CIRCULATION 



235 



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 con- xK ; /\ 



traction the bulbus is seen 

 to be distended and to 

 become of a darker colour. 

 The ventricular contrac- 

 tion passes off suddenly, 

 the ventricle again be- 

 coming larger and of a 

 deep red colour. At this 

 moment the bulbus aortee 

 contracts and becomes 

 pale and then relaxes 

 before the next ventricu- 

 lar contraction. (Practical 

 Physiology.) 



3/NU3. 



AURICLES 



VBTMOf, 



BULBU5 



FIG. 112. Scheme of the Cardiac Cycle in 

 the Frog. S.S., sinus systole ; A.S., auri- 

 cular systole ; V.S., ventricular systole ; 

 B.S., bulbus systole ; P. , rest of all cham- 

 bers. 



Each chamber of the heart thus passes through two phases 

 a contraction phase, a systole of short duration, and a longer re- 

 laxation phase, the diastole. And the sequence of events in the 

 frog's heart might be schematically represented as in fig. 112. 



B. Mammal. 1. Rate of Recurrence. The rate of recurrence 

 of the cardiac cycle varies with the animal examined. In 

 the adult horse it is about 36 to 40 per minute. Many 

 factors modify the rate of the heart. 



Eate of heart per minute in different animals: 



Horse . . . 36 to 40 



Ox . . . . 45 to 50 

 Sheep 70 to 80 



Dog . 90 to 100 



Rabbit . . 120 to 150 



1. Period of Life. The following table shows the average 

 rate of the heart at different ages : 



HORSE 



New born . 

 Under 1 year 

 4 years 



92 to 132 per minute. 

 50 to 68 

 50 to 56 



