376 LUDWIG AND MODERN PHYSIOLOGY. 



the interpretation of the processes of life by the circumstance that 

 chemical and physical knowledge was in itself too little advanced. 

 Comparison was impossible, for the standards were not forthcoming. 



Vitalism in its original form gave way to the rapid advance of knowl- 

 edge as to the correlation of the physical sciences which took place in 

 the forties. Of the many writers and thinkers who contributed to that 

 result, J. E. Mayer and Helmholtz did so most directly, for the con- 

 tribution of the former to the establishment of the doctrine of the 

 conservation of energy had physiological considerations for its point of 

 departure; and Helmholtz, at the time he wrote the Erhaltnng der 

 Kraft, was still a physiologist. Consequently when Ludwig's cele- 

 brated Lehrbuch came out in 1852, the book which gave the coup de 

 grace to vitalism in the old sense of the word, his method of setting 

 forth the relations of vital phenomena by comparison with their physical 

 or chemical counterparts, and his assertion that it was the task of 

 physiology to make out their necessary dependence on elementary con- 

 ditions, although in violent contrast with current doctrine, were in no 

 way surprising to those who were acquainted with the then recent 

 progress of research. Ludwig's teaching was indeed no more than a 

 general application of principles which had already been applied in 

 particular instances. 



The j>roof f the nonexistence of a special "vital force" lies in the 

 demonstration of the adequacy of the known sources of energy in the 

 organism to account for the actual day by day expenditure of heat and 

 work; in other words, on the possibility of setting forth an energy bal- 

 ance sheet in which the quantity of food which enters the body in a 

 given period (hour or day) is balanced by an exactly corresponding- 

 amount of heat produced or external work done. It is interesting to 

 remember that the work necessary for preparing such a balance sheet 

 (which Mayer had attempted, but from want of sufficient data failed 

 in) was begun thirty years ago in the laboratory of the Eoyal Institu- 

 tion by the foreign secretary of the royal society. But the determina- 

 tions made by Dr. Frankland related to one side of the balance sheet, 

 that of income. By his researches in 1866 he gave physiologists for 

 the first time reliable information as to the heat value (i. e., the amount 

 of heat yielded by the combustion) of different constituents of food. 

 It still remained to apply methods of exact measurement to the expendi- 

 ture side of the account. Helmholtz had estimated this, as regards 

 man, as best he might, but the technical difficulties of measuring the 

 expenditure of heat of the animal body appeared until lately to be 

 almost insuperable. Now that it has been at last successfully accom- 

 plished, we have the experimental proof that in the process of life there 

 is no production or disappearance of energy. It may be said that it 

 was unnecessary to prove what no scientifically sane man doubted. 

 There are, however, reasons why it is of importance to have objective 

 evidence that food is the sole and adequate source of the energy which 



