CHAPTER XV. 



v^^ 



THE WORKING OF THE HEART AND BLOOD-VESSELS. 



The Beat of the Heart. It is possible with some little 

 skill and care to open the chest of a living narcotized ani- 

 mal, such as a rabbit, and see its heart at work, alternately 

 contracting and diminishing the cavities within it, and relax- 

 ing 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 is expanding. The contraction of any part of the heart 

 is known as its systole and the relaxation as its diastole, and 

 since the two sides of the heart work synchronously, the au- 

 ricles together and the ventricles together, we may describe a ' 

 whole "cardiac period" or "heart-beat" as made up succes- 

 sively of auricular 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 muscle-fibres of 

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

 tween the finger and thumb feels soft and flabby, but during 

 the systole it (especially its ventricular portion) becomes hard 

 and rigid. 



Change of Form of the Heart. During its systole the 

 heart becomes shorter and rounder, mainly from a change in 

 the shape of the ventricles. ' A cross-section of the heart at 

 the base of these latter during diastole would be elliptical in 

 outline, with its long diameter from right to left; during the 

 systole it is more circular, the long axis of the ellipse becom- 

 ing shortened, while the dorso-ventral diameter remains little 



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