A CRITIQUE OF THE BODY-SURFACE LAW. 197 



There are even very real purely physical difficulties in the way of 

 assuming that the superficial body-area can be considered a true meas- 

 ure of the heat-loss which is assumed to bear a causal relation to heat- 

 production. Heat-loss does not occur exclusively from the skin. A 

 considerable proportion of the total heat generated is given off from the 

 lungs through the warming of the air and through the vaporization of 

 water. From a large number of experiments with human subjects at 

 rest, either with or without food, it is found that on the average 2.3 

 per cent of the total heat for 24 hours is required to warm the inspired 

 air; 10 per cent is lost as the result of vaporization of water from the 

 lungs and 12.3 per cent from the vaporization of water from the skin. 89 

 A recent critical study by Soderstrom and Du Bois 90 indicates that 

 with normal individuals somewhat more than 25 per cent of the total 

 heat is lost in the vaporization of water from the lungs and skin. 



Turning from purely experimental tests to those in which the results 

 of experimentation are subjected to statistical analysis, we may first 

 note that the estimates of body-surface area upon which most of the 

 conclusions have been based have been shown to be open to serious 

 criticism. It is to the credit of D. and E. F. Du Bois that they have 

 made possible greater precision in this phase of the work. 



In testing by statistical methods the validity of this "law" which 

 has held a conspicuous place in the literature of metabolism for over a 

 quarter of a century, we have started out from two interdependent 

 fundamental assumptions which seem axiomatic. 



(a) The primary requisite in testing any biological law is to deter- 

 mine quantitatively the degree of interdependence of the magnitudes 

 of the variables which it connects. 



(6) The true test of the validity of a law is its capacity for predict- 

 ing an unknown result. 



The chief argument used in the past in support of the body-surface 

 law has been that heat-production shows the least variation from 

 individual to individual when expressed in calories per square meter 

 of body-surface. We have shown that this argument is nullified by 

 the simple physical relationship between body- weight and body-surface. 

 The surface areas of similar solids are not directly proportional to their 

 weights, but to the two-thirds powers of their weights. Thus, in a 

 series of individuals whose body-surface area has been determined by 

 the Meeh formula, body-surface area must necessarily be less variable 



than body- weight. The ratio ^^surface must ' therefore > also be less 



ui j.i_ Total heat 



variable than Body . weight ' 



Since the body-surface measurements by the Meeh formula and 

 by the Du Bois height-weight chart are very closely correlated, the 



88 Benedict, Carnegie Inst, Wash. Pub. No. 77, 1907, p. 476. 

 90 Soderstrom and Du Bois, Arch. Intern. Med., 1917, 19, 946. 



