452 NITROGENOUS METABOLISM. [Boon 11. 



itself. And we may readily conceive of the living unit effecting 

 changes, and even profound changes, in this its internal medium, 

 without the substances thus changed ever becoming an integral part 

 of the living unit itself. Moreover we can also conceive of the unit 

 as a whole producing changes in the lymph surrounding it, much 

 in the same way, as, according to some observers, the yeast-cell 

 produces changes in the molecules of sugar which surround it, but 

 which never become part of itself. We may therefore, in the case 

 of proteids, follow Voit and others in distinguishing between the 

 proteids 011 the one hand which actually become part of living 

 units and which may be called " tissue-proteids " or " morphotic 

 proteids," and those on the other hand which are found in the 

 internal meshes of the unit or in the surrounding lymph or in the 

 blood, and which, since they probably pass readily from one medium 

 to the other, may be spoken of as ''circulating proteids." By 

 older physiologists at a time when the energy of bodily movements, 

 of which we shall speak directly, was supposed to come from the 

 direct metabolism of the morphotic proteids of muscle, the increase 

 of urea due to food independent of exertion was regarded as simply 

 arising from proteids metabolized in the blood, and so cast out as 

 useless ; hence the phrase, to which we have already referred, of 

 luxus-consumption. We now know however, as will presently be 

 pointed out, that the energy of bodily movements does not come 

 from the metabolism of the proteids of muscle, and we have 

 already seen that oxidations on a large scale do not take place 

 in the blood. Hence this view of a luxus-consumption is no 

 longer tenable. There still remains, however, the difficulty of 

 supposing that every grain of urea which passes from the body 

 after a rich proteid meal is the issue of the metabolism of a 

 quantity of proteid previously existing as an integral part of some 

 tissue unit ; in other words, it seems unlikely that, simply as the 

 result of such a meal, the actual living proteid framework of the 

 body should be so largely renewed. Moreover the contrast between 

 that part of the daily urea which is variable and fluctuating and 

 that part which is more constant has to be explained. Hence has 

 arisen the view that the sources of urea are twofold, corresponding 

 to the metabolism of two distinct categories of the proteids of the 

 body. On the one hand, part of the urea, especially that which 

 appears as the immediate result of food, is supposed to be derived 

 from the metabolism of what we defined above as "circulating 

 proteids;" while, on the other hand, a certain (presumably smaller) 

 portion is really due to the metabolism of the "tissue-proteids," 

 i.e. of the actual living framework of the body. 



We must not attempt to discuss this view at length, and indeed 

 our knowledge is inadequate for the purpose. We have not as yet 

 a measure of the rate at which the metabolism of the living unit 

 does or may take place, and the arguments in favour of a metabolism 

 of circulating proteids are, at present, of a very indirect character. 



