THE CARDIAC CYCLE (rEVOLUTIO CORDIS) 273 



through the system. Its action, however, is not comparable to that 

 of a piston-pump, but rather to that of a rubber bulb when compressed 

 by the hand. The contraction of its muscular substance diminishes 

 the size of its cavities so that the blood contained therein is subjected 

 temporarily to a high degree of pressure. Each contraction of the 

 heart, or systole, is immediately followed by a period of relaxation, or 

 diastole, and the latter in turn by a period of rest. These three phases 

 together constitute the cardiac cycle. 



The general rule, that the frequency of the heart is indirectly 

 proportional to the size of the body, finds its application throughout 

 the animal kingdom, but particularly among the warm-blooded ani- 

 mals. This fact is clearly brought out by the following compilation: 



Elephant 25 cycles in a minute 



Camel 30 cycles in a minute 



Lion, horse, ox 40 cycles in a minute 



Donkey 50 cycles in a minute 



Panther 60 cycles in a minute 



Sheep 70 cycles in a minute 



Man 70 cycles in a minute 



Dog 100 cycles in a minute 



Rabbit 150 cycles in a minute 



Mouse 175 cycles in a minute' 



Among the cold-blooded animals this relationship is not always 

 apparent, because their bodily functions are more markedly influenced 

 by outside conditions. The heart of the frog or turtle beats 40 to 50 

 times in a minute, a rather slight frequency for such small animals. 

 The fact that the cardiac frequency is greater in small animals, need not 

 surprise, because their metaboHsm is greater on the whole than that of 

 larger animals. This must necessarily be so, because as the former 

 present a proportionately larger surface to the medium in comparison 

 with their mass, they must lose larger amounts of heat. This greater 

 loss is counteracted by more intense metaboUc changes. 



The human heart is subject to various influences, such as age, sex, 

 temperature, barometric pressure, posture, muscular movements, 

 emotions, etc. Before birth, the heart of the female beats about 140 

 to 145 times in a minute,' and that of the male about 130 to 135 per 

 minute. Conditions being favorable, it is posible to make use of this 

 fact in foretelHng the sex of the fetus. It is still very frequent at 

 birth, but its rate is markedly decreased during the first year of extra- 

 uterine life and more gradually during the subsequent years. Late in 

 life its frequency is again increased. 



At birth 140 



Infancy 120 



Childhood 100 



Youth 90 



Adult age 75 



Old age 70 



Extreme old age 75-80 



18 



