CIRCULATION. THE HEART. 133 



Harvey finally (1615-1628) established the theory of the 

 circulation of the blood as we hold it at this day. 



I. OF THE CENTRAL ORGAN OF THE CIRCULATION ; OF THE 



HEART. 



IN order to comprehend the functions of the heart we 

 must not think of it as we find it in the dead body, for there 

 is no trace there of the muscular elasticity, which is one of 

 the most important properties of a muscle, just as important 

 as the contractility, and having a special purpose in that cav- 

 ity of the heart called the auricle. 



Auricle. The chief function of the auricle, on account of 

 its power of dilatation, is to facilitate the flow of the venous 

 blood ; it may be said to have the same effect as blood-letting 

 at the extremity of the venous system, by which the pressure 

 of the fluid is consequently diminished. During four-fifths 

 of the time occupied by a cardiac revolution the auricle is in 

 a state of repose, and fills with blood, or rather, allows itself 

 to be filled, for it exercises little or no active aspiration on 

 the venous blood (see Respiration). It resembles, at this 

 moment, a soap-bubble, distended by air blown into it : thus 

 it becomes the receptacle of the blood, the ante-chamber of 

 the ventricle, a receptacle wherein a large quantity of blood 

 accumulates; and the auricular capacity being greater than 

 that of the ventricle, which it can immediately fill without 

 itself becoming completely emptied. 



When the auricle is full of blood it contracts suddenly, 

 and drives the blood towards the ventricle, as it were, in the 

 twinkling of an eye. Its contraction lasts from one-fourth 

 to one-fifth of the total cycle. Supposing that the heart con- 

 tracts sixty times in a minute, the contraction of the auricle 

 would last only one-fourth or one-fifth of a second, the rest 

 of the time being in a state of repose. By computing its 

 times of activity and repose, we might say that the auricle 

 is relaxed during eighteen hours out of the twenty-four. 



The contraction of this cavity tends to throw its contents 

 towards the ventricle, or to return them to the veins. There 

 are no valves in the direction of the veins (Eustachian valve 

 being excepted), or they are placed at a distance, and are 

 consequently incapable of preventing this reflux; but the 

 veins are full of blood, under feeble pressure, it is true, but, 

 nevertheless, some resistance is thus offered to the return of 

 the auricular contents. The condition of the ventricle is at 



