THE PULSE-RATE IN VERTEBRATE ANIMALS 65 



What is true of the cold-blooded vertebrates is true of the 

 embryo of the warm-blooded animal in respect of want of constant 

 percentage of oxygen in the blood-supply to the body as in so 

 many other respects. Although the blood leaving the left ventricle 

 is blood brought straight from the respiratory organ of the 

 embryo, the allantois, this serves only to supply the head, the 

 great aorta through which it flows being joined, after having 

 given off the vessels to the head, by the ductus Botalli bringing 

 blood (through what is afterwards the pulmonary artery) from 

 the right ventricle, which has received it from all organs and 

 kept all but that from the respiratory organs. Thus the per- 

 centage of oxygen, which may have been constant in the blood 

 leaving the left ventricle, is no longer so by the time it reaches 

 the body of the animal and this must continue to be the case so 

 long as the ductus Botalli remains open, which it does until 

 the time of hatching or birth. When it closes, the blood from 

 the right ventricle which would otherwise have gone along it, 

 can only go to the lungs, and the channels from the lungs to 

 the left auricle, the pulmonary veins, become functional with 

 the lungs themselves, so that now blood saturated with oxygen 

 enters the left auricle from the respiratory organ of the adult, 

 and (the septum between the two auricles being now complete) 

 passes unaltered into the left ventricle, whence it is driven to 

 supply not only the head but now also the body. It would be 

 interesting to know whether in the young guinea-pig and chick, 

 which are able to regulate their temperature as soon as they 

 come into the world, the ductus Botalli closes earlier than it 

 does, e.g., in young mice, rats and pigeons which can only regu- 

 late their temperature very imperfectly when born or hatched 

 and take a week or more to develop this power. It would help 

 us to find out whether, or to what extent, the want of power to 

 regulate temperature depends upon the fact that any attempt of 

 the heart to adapt itself to meet special demands made upon it 

 for oxygen, potentially or actually, could meet with only 

 imperfect success. 



Let us now see in how far the hearts of birds and mammals, 

 having the power to regulate the oxygen-supply by regulating 

 the volume of blood expelled in unit time, succeed in doing so 



meeting permanent differences of oxygen-requirement, did these exist, in the 

 different species of reptiles and amphibians, in the way that alteration of volume- 

 rate lends itself in birds and mammals. 



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