544 THE PHYSIOLOGY OF ALIMENTATION. 



From the point of view of their production of heat, these cycles are 

 all equivalent. But are they equivalent from the vital point of view"? 



Consider an ordinary alternative. The aliment passes from the 

 initial to the final state after being incorpotated with the elements of 

 the tissues and having participated in the vital operations. Here the 

 linieutary potential is transferred into thermal energy after having 

 traversed the intermediate phase of vital energy. This is a normal 

 case, the regular type in the alimentary evolution. In this case it 

 might be said that the aliment had fully performed its function; it had 

 served vital purposes before being changed in the heat, and had been 

 biothermogenic. . Now consider the most simple case of the irregular 

 or aberrant type. The aliment passes from the initial to the final state 

 without being incorporated in the living cellules of the organism, and 

 without taking part in its vital functions. It remains confined in the 

 blood and circulating fluids, but finally undergoes the same molecular 

 disintegration and liberates the same quantity of heat. Its chemical 

 energy changes at the first attack into thermal energy. The aliment is 

 a pure thermogene. It has abdicated a portion of its functions and 

 has been of less vital utility. 



Does this case present itself in reality? Can the same aliment be, 

 as supposed, a biothermogene and a pure thermogene 1 ? Some physi- 

 ologists, among them Fick of Wurtzburg, have maintained that it was 

 actually so with most aliments; the nitrogenous substances, hydrocar- 

 bons and fats, all being capable of either of these transformations. On 

 the other hand Zuntz and von Mering have absolutely denied the 

 existence of the aberrant type of pure thermogene, maintaining that 

 no substances whatever are directly decomposed into organic liquids 

 without the functional intervention of the histological elements. Still 

 other authors, finally, teach that a small number of alimentary sub- 

 stances thus suffer direct decomposition, and among them alcohol. 



The " theory of luxurious expenditure," of J. Liebig, and the "theory 

 of circulating albumen," of Voit, affirm that the proteid aliments suffer 

 in part a direct combustion in the blood vessels. This subject has occa- 

 sioned a celebrated discussion, and the opinions of physiologists are 

 still divided upon it. Disengaging the main object of discussion from 

 all the side issues which have been raised, the question is whether an 

 aliment always follows the same course of evolution whatever be the 

 circumstances, and in particular if it be introduced in great excess. 

 Liebig held that the superabundant j>ortion escaping by ordinary pro- 

 cesses was destroyed by direct combustion. He affirmed, for instance, 

 that substances containing an excessive amount of nitrogen instead of 

 running the usual cycle of vital operations suffered direct combustion 

 in the blood. We express the same idea to-day when we say that they 

 sustain an accelerated evolution, and that their energy, omitting the 

 intermediate stage, passes at once from the chemical to the thermal 



