248 VETERINARY PHYSIOLOGY 



it is usual to go to the right extremity of the area in order, 

 as far as possible, to eliminate the mitral sound. The best 

 situation to select is at the junction of the fifth right costal 

 cartilage with the sternum. 



Cardiac Murmurs. When the valves are diseased and fail to 

 act properly, certain continuous sounds called cardiac murmurs 

 are heard. 



These owe their origin to the fact that, while a current of 

 fluid passing along a tube of fairly uniform calibre is not 

 thrown into vibrations and therefore produces no sound, 

 when any marked alterations in the lumen of the tube 

 occur either a sudden narrowing or a sudden expansion 

 the flow of fluid becomes vibratory, and, setting up vibrations 

 in the solid tissues, produces a musical sound. 



Such changes in the calibre of the heart are produced in 

 two ways : 



1st. By a narrowing, either absolute or relative, of the 

 orifices between the cavities stenosis. 



2nd. By a non-closure of the valves incompetence. 



Stenosis. If one of the aurieulo-ventrieular orifices . is 

 narrowed, a murmur is heard during the period at which 

 blood normally flows through this opening. A reference 

 to fig. 116 at once shows that this occurs during the 

 whole of ventricular diastole, and that the flow is most 

 powerful during the first period of ventricular diastole and 

 during auricular systole. 



If the aortic or the pulmonary valve is narrowed the murmur 

 will be heard (fig. 116) during ventricular systole. 



The narrowing need not be absolute. A dilatation of the 

 artery will make the orifice relatively narrow, and will produce 

 the same result. 



Incompetence. If the aurieulo-ventrieular valves fail to 

 close properly, then, during ventricular systole, blood will be 

 driven back into the auricles, and a murmur will be heard 

 during this period. 



If the aortic or pulmonary valve fails to close, the blood will 

 regurgitate into the ventricle from the arteries during ventricu- 

 lar diastole, and a murmur will be heard during this period. 



By the position at which these murmurs are best heard the 

 pathological condition producing them may be determined. 



