CH. xxxix.] LUXUS CONSUMPTION. 593 



is at first a little difficult for him to grasp the fact, that although 

 the amount of nitrogen and carbon ingested is equal to the 

 amount of the same elements which are eliminated, yet the 

 eliminated carbon and hydrogen are not derived from the food 

 direct, but from the tissues already formed ; the food becomes 

 assimilated and takes the place of the tissues thus disintegrated. 

 Let us suppose we have a tube open at both ends and filled with 

 a row of marbles ; if an extra marble is pushed in at one end, a 

 marble falls out at the other ; if two marbles are introduced 

 instead of one, there is an output of two at the other end; if 

 a dozen, or any larger number be substituted, there is always a 

 corresponding exit of the same number at the other end of the 

 tube. This very rough illustration may perhaps assist in the 

 comprehension of the metabolic exchanges. 



The difficulty just alluded to, which a student feels, was also 

 felt by the physiologists who first studied metabolism ; and Voit 

 formulated a theory, of which the following is the gist : All pro- 

 teid taken into the alimentary canal appears to affect proteid 

 metabolism in two ways ; on the one hand, it excites rapid dis- 

 integration 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 mani- 

 festations other than heat had their origin in proteid metabolism, 

 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 " In MI- 

 consumption," a wasteful consumption. There were many deduc- 

 tions from this general theory to explain particular points : of 

 these two may be mentioned: (i) In inanition, the urea dis- 

 charged for the first few days is much greater than it is subse 

 quently : this was supposed to be due to the fact that in the first 

 K.P. Q Q 



