CHAP, v.] NUTRITION. 451 



The characteristic metabolic effects of proteid food are shewn 

 not only by these calculations of what is supposed to take place 

 in the body, but also by direct analysis. Lawes and Gilbert 

 laboriously analysing the body of a pig, which had been fed on a 

 known diet, and comparing the analysis with that of another pig of 

 the same litter, killed at the time when the first was put on the 

 fixed diet, found that of the dry nitrogenous material of the food 

 only 7'3-i p. c. was laid up as dry proteid material during the 

 fattening period, though the amount of proteid food was low ; in 

 the sheep the increase was only 4' 14 p. c. 



It may be worth while to consider briefly here what is exactly 

 meant by the proteid metabolism of which we are speaking. 

 In the first place, in dealing with the changes taking place in the 

 body we may distinguish between morphological and physiological 

 destruction and renewal. We know that an epithelium cell, as 

 notably in the case of the skin, may be bodily cast off and its 

 place filled by a new cell ; and probably a similar disappearance of 

 and renewal of histological units takes place in all the tissues of 

 the body to a variable extent. But, in the adult body, these 

 histological transformations are, in the cases of most of the tissues, 

 slow and infrequent. A muscle for instance may suffer very 

 considerable wasting and recover from that wasting without any 

 loss or renewal of its elementary fibres. And it is obvious that 

 the metabolism of which we are now speaking does not involve any 

 such shifting of histological units. On the other hand we find 

 these histological units, the muscle-fibre or the gland-cell for 

 instance, living on their internal medium the blood, or rather on 

 the lymph which is the middleman between themselves and the 

 actual blood flowing in the vascular channels. Now we have 

 previously insisted at length on the view that no oxidative 

 changes on a large scale take place, as was once thought, in the 

 blood. The proteid metabolism which we are now considering or 

 rather the destructive part of that metabolism (and to avoid the 

 introduction of a new word we may venture, in using the word 

 metabolism, to leave the context to explain, whether the whole 

 series of changes constructive and destructive, or the constructive 

 changes alone, or the destructive changes alone, are intended) is 

 fundamentally oxidative in character; and we may therefore assume 

 that the large proteid metabolism which we are considering does 

 not go on in the blood. In other words, the metabolism of proteids 

 and the reduction of their nitrogenous residues into urea or into 

 immediate antecedents of urea is carried out by the agency of the 

 elements of the tissues. In a tissue unit however, such as a muscle- 

 fibre or gland-cell, we must distinguish between the actual living 

 protoplasm or modified protoplasm, the morphological framework so 

 to speak, and the material or substances, solid or in solution, which 

 are lodged in the spaces of the framework, which are not part of the 

 living unit but rather form a sort of internal medium to the unit 



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