OH. XL.] ANIMAL HEAT. 595 



The place where Voit situates his circulating proteid is beyond 

 the ken of the anatomist ; it is in a mysterious space between 

 tissue-elements, blood-vessels, and lymph-vessels ; the chemist 

 meets with equal difficulties, as there is apparently no chemical 

 difference between tissue proteid and circulating proteid. I can, 

 therefore, arrive at no other conclusion than that these terms 

 are not only useless, but unscientific, and are the outcome of 

 speculations in a region where there is as yet no positive know- 

 ledge. These criticisms on Voit's theories do not, however, 

 by any means, lessen the importance and high value of the 

 immense amount of practical research carried on by Voit and 

 his pupils." 



I have placed Sir Michael Foster's view first because it takes 

 into account certain facts which tend to show that there are degrees 

 in metabolism. The most important of these is the formation 

 of amido-acids in the intestine. It is an undoubted fact that 

 by feeding an animal on leucine and other amido-acids, the 

 urea is increased. This transformation of leucine into urea 

 occurs in the liver. It can hardly be supposed that leucine 

 becomes to any great extent an integral part of the living frame- 

 work of the liver cells, but like other extractives, and like 

 aromatic compounds absorbed from the alimentary canal, it 

 becomes a part of what Foster terms the intercalated material. 

 Here it undergoes the final change, and is ultimately and 

 apparently very rapidly discharged in the urine. Dr. Sheridan 

 Lea discussing the probable role of the amido-acids in the animal 

 economy, compares it to the part played by the salts of the food. 

 Neither salts nor extractives simply pass into the urine without 

 fulfilling a useful purpose on their way ; but the exact and 

 specific use of each, whether on the synthetic or analytic side of 

 metabolic phenomena, must be the subject of renewed research. 



CHAPTER XL. 



ANIMAL HEAT. 



AMONG the most important results of the chemical processes 

 we sum up under the term metabolism, is the production of heat. 

 Heat, like mechanical motion, is the result of the katabolic side 

 of metabolic processes ; the result, or accompaniment, that is to 

 say, of the formation of carbonic acid, water, urea, and other 

 excreted products. 



Q Q 2 



