THE CIRCULATION 



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s/nus 



B. Physiology of the Heart. 



Cardiac Cycle. If any one part of the heart of a frog is 

 watched it is seen to undergo contractions and relaxations at 

 regular rhythmical intervals. 



1. General Description. In the frog a contraction, starting 

 from the openings of the veins, suddenly involves the sinus 

 venosus, causing it to become smaller and paler. This con- 

 traction is rapid and of short duration, and is followed by a 

 relaxation, the cavity again regaining its former size and 

 colour. As this relaxation 

 begins, the two auricles 

 are suddenly contracted 

 and pulled downwards 

 towards the ventricle at 

 the same time becoming 

 paler, while the ven- 

 tricle becomes more dis- 

 tended and of a deeper 

 red. The rapid brief auri- 

 cular contraction now gives 

 place to relaxation, and, 

 just as this begins, the 

 ventricle is seen to be- 

 come smaller and paler, 

 and if held in the finger 

 is felt to become firmer. This event takes place more slowly 

 than the contraction of either sinus or auricles. The chief 

 change in the ventricle is a diminution in its lateral diameter, 

 though it is also decreased in the antero- posterior and 

 vertical directions. During ventricular contraction the 

 bulbus is seen to be distended and to become of a darker 

 colour. The ventricular contraction passes off suddenly, the 

 ventricle again becoming larger and of a deep red colour. 

 At this moment the bulbus aortse contracts and becomes pale 

 and then relaxes before the next ventricular contraction. 



Each chamber of the heart thus passes through two 

 phases a contraction phase, a systole of short duration, 

 and a longer relaxation phase, the diastole. And the 



FIG. 110. Scheme of the Cardiac Cycle in 

 the Frog. S.S., Sinus Systole; A.S., 

 Auricular Systole; V.S., Ventricular 

 Systole; B.S., Bulbus Systole; P., Rest 

 of all chambers. 



