264 H. A. KREBS VOL. 4 (1950) 



values for brain in Table VI, where the brains of the 2 smaller sheep show higher values 

 than the 3 brains from the larger animals. 



Brain cortex. In the largest species (horse) the average Q02 of brain cortex was about 

 half the average Q02 value of the smallest species (mouse) nam.ely 48 % for the measure- 

 ments in medium II, and 46% for the measurements in medium III. In contrast, the 

 basal heat production per kg bodyweight of the horse is only 11% and 12% respectively 

 of that of the mouse. 



Kidney cortex. The changes of the Q02 values from species to species in this tissue 

 were similar to those of brain cortex. The average Q02 value of horse kidney cortex was 

 47% of that of mouse kidney. The average Q02 value for sheep kidney was only 14% 

 below that for guinea pig kidney, whilst the basal heat production per kg. bodyweight 

 of the sheep is only 37% of the guinea pig. Thus the decrease of the Q02 values with body 

 size was again much smaller than the decrease in the rate of the basal heat production. 



Spleen, Mug. For the horse the Q02 value of spleen tissue was about a quarter, and 

 for lung about one third, of the corresponding values for the mouse. In these two tissues 

 the discrepancies between the changes in Q02 and the changes in basal heat production 

 in relation to body size are thus not as great as in brain and kidney, but they are still 

 considerable. 



Liver. Liver shows greater Q02 changes with body weight than any other tissue 

 tested, especially in medium III (Tables V and VII). In medium II Q02 of horse liver 

 was 23%, and in medium III it was 13.5% of that of mouse liver. Thus, when comparing 

 the Q02 values obtained in medium II for these two species, about the same percentage 

 change is found as for the basal rate of heat production. But the parallelism over the 

 whole series of species is very imperfect. For example, the Q02 values for guinea pig, cat 

 and dog are about the same (-9.5; -10.2; -10.8), whilst the basal rate of heat production 

 shows a progressive fall with body weight (85; 50; 31). 



The changes of Q02 of liver with body weight reported in this paper are similar to 

 those found by Kleiber^, but owing to the differences in the media used the present 

 Q02 values are all higher than those reported by Kleiber. 



4. Rdle of muscle tissue in chemical temperature control 



As the rate of respiration of a number of homologous tissues of animals of different 

 sizes fails to show a strict parallelism with the basal rate of heat production of the intact 

 body, it remains to be explained how the characteristic differences in the basal rates of 

 heat production of animals of different sizes arise. One kind of explanation is contained 

 in various publications by Kestner^''^' '^^^ and Blank^"'*, who stated that the proportion 

 of highly active organs is somewhat greater in the body of small animals than in that 

 of large animals. He expressed the view that the "relative size of the brain and the large 

 glands can give a complete explanation of the different heights of metabolism in different 

 animals^"^". This view is not substantiated by quantitative measurements and such 

 data as are available cannot be reconciled with Kestner's hypothesis (see Kleiber*). 



An alternative explanation is offered by the conception that the relation between 

 Q02 and body weight found for the 5 tissues tested does not hold for every tissue ; that 

 there is at least one major tissue whose "basal" Q02 changes with the body weight 

 approximately parallel with the basal heat production; that this organ is the striated 

 musculature. 



The substance of this conception is, of course, not new in that it is commonly 

 References p. 262-269. 



