I.— PHYSIOLOGY 193 



±0-005) ^rid between O2 and calories (correlation coefficient 0-92 

 ± o-oo8). The lines C represent the relation of the carbon dioxide 

 output and oxygen intake to the heat produced, when pure carbohydrate 

 is burnt in a calorimeter ; they were calculated from the value given by 

 Lusk, viz. 5 -047 Cal. per litre of oxygen and carbon dioxide. The lines 

 K and M represent the relation of the heat to the carbon dioxide output 

 on the left-hand side and to the oxygen intake on the right when two 

 different kinds of human fat are burnt. These two fats, from liver and 

 skeletal muscle analysed by Cathcart and Cuthbertson, were chosen 

 because they represent extremes, and the lines of the other nine human 

 fats analysed by them lie in between. Most of the carbon dioxide points 

 lie between the theoretical carbohydrate and fat lines which are widely 

 spaced ; but this is certainly not the case with the oxygen points, as the 

 theoretical limits are much narrower. It is possible to look on the carbon 

 dioxide as the product of combustion of a mixture of carbohydrate and 

 fat, but, because the oxygen points lie for the most part outside the 

 theoretical limits, the oxygen intake cannot be entirely used up in 

 combustion but must in part be concerned in some kind of conversion. 



The equation of the regression line to be used for determining the heat 

 from the COj is 



Calories (per hour) = 3 •18CO2 (grm. per hour) — 4*92 . . (i) 



The Theory of the Constant Combustion Ratio. 



There is, then, a fundamental error in the theory that the respiratory 

 quotient indicates the proportion of carbohydrate and fat that is being 

 burnt in the body under basal conditions, the theory of the variable 

 combustion ratio ; this theory (No. i) must be abandoned. There 

 are only two other possible theories (Nos. 2 and 3) to be considered. 

 The simplest one (No. 2) is that carbohydrate and fat are always burnt 

 in the body in a fixed proportion, and the respiratory quotient, when it is 

 high, measures the amount of conversion of carbohydrate into some less 

 oxygenated material such as fat, and, when it is low, measures the 

 reverse change. 



The other possibility (No. 3), which is more complicated, is intermediate 

 between Nos. i and 2. The change in the respiratory quotient is the 

 resultant of two independent variables : a variation in the combustion 

 ratio and a variation in the amount of conversion of one foodstuff towards 

 the other. As the respiratory quotient rises there are two opposed 

 processes of gradually increasing intensity simultaneously at work, one 

 an increasing combustion of carbohydrate relative to fat, which tends to 

 make the heat, calculated by Zuntz-Schumburg, too low, the other an 

 increasing conversion of carbohydrate into fat which tends to make it 

 too high, as it is an endothermic reaction. Against this view there is a 

 fundamental objection. The conversion of carbohydrate and fat may be 

 represented by a reversible equation : 



4C27H6o04 + 4202;^i8C6Hio06 + ioH20 . . (2) 



