SOUNDS OF THE HEART 370 



therefore, produced to allow of this extra work being done. In 

 a case like this, it is not surprising to find later that the heart is 

 unable to respond to the call for any extra effort, and that even 

 slight exercise results in distress. 



(2) Incompetence. The failure of any of the valves to close 

 completely allows blood to trickle back into the empty expelling 

 chamber. This regurgitation throws the tightly stretched cusps 

 into vibration and produces a murmur. If this sound is heard 

 during ventricular systole it may be ascribed to incompetence 

 of either of the atrio- ventricular valves— if during ventricular 

 diastole, the aortic or pulmonary valves are at fault. 



In aortic incompetence the sound will be best heard where the 

 aorta comes nearest to the surface, viz. at the second right costal 

 cartilage ; in pulmonary incompetence the murmur will be best 

 heard over the second left interspace just external to the margin 

 of the sternum. 



The sound of the mitral valve is heard at its best just over the 

 apex of the heart ; that of the tricuspid valve at the junction of 

 the fourth right costal cartilage with the sternum. 



By means of a recording microphone, a tracing may be obtained 

 representing the values of these sound waves. Such a phono- 

 cardiogram (Fig. 93, Chap. XXVI.), if taken simultaneously with 

 a tracing of the mechanical or electrical changes of the heart, 

 is of great use to the physician as an indication of cardiac 

 efficiency. 



If any of the large arteries be compressed, say by the imposition 

 on the overlying skin of the stethoscope, murmurs will be heard. 

 These sounds are caused by the sudden narrowing of the lumen 

 of the artery by the pressure of the instrument. The blood rushes 

 through the narrowed part into the comparatively wide part of 

 the vessel beyond the point of pressure and so sets up eddies. 

 The vibratory movement of the fluid is transmitted to the arterial 

 walls and passed on to the internal ear (Part II.). 



Considering the circulatory mechanism as a whole, one is struck 

 by the extraordinary efficiency of this method of transport. 

 Comparatively little energy is wasted. Fluid leaves the ventricle 

 under a pressure of over 1 00 mm. Hg, passes through a system of 

 large and small tubes and returns to the reservoir of the central 

 pump with no surplus pressure. Just enough blood pressure is 

 provided to carry the fluid within range of the atrial suction and no 

 more. It has been stated that by the rhythmic contractions 

 (peristaltic waves) of the muscular coat of the vessels, the blood is 

 helped along its course. The mechanics of peristalsis will be 

 considered shortly (Chap. XXVIII.). 



