234 HUMAN PHYSIOLOGY 



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 auriculo-ventricnlar orifices is 

 narrowed, we then hear the murmur during the period at 

 which blood normally flows through this opening. A refer- 

 ence to Fig. 114 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 pulmonary valve is narrowed the murmur 

 will be heard (Fig. 114) 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 auriculo-ventricular 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 valves fail to close, the blood 

 will regurgitate into the ventricles from the arteries during 

 ventricular diastole, and a murmur will be heard during this 

 period. 



By the position of these murmurs the pathological con- 

 dition producing them may be determined. 



9. Work of the Heart. The heart in pumping blood 

 through it is doing work, and the amount of work may be 

 expressed in work units e.g. kilogram metres. With each 

 beat something under 80 grms. of blood are thrown from 

 each ventricle into the aorta and pulmonary artery. Thus 

 the weight lifted may be 0*08 kilos. The output of blood 

 at each beat of the heart of the dog is measured by Roy's 

 cardiometer, a rigid walled air-tight case, which is placed 

 round the heart and connected with a piston-recorder, so 

 that the decrease in the volume of the enclosed heart due 



