CH. XXXIX.] LUXUS CONSUMPTION 595 



ways: on the one hand, it excites rapid disintegration of proteids, 

 giving rise to an immediate increase of urea ; on the other hand, it 

 serves to maintain the more regular proteid metabolism continually 

 taking place in the body, and so contributes to the normal regular 

 discharge of urea. It has been, therefore, supposed that the proteid 

 which plays the first of these two parts is not really built up into 

 the tissues, does not become living tissue, but undergoes the changes 

 that give rise to urea, somewhere outside the actual living substance. 

 The proteids are therefore divided into " tissue-proteids," which are 

 actually built up into living substance, and " floating or circulating 

 proteids/' which are not thus built up, but by their metabolism 

 outside the living substance set free energy in the form of heat only. 

 It was at this time erroneously supposed that the exclusive use of 

 proteid food was to supply proteid tissue elements, and that vital 

 manifestations other than heat had their origin in proteid meta- 

 bolism, the metabolism of fats and carbohydrates giving rise to heat 

 only. Hence, when it was first surmised that a certain proportion of 

 proteids underwent metabolism, which gave rise to heat only, this 

 appeared to be a wasteful expenditure of precious material, and the 

 metabolism of this portion of food was spoken of as a " luxus con- 

 sumption," a wasteful consumption. There were many deductions 

 from this general theory to explain particular points, of these two 

 may be mentioned : (1) In inanition, the urea discharged for the first 

 few days is much greater than it is subsequently : this was supposed 

 to be due to the fact that in the first few days all the floating capital 

 was consumed; (2) the effect of feeding with a mixture of gelatin 

 and proteid was supposed to be due to the fact that gelatin was able 

 to replace " floating proteid," but not " tissue proteid." 



This theory of Voit's, ingenious and plausible at first sight, has 

 met with but little general acceptance, because so many observed 

 facts are incompatible with it. 



Sir Michael Foster writes as follows: "The evidence we have 

 tends to show that in muscle (taking it as an instance of a tissue) 

 there exists a framework of what we may call more distinctly living 

 substance, whose metabolism, though high in quality, does not give 

 rise to massive discharges of energy, and that the interstices, so to 

 speak, of this framework are occupied by various kinds of material 

 related in different degrees to this framework, and therefore deserv- 

 ing to be spoken of as more or less living, the chief part of the 

 energy set free coming directly from the metabolism of some or other 

 of this material. Both framework and intercalated material undergo 

 metabolism, and have in different degrees their anabolic and kataboKc 

 changes ; both are concerned in the life of the organism, but one more 

 directly than the other. We can, moreover, recognise no sharp 

 break between the intercalated material and the lymph which bathes 



