644 PHYSIOLOGY 



will be increased under any conditions which bring about an augmented tissue 

 disintegration, such as the toxaemia of fevers, poisoning by arsenic or phos- 

 phorus, or partial asphyxia by deprivation of oxygen, as after inhalation of 

 carbon monoxide gas. In this function protein cannot be replaced by either 

 of the other food-stuffs. With regard to its second fate, viz. the furnishing 

 of energy to the body, protein stands on the same level as carbohydrates or 

 fats, its relative value as compared with these two classes of substances being 

 represented by its physiological heat- value. Owing to the inability of the 

 body to store protein to any marked extent, any protein which is absorbed 

 in the alimentary canal in excess of the nutritional requirements of the body 

 is at once broken down and oxidised to satisfy the energy requirements of the 

 cells. The NH and NH 2 groups in the protein molecule add little or nothing 

 to its chemical or potential value. The energy value of the protein depends 

 on its carbon and hydrogen content, and it seems probable that the greater 

 part of the nitrogen is split of! as ammonia from the protein molecule during 

 or shortly after its absorption from the alimentary canal. It is on this account 

 that an increased excretion of urea is the almost immediate consequence of 

 the ingestion of protein. The point made by Pfliiger against Voit, viz. that 

 no oxidation occurs in the lymph or blood, is really beside the mark. The 

 living cell is a complex system which may include food in all degrees of oxida- 

 tion or chemical change, without this food material necessarily forming part 

 of the living framework. Every histologist distinguishes the paraplasma 

 from the more active and essential protoplasm, and we must assume that 

 it is in the ultra-microscopic interstices of the foam-like protoplasm that the 

 chief processes of chemical change and oxidation occur. A proof therefore 

 that oxidation depends on the condition of the cells and not on that of the 

 blood does not justify the conclusion that the whole nitrogenous metabolism 

 of the body is confined to the living protoplasm itself. 



Folin has lately brought forward a number of facts which point not only 

 to a twofold origin of the nitrogen of the urine but also to a qualitative 

 difference in the two orders of protein metabolism. Whereas in the urine 

 of man on a normal diet the urea nitrogen forms 85 per cent, or more of the 

 total nitrogen, a reduction of the protein ration to the minimum necessary to 

 meet the nutritional requirements of the body causes not only an absolute 

 diminution of the urea but a large relative diminution when compared with 

 the other constituents of the urine, such as creatinin. He concludes therefore 

 that the nitrogenous end-products of nutritional metabolism are different 

 from those of the energy metabolism. As Speck has pointed out, there is 

 also a difference in the time-relations of the two orders of metabolism. 

 Whereas the nitrogen, which furnishes no energy to the body, is rapidly 

 eliminated when protein is being utilised for the supply of energy to the 

 body, the occurrence of increased tissue waste causes a rise of nitrogenous 

 excretion, which comes on slowly, often after the lapse of a day, and may 

 last two or three days. The process of protoplasmic disintegration appears 

 therefore to occur in a series of stages, which occupy a considerable time 

 and end in the production of substances qualitatively distinct from that 



