CHAPTER XV. 



THE WORKING OF THE HEART AND BLOOD. 

 VESSELS. 



The Beat of the Heart. It is possible by methods known 

 to physiologists to open the chest of a living narcotized 

 animal, such as a rabbit, and see its heart at work, alter- 

 nately contracting and diminishing the cavities within it 

 and relaxing and expanding them. It is then observed 

 that each beat commences at the mouths of the great veins; 

 from there runs over the rest of the auricles, and then 

 over the ventricles; the auricles commencing to dilate the 

 moment the ventricles commence to contract. Having 

 finished their contraction, the ventricles also commence to 

 dilate and so for some time neither they nor the auricles 

 are contracting, but the whole heart expanding, f The con- 



wn_as its systole 

 and the relaxation as its diastole, and since theTTwo aides 

 of the heart work synchronously7"the auricles together and 

 the ventricles together, we may describe a whole "cardiac \ 

 period" or " heart-beat" as made up successively of auricu- \ 

 lar systole, ventricular systole, and pause. This cycle is / 

 repeated about seventy times a minute; and if the whole 

 time occupied by it be subdivided into 100 parts, about 

 9 of these will be occupied by the auricular systole, about 

 '30 by the ventricular systole, and 61 by the pause: 

 during more than half of life, therefore, the muscles of 

 the heart are at rest. In the pause the heart if taken be- 

 tween the finger and thumb feels soft and flabby but dur- 

 ing the systole it (especially in its ventricular portion) be- 

 comes hard and rigid. 



Change of Form of the Heart. During its systole the 



