THE CIRCULATION. 245 



duced a feeble sound of rather low pitch. This sound is believed to 

 be produced by the contraction of the muscular fibres of ventricular 

 walls, and has been termed "muscle-sound" 



Any muscle whatever, during its contraction, gives rise to a dull 

 sound. It is evident then that, during contraction of the ventricle, 

 this same phenomenon must occur and so contribute its part to the 

 production of the first sound. 



From new experiments it appears that the role of the muscular 

 contraction is more important than it has generally been thought to 

 be. For verification of this the following experiment seems to be 

 decisive : 



The heart is exposed in a dog which has been poisoned with 

 curare and in which artificial respiration has been maintained dur- 

 ing two hours. The left ventricle is cut open in front and at the 

 back with scissors along the intraventricular partition. The incisions 

 are rapidly lengthened from the apex toward the base in such a 

 manner as to turn completely outside all the ventricular wall. This 

 portion is no longer held to the rest of the heart except by the auricle. 



The suspended piece of ventricular wall, under these conditions, 

 continues to contract with force and rhythm for some seconds. If the 

 stethoscope be applied to the internal face of the stump, it per- 

 mits us to hear at the moment of each contraction a sound that is 

 exactly like the one which had been perceived in the nonmutilated. 

 There is, however, a vast difference in intensity, the sound emitted 

 from the experimental heart-muscle being very weak. 



The contraction of the auricles is not considered at all as being 

 a factor in the production of cardiac sounds. Repeated experiments 

 have proved the auricular contractions to be inaudible. 



Position of Valves and the Areas of Audibility. The pulmonary 

 and tricuspid of the right side lie nearer the surface than the aortic 

 and mitral of the left. 



The best point to hear the pulmonary valve is chiefly behind 

 the third left costal cartilage. For the aortic valve it is behind the 

 left half of the sternum, on a level with the third interspace. For the 

 mitral valve it is behind the left half of the sternum, on a level with 

 the fourth and upper border of the fifth cartilage. For the tricuspid 

 valve, behind the lower fourth of the sternum, to the right of the 

 middle line from the fourth right cartilage to a point behind the 

 junction of the sixth right cartilage to the sternum. 



Variations in Heart-sounds. Increase in the intensity of the 

 first sound of the heart is indicative of a more vigorous contraction 



