410 VETERINARY PHYSIOLOGY 



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. 



1st. Stenosis. — If one of the auriculo-ventricular orifices is 

 narrowed, a murmur is heard during the period at which 

 blood normally flows through this opening (p. 405). A 

 reference to fig. 176 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 svstole. 



If the aortic or the pulmonary valve is narrowed the 

 murmur will be heard (fig. 176) during ventricular systole. 



The narrowing need not be absolute. A dilatation of the 

 artery will make the orifice relatively narrow, and will pro- 

 duce the same result. 



2nd. 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 (fig. 176). 



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

 will regurgitate into the ventricle from the arteries during 

 ventricular diastole, and a murmur will be heard during this 

 period (fig. 176). 



By the position at which these murmurs are best heard 

 the pathological condition producing them may be deter- 

 mined. 



II. The Work of the Heart. 



The enormous variations in metabolism which the muscles 

 undergo between rest and activity have already been 

 considered (p. 266). The oxygen intake and output of 

 carbon dioxide mav increase tenfold during muscular work. 



To supply this oxygen the flow of blood through the 

 muscles must be proportionately increased, and this increase 

 is secured by (1) a dilatation of the blood-vessels going to the 

 muscles with, at the same time (2), a constriction of the 

 blood-vessels of the abdomen. 



