426 TEE HUMAN BODY. 



into parts of a living tissue, and were simply burnt at once 

 in order to maintain the bodily warmth. It haying been 

 proved, however, that more fat might accumulate in the 

 body of an animal than was taken in its food, this excess 

 was accounted for by supposing it was due to excess of com- 

 bustible foods, converted into fats and stored away as oil- 

 droplets in various cells; but not actually built up into truo 

 living adipose tissue. Liebig, somewhat similarly, classed 

 all foods into plastic, concerned in making new tissues, and 

 respiratory, directly oxidized before they ever constituted 

 a tissue. The plastic foods were the proteids, but these 

 also indirectly gave rise to the energy expended in muscu- 

 lar work, and to some heat: the proteid muscular fibre 

 being broken first into a highly nitrogenous part (urea, or 

 some body well on the road to become urea) and a non- 

 nitrogenized richly hydrocarbonous part; and this latter 

 was then oxidized and gave rise to heat. Several facts may 

 be urged against this view (1) Men in tropical climates 

 live mainly on non-proteid foods, yet their chief needs are 

 not heat production, but tissue formation and muscular 

 work : according to Liebig's view their diet should be 

 mainly nitrogenous. (2) Carnivorous animals live on a 

 diet very rich in proteids, nevertheless develop plenty of 

 animal heat, and that without doing the excessive muscu- 

 lar work which, on Liebig's theory, must first be gone 

 through in order to break up the proteids, with the produc- 

 tion of a non-azotized part which could then be oxidized for 

 lieat-production. (3) Great muscular work can be done on 

 a diet poor in proteids; beasts of burden are for the most 

 part herbivorous. (4) Further, we know exactly how much 

 energy can be liberated by the oxidation of proteids to that 

 stage which occurs in the Body; and it is perfectly possible 

 to estimate pretty accurately the amount of urea and uric 

 acid excreted in a given time; from their sum the amount 

 of proteid oxidized and the amount of energy liberated in 

 that oxidation can be calculated; if this be done it is found 

 that, nearly always, the muscular work done during the 

 same period represents far more energy expended than could 

 be yielded by the proteids broken down. 



