COURSE OF CIRCULATION IN MAMMALS 157 



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so as to entirely fill the ventricle. The sound of the tricuspid valve 

 is heard loudest at the junction of the fourth right costal cartilage 

 with the sternum, that of the mitral over the apex beat, that of the 

 aortic semilunar valves in the direction of the aorta, where it comes 

 nearest to the surface at the second right costal cartilage the pul- 

 monary to the left and external to the margin of the sternum. The 

 sounds are changed in character by valvular lesion (Figs. 62 and 63) 

 or muscular weakness of the heart, and afford important signs to the 

 physician. 



Murmurs are produced by eddies setting some part of the mem- 

 branous walls or valve flaps in vibration. Thus, if a fine instrument 

 provided with a hook be passed down the carotid into the aorta, 

 and the aortic valves are torn and rendered incompetent, a murmur 

 results. Inflammation, shrinkage, and incompetence of the valves 

 results from rheumatic and other infections. 



If a stethoscope be placed over a large artery, a murmur will be 

 heard, caused by the blood rushing through the vessel narrowed by 

 the pressure of the instrument. The fluid escapes into a wider portion 

 of the vessel beyond the point of pressure, and the. sound is caused 

 by the eddies set up there throwing the membranous wall of the vessel 

 into vibration. Such a sound is heard over an aneurism. The 

 placental bruit heard during pregnancy is a sound of this kind, arising 

 from pressure on the uterine arteries. In cases of insufficient aortic 

 valves a double blowing murmur may be heard, the first being due to 

 the rush of blood into the vessel, and the second by the regurgitation 

 of the blood back into the ventricle. These murmurs are produced by 

 eddies of blood setting the membranous parts into vibration. 



Occasionally a murmur seems to be produced by the displacement 

 of air in the bronchial vessels by the beat of the heart, and may simulate 

 the murmur of aortic incompetence. By placing a stethoscope over 

 the jugular vein on the right side, and above the collar-bone, a murmur 

 is heard -the bruit de diable particularly if the subject turn his head 

 to the left. This is held to be due to the vibration of the blood in the 

 jugular vein rushing from the dilated to the contracted part. It is 

 more marked during auricular diastole and during inspiration. 



