490 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



of the body are greater, since it may liajopen even in the ventricle. In 

 the dipnoi and amphibians, moreover, 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 simulta- 

 neously, the percentage of oxygen in all the blood leaving the ventri- 

 cle 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 the blood from the lungs, by entering and 

 leaving the ventricle 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. In all 

 these classes of lower vertebrates, therefore, the heart itself could not 

 regulate the rate of oxygen supply to meet different demands by alter- 

 ing either the volume given out per beat or the frequency 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 



/; 



A B 



Fig. I. — Diagrams to show the sort of relation of tlie oxygen to the blood-volume in the 



systemic circulation. 

 A, warm-blooded vertebrate. B, reptile and amphibian. 



blood which supplies the body has to pass first through the respira- 

 tory organs, and so would contain a constant percentage of oxygen, 

 even if its haemoglobin did not become fully saturated. 



In fish, amphibians, and snakes, the attempt seems occasionally to 

 be made to maintain a temperature above that of the environment 

 (16a), but in how far it approaches to being constant we only know 

 for two specimens of the Indian python (13a). It would be inter- 

 esting to find out whether in a species of Thyrinus, the bonito, which 

 may have an internal temperature as much above that of the environ- 

 ment as the python, it is more nearly constant, and how far the de- 

 mand for oxygen in the one and in .the other varies both with the ex- 

 ternal temperature and with the size of the individual ; moreover, if 

 it so varies, in what way the supply is regulated to meet the different 

 demands. 



The difference obtaining between the warm-blooded vertebrates on 

 the one hand, and all the cold-blooded except fish, on the other, with 

 regard to the relation of the oxygen to the volume of the blood in the 

 systemic circulation, is illustrated in figure 1. Regulating the vol- 

 ume rate would regulate the oxygen supply only with arrangement A. 



