394 



VETERINARY PHYSIOLOGY 



contracted and pulled downwards towards the ventricle, at 

 the same time becoming paler, while the ventricle becomes 

 more distended and of a deeper red. The rapid brief 



auricular contraction now 

 gives place to relaxation, 

 and, just as this begins, 

 the ventricle is seen to 

 become smaller and paler, 

 and, if held in the fingers, 

 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 



S/ffUS. 



aukicle::> 



V£fiTniat5^ 



BULBU5 



is a diminution in its 

 lateral diameter, though 

 it is also decreased in the 

 antero-posterior and verti- 



During 



Fig. 169. — 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. The upstrokes 

 represent systole, the downstrokes 

 diastole. 



cal directions 



ventricular contraction the bulbus is seen to be dis- 

 tended and to become of a darker colour. The ventricular 

 contraction passes off suddenly, the ventricle again becom- 

 ing 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 {Practical 

 Physiology). 



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. The sequence of 

 events in the frog's heart might be schematically represented 

 as in fig 169. 



B, Mammal. 



1. Rate of Recurrence. — The rate of recurrence of the 

 cardiac cycle varies with the animal examined. In man it 

 is, in adult life, about 72 per minute. In the adult horse 

 it is about 36 to 40 per minute. 



