454 THE HUMAN BODY. 



these also indirectly gave rise to the energy expended ii> 

 muscular 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-nitro- 

 genized 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 pro- 

 duction, 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 muscular work which, on Liebig's theory, 

 must first be gone through in order to break up the proteids, 

 with the production of a non-azotized part which could then 

 be oxidized for heat-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 pro- 

 teids to that stage which occurs in the Body; and it is pos- 

 sible to estimate with considerable accuracy the amount of 

 urea and uric acid excreted in a given time; from their sum 

 the amount of proteid oxidized arid 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* dur- 

 ing the same period represents far more energy expended 

 than could be yielded by the proteids broken down. 



The Source of the Energy Expended in Muscular Work. 

 This important question, which was postponed in the chap- 

 ters dealing with the physiology of the muscular tissues, 

 needs now consideration. It may be put thus : Does a 

 muscle-fibre work by the oxidation of its proteids, i.e. by 

 breaking them down into compounds which are then re- 

 moved from it and conveyed out of the Body ? or does it 

 work by the energy liberated by the oxidation of carbon and 

 hydrogen compounds only ? The problem may be attacked 

 in two ways : first, by examining the excretions of a man, or 

 other animal, during work and rest; second, by examining 

 directly the chemical changes produced in a muscle when it 

 contracts. Both methods point to the same conclusion, viz., 

 that proteid oxidation is not the source of the mechanical 

 energy expended by the Body. 



