1867.] Food as a Motive Tower. 337 



in the interspaces between the capillaries. In its passage through 

 the lymphatic glands, all of the lymph that is susceptible of the 

 process is reconverted into blood. 



Closely connected with Liebig's food theory, is his theory of the 

 origin of muscular work. The final result of the disintegration of 

 muscular tissue is undoubtedly its oxidation; but Liebig assumed 

 that this was the immediate result of it. He supposed, and it is 

 still generally supposed by physiologists, that a portion of oxygen 

 separated from its combination in the blood and traversed the 

 walls of the capillaries with the nutritive fluid, and that as fast as 

 the new cells were built up by the one, the old ones were oxidized 

 by the other ; the oxidized and now lifeless products being carried 

 back to the blood to be ultimately excreted from the body as capable 

 of no further use. The force liberated in this oxidation was the force 

 which contracted the muscle, and so did the work. Lastly, it was 

 assumed by Liebig that inasmuch as the muscular tissue was formed 

 from the flesh-formers of the food, and was almost identical in 

 composition with them, the whole work of the body was derived 

 from the flesh-formers, which were therefore a true measure of the 

 amount of work which the body could accomplish. When muscle, 

 or the flesh -formers of food, are oxidized in the body, their nitrogen 

 is chiefly converted into a crystalline substance called urea, the 

 great bulk, if not the whole, of which is excreted through the 

 kidneys. Hence the amount of urea excreted has been adopted by 

 Haughton,* and subsequently by Playfair,f as a measure of ^the 

 amount of work effected by the organism. 



But these brilliant and beautiful theories encountered a certain 

 amount of opposition even from the very first. Lawes and Gilbert, 

 the well-known agricultural chemists, in describing some experi- 

 ments on the fattening of cattle made twenty years ago, pointed out 

 that the amount of urea which a pig excreted could be doubled 

 merely by doubling the amount of nitrogen in its food, and this 

 without any alteration in the quantity of work it did. Mayer, in a 

 work J which will for ever remain a landmark in the history of 

 science, combatted some portions of the theory with irresistible force, 

 and more recently Voit in Germany and Dr. Edward Smith in Eng- 

 land showed that the work of an animal could be enormously in- 

 creased without any proportionate augmentation in the excretion of 

 urea. This, with the important observation of Lawes and Gilbert, 

 proved conclusively that the amount of work done could not be 

 measured by the urea excreted. The theory which Mayer supported 

 with such ability and which had indeed, as Frankland points out, 



* ' Dublin Quarterly Journal of Medical Science,' vol. xxviii. et seq. 

 t ' On the Food of Man, iu relation to his Useful "Work.' 



X 'Die Organische Bewegung in ihrem Zusammenhange mit dcm Stoffwechsel. 

 Heilbronn,' 1845. 



