292 CIRCULATION 



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 stethoscope will be 

 placed 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 (Figs. 66 and 67), 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 100 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 is provided to 

 carry the fluid within range of the auricular 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. XXVII.). 



One further point making for the economical working of the 

 inland transport service, owes its enunciation to John Hunter. 

 He wrote, " To keep up a circulation sufficient for the part and 

 no more, Nature has varied the angle of the origin of the arteries 

 accordingly." Suppose a point C is h units vertically distant 

 from an artery AB, the problem is to find out the route by which 

 the blood could be conveyed from A to C with the least possible 

 loss of energy. This is not necessarily by the shortest route or 

 by the route using the shortest piece of branch tubing. The 

 shortest route would be h units long and would arise from AB at 



