292 



THE CIRCULATION 



papillary muscles and fleshy columns during the first portion of the systole. 

 It cannot be doubted that the friction of the heart's apex against the 

 chest-wall plays a part in producing this sound, and probably also the 

 vibration of the columns of blood both within the ventricles and in the 

 first part of the great arteries. When one considers the extreme and 

 sudden vigor with which the ventricles contract, becoming very tense 

 and hard, it is not difficult to understand how a sound is produced. 

 The valves' closure has comparatively little to do with it, for when the 

 valves are mechanically kept from closing, the gross sound is altered 

 but little, and the main tone persists in its entirety. That many elements 

 enter into the production of the first sound is certain, and of these proba- 

 bly the more important have now been described. 



FIG. 156 



Nutrition of the heart-muscle in the pig; Golgi's method: I, intermuscular spaces from which 

 numerous nutrition-canals (s) pass into the muscle-fibers; k, blood-capillaries; m, sectioned 

 muscle-fibers. (Nystrom.) 



The second sound follows the first after a very brief interval, perhaps 

 0.1 second, the gradual dying-away of the first sound including most 

 of this period. There has been less discussion as to the cause of the 

 second sound, for it is obviously the kind of noise made by the sudden 

 closure of a valve at the end of a tube in which the pressure of a liquid is 

 high. Furthermore, it occurs just at the time when the two semilunar 

 valves are closing namely, just after the end of ventricular systole. 

 It is then undoubtedly made by the sudden closure of these valves. Did 

 the pulmonary and the aortic valves close exactly at the same time, this 

 sound would be still shorter and sharper than it is. Owing, however, 

 probably to local variations in the pressures within the aorta and the pul- 



