I.— PHYSIOLOGY 187 



closely approaching unity, for its chief food, the carbohydrates, contains 

 enough oxygen to combine with the hydrogen to form water ; that a 

 carnivorous animal has a quotient about o • 74 and an omnivorous animal 

 such as man a somewhat higher quotient ; and finally, that even a herbi- 

 vorous animal has a low quotient during starvation, for it then lives upon 

 its own tissues.' The complete evolution of the theory may be said to 

 date from Zuntz and Schumburg's calculations, for since that time the 

 respiratory quotient has been generally regarded as providing a quantita- 

 tive measure of the combustion ratio in all circumstances— after food, in 

 the post-absorptive state and during starvation. For convenience this 

 may be called the theory of the ' variable combustion ratio,' since this 

 varies with the respiratory quotient. 



Direct Calorimetry. 



Direct determinations of the output of heat in calorimeters have been 

 made over many years ; but up to the end of last century, when the 

 Atwater-Rosa respiration calorimeter was constructed, the complete 

 respiratory exchange was not measured simultaneously with the direct 

 calorimetric measurements. The heat output of man and animals, under 

 basal conditions, has been measured directly in a calorimeter by Benedict 

 and Carpenter, DuBois and his colleagues and Murlin and Lusk, and 

 compared with the heat output calculated indirectly by means of the 

 Zuntz-Schumburg figures from the oxygen intake and the carbon dioxide 

 output, which were measured simultaneously. The results obtained have 

 been held by their authors to justify the conclusion from average values 

 that the agreement between the two methods was sufficiently satisfactory 

 to give support to the theory of the ' variable combustion ratio ' outlined 

 above. 



However, this theory has not escaped criticism. Benedict and 

 Carpenter, when describing their observations on the metabolism of 

 subjects after a meal of carbohydrate, state (p. 173) that in many instances 

 the agreement betv/een direct and indirect calorimetry is extremely 

 unsatisfactory because of ' the abnormal conditions previously outlined 

 which obtain when excessive amounts of carbohydrate are ingested.' 

 Respiratory quotients below o • 707 — the theoretical quotient of the 

 combustion of fat — have not infrequently been obtained ; these are 

 impossible on the theory, since they can only be explained by the con- 

 version of fat into some more highly oxygenated substance such as carbo- 

 hydrate. These low quotients have been attributed by staunch upholders 

 of the theory to experimental error. 



The partial conversion of fat to carbohydrate was supported by M. S. 

 Pembrey, who became one of the earliest critic^ of the theory, after ob- 

 serving very low quotients in hibernating animals. Observations of this 

 kind date back to Regnault and Reiset who, when investigating the meta- 

 bolism of marmots, found, in the case of marmot C, for example, a respira- 

 tory quotient of 0399, when the animal was asleep and a quotient of 



