SIGNIFICANCE OF PULSE KATE BUCHANAN. 491 



With any other arrangement, such as that in B, the absolute amount 

 of oxygen supplied in unit time could be increased by increasing the 

 frequency of the beat, but it could not in that way be regulated at all 

 accurately to suit special demands. 1 



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 re- 

 spects. 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 pul- 

 monary artery) from the right ventricle, which has received it from 

 all the organs and reserved all but that from the respiratory organs. 

 Thus the percentage 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 regulate their tempera- 

 ture 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 



1 Since this paper went to press, Krogh has published a series of articles in the Skand. 

 Arch. f. Physiol. (1910) in which, amongst other things, it is shown that a method of 

 regulating the oxygen supply to the body does exist in reptiles and amphibians. This 

 consists in adjusting the relative volumes of blood in the pulmonary and systemic arches 

 by alteration of resistance in the pulmonary arteries, this being effected by variations 

 in the tonus of their vaso-constrictor nerves. Thus, while the blood per beat driven into 

 the systemic circulation becomes less in volume the more oxygen the tissues consume, 

 its oxygen-tension becomes not only relatively, but absolutely, greater in consequence of 

 the increase in the volume going per beat through the lungs, which naturally involves a 

 greater absolute absorption of oxygen. Although a convenient way of meeting dif- 

 ferences of oxygen requirement in the individual, it is not one that would lend itself to 

 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. 



