coincides with the closure of the auriculo-ventricular valves, and is 

 largely due to the action of these valves. 



The character ot the first sound of the heart is not, however, purely 

 valvular in nature, and is not what would be expected from the sudden 

 closure of a membranous valve. That it is, however, largely due to tiic 



Fig. 211.— Base of the Heart of the Horse, 

 Both Auricles beixg Removed. (Miiller.) 



r V, right auricle ; IV, left auricle ; rK, right ventricle ; IK, 

 left ventricle ; 1, coronary furrow ; 2, right, 3, left auriculo-ven- 

 tricular orifices; 4, origin of the aorta; 5, aortic semi-lunar 

 valves ; 6, origin of the pulmonary artery ; 7, its semi-lunar 

 valves ; 8, auricular septum. 



