140 THE BLOOD AND ITS CIRCULATION. 



thorax of an animal be opened, and the heart taken out with 

 the hand, this change of consistency, coinciding with the 

 ventricular systole, may be felt over the whole surface : the 

 pulsation of the heart is then felt, as when the hand, placed 

 over the cardiac region, feels it through the wall of the chest. 

 The displacement, the recoil, and even the torsion of the 

 heart thus have little to do with producing the shock felt ; it 

 is principally owing to the change in the condition of the 

 ventricle, which, at first flabby and soft, stiffens throughout 

 in order to expel its contents. 



In the auscultation of the heart we hear, during one of its 

 contractions, two sounds succeeding each other at short inter- 

 vals. It has been demonstrated by a long series of vivisec- 

 tions that the first sound is produced during the systole of 

 the ventricle, and the second immediately after the systole, 

 when the heart enters a state of complete repose. We are 

 agreed as to the explanation of the second sound: as it is 

 produced during the repose of the heart, it is evidently not 

 caused by any movement in that organ. It is, therefore, in 

 general, rightly attributed to the movements of the aortic 

 and pulmonary (semilunar) valves, which stiffen suddenly in 

 arresting the backward flow of the blood. This sound is 

 short and sharp (theory of Rouanet). 



It is more difficult to explain the first sound. It is gener- 

 ally supposed to be owing to the play of the auriculo-ventri- 

 cular valves ; but if these membranous folds really act as 

 valves, they ought to stiffen suddenly ; and as, moreover, 

 the first sound lasts a certain time, nearly corresponding 

 with that of the contraction of the ventricle, its intensity 

 and its length can only be explained by supposing it to be 

 caused by the muscular contraction of the walls of the ven- 

 tricle. If, on the other hand, we call to mind the description 

 given of the working of the auriculo-ventricular apparatus, 

 and take into account the resemblance of this sound to that 

 of a sail flapping in the wind, or of a towel suddenly taut- 

 ened when stretched out by the four corners, its explanation 

 becomes simple. It is a sonorous manifestation of the work- 

 ing of the membranous auriculo-ventricular sails, stretched 

 out by the papillary muscles and their tendons, as long as 

 the ventricular systole lasts. These long, jerky, and ener- 

 getic tensions are exactly what would produce the sound 

 which we have described. 



In order to sum up the relative length of the auricular and 

 ventricular systoles and diastoles, we will, with a line divided 



