236 VETERINARY PHYSIOLOGY 



2. Temperature of the Body. The pulse varies with the body 

 temperature, being increased as the temperature rises. 



3. Muscular Exercise increases the rate of the heart, first by 

 driving the blood from the muscles into the great veins (p. 291), 

 and second, by developing substances such as C0 2 , which act 

 directly upon the cardiac and respiratory mechanisms. 



4. Stimulation of certain nerves especially those of the 

 abdomen tends to cause a retardation in the rate of the 

 heart (p. 256). 



2. Sequence of Events. The sequence of events making up 

 the cardiac cycle is simpler in the mammal than in the frog. 



The contraction starts in the great veins which enter the 

 auricles, and spreads down along them to these chambers. 

 This corresponds to the contraction of the sinus in the frog's 

 heart. It is followed by a short sharp contraction of the 

 auricles, which become smaller in all directions and seem to 

 be pulled down towards the ventricles. The contraction of 

 the auricles in mammals is not accompanied by so marked a 

 dilatation of the ventricles as in the frog. 



After the auricles have fully contracted, the contraction of 

 the ventricles' begins, and immediately the auricles relax and 

 resume their original size. 



The ventricular contraction develops suddenly, lasts for 

 some time, and then suddenly passes off. 



The contraction of the ventricles is followed by a period 

 during which both auricles and ventricles remain relaxed. 

 This is called the pause of the cardiac cycle. 



The cardiac cycle in mammals may be represented as in fig. 113. 



3. Duration of the Phases. Ventricular systole lasts three 

 times as long as auricular systole. 



The duration of these two phases in relationship to the 

 pause varies very greatly. Whatever may be the rate of 

 the heart, the auricular and ventricular systoles do not vary, 

 but in a rapidly acting heart the pause is short, in a slowly 

 acting heart it is long. 



4. Changes in the Shape of the Chambers. 



1. Auricles. These simply become smaller in all directions 

 during systole. 



