THE PULSE-RATE IN VERTEBRATE ANIMALS 6^ 



The heart therefore, being asked to replenish the supply, must, 

 if it respond, give out the larger relative volume of oxygen- 

 containing blood in unit time the smaller the animal, and it 

 might do so either by expelling a larger amount with each beat 

 or by increasing the frequency of the beat. 



But by regulating the volume of blood supplied to the 

 muscles in unit time, the heart can only regulate the rate of 

 supply of oxygen if the oxygen is present in a constant per- 

 centage. This is the case in birds and mammals, in which the 

 blood in the systemic circulation leaves the left ventricle of the 

 heart with its haemoglobin saturated with oxygen. It is not 

 the case in the lower vertebrates, not even in crocodiles ; for 

 although they, like birds and mammals, have the oxygenated 

 blood completely separated from the rest in the heart, it becomes 

 mixed with other blood in the dorsal aorta and may become 

 so even in the conus. In other reptiles facilities for the 

 admixture of the blood coming from the lungs with blood 

 coming from other parts of the body are greater, since it may 

 happen even in the ventricle. In the dipnoi and amphibians, 

 other organs besides the lungs have respiratory functions and 

 the blood from the rest of the organs in the body may mix 

 with oxygenated blood elsewhere than in the ventricle and the 

 arterial system. Where there is only one auricle, as in the 

 dipnoi, and blood of all qualities enters the ventricle simul- 

 taneously, the percentage of oxygen in all the blood leaving the 

 ventricle must be variable. Where there are two auricles, the 

 one of which receives only oxygenated blood, as in amphibians 

 and reptiles, this need not be the case, since even where there 

 is only one ventricle the blood from the lungs, by entering 

 and leaving the heart after the rest, may remain very nearly 

 saturated. But such blood is by special arrangement supplied 

 to the head only, and the blood to the limbs and other muscles 

 is unsaturated, or very soon becomes so. In all these classes 

 of lower vertebrates, therefore, the heart itself could not 

 regulate the rate of oxygen-supply to meet different demands 

 by altering either the volume given out per beat or the fre- 

 quency of the beat. In fish, on the other hand, there is the 

 possibility of regulating it either by altering the frequency of 

 the respiratory movements or by altering the volume of blood 

 expelled in each heart-beat, since all the blood which supplies 

 the body has to pass first through the respiratory organs and 



