596 GENERAL METABOLISM [CH. XXXIX. 



it ; hence such phrases as ' tissue proteid ' and ' floating proteid ' are 

 undesirable if they are understood to imply a sharp line of demarca- 

 tion between the ' tissue ' and the blood or lymph, though useful as 

 indicating two different lines or degrees of metabolism." 



Sir John Burdon-Sanderson writes as follows : " The production 

 of urea and other nitrogenous metabolites is exclusively a function 

 of ' living material ' ; and this process is carried on in the organism 

 with an activity which is dependent on the activity of the living 

 substance itself, and on the quantity of material supplied to it. No 

 evidence at present exists in favour of a 'luxus consumption' of 

 proteid." 



Professor Hoppe-Seyler, after stating that he can make out no 

 clear distinction between the two varieties of proteid from Voit's own 

 writings, proceeds as follows : " Voit states that the circulating proteid 

 is no other than that which is dissolved in the tissue juice, which is 

 derived from the lymph-stream, and ultimately from the circulating 

 blood. He (Voit) further says: 'As soon as the proteid of the 

 blood-plasma leaves the blood-vessels, and circulates among the 

 tissue elements themselves, it is then the proteid of the nutrient 

 fluid or circulating proteid. It is no longer proteid of the blood- 

 plasma, nor yet is it the proteid of the lymph-stream/ 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 conclu- 

 sion 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 knowledge. 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 

 amino-acids in the intestine. It is an undoubted fact that by feeding 

 an animal on leucine and other amino-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 framework of the liver cells, but like other 

 extractives, and like aromatic compounds absorbed from the ali- 

 mentary canal, it becomes a part of what Foster terms the inter- 

 calated material. Here it undergoes the final change, and is ultimately 

 and apparently very rapidly discharged in the urine. Sheridan 

 Lea, discussing the probable role of the amino-acids in the animal 



