834 



PHYSIOLOGY 



urea, carbonic acid, sulphuric acid, and water, nearly 2 grm. of oxygen 

 would be required in the twenty-four hours, i. e. about 1 c.c. per minute. 

 Cetti's total oxygen consumption was at the rate of 5 c.c. per kilo per 

 minute, so that four-fifths of the oxygen absorbed was required for the 

 oxidation of non-nitrogenous substances, and these, as we have seen, 

 could only have been fats. In animals with a large store of fat the pro- 

 portion of the energy obtained at the cost of the fats may be still greater. 

 In dogs Rubner and Voit reckoned that only 10 to 16 per cent, of the total 

 energy was derived from proteins, the rest, i. e. 84 to 90 per cent., being 

 obtained from the oxidation of fats. 



The oxidation of fats supplies energy not only for the production of 

 heat but also for the performance of mechanical work, and it seems probable 

 that the utilisation of the fat occurs in the muscular tissues themselves. 

 Fat is found as a normal constituent of all muscle fibres, and the amount 

 of this substance is greater in proportion to the activity of the muscles 

 concerned. Thus the ever-active heart muscle, and the red muscles of 

 the diaphragm, contain larger amounts of fat than the pale voluntary 

 muscles which have to undertake only short periods of activity. In the 

 human heart muscle 15 per cent, of the solids are soluble in ether, and 

 more than one-half of the ether extract is composed of fat, and is suffi- 

 cient to supply the energy of the contracting heart for six or seven hours' 

 work. 



The degree to which the muscles during contraction call upon each 

 class of foodstuffs may be judged from the respiratory quotient. If the 

 body has previously supplied the greater part of its needs at the expense 

 of fats, it will continue to do so during muscular work. This is well shown 

 in the following Table, in which the oxygen consumption and' respiratory 

 quotient are compared in a man resting and working on three different 

 diets, one principally fat, one principally carbohydrate, and the other 

 principally protein : 



We may conclude then that the tissues of the body are able to obtain 

 their energy by the direct utilisation of the fats which they contain. The 

 changes in the fat molecules, which are involved in the utilisation of their 

 energy, are still to be determined. The energy of fat is available only on 

 its oxidation. The transformation of fats into fatty acids or glycerine, 

 or the synthesis of fats from aldehydes or from carbohydrates, which we 



