CHAP, v.] NUTRITION. 459 



most intense mental action, unaccompanied by any muscular mani- 

 festations, the most energetic action of the heart or of the bowels, 

 with the slight exceptions mentioned above, the busiest activity of 

 the secreting or metabolic tissues, all these end simply in aug- 

 menting the expenditure of income in the form of heat. 



A normal daily expenditure in the way of mechanical labour 

 can be easily determined by observation. Whether the work take 

 on the form of walking, or of driving a machine, or of any kind 

 of muscular toil, a good day's work may be put down at about 

 150,000 kilogramme-metres. The normal daily expenditure in 

 the way of heat cannot be so readily determined. Direct calori- 

 metric observations are attended with this difficulty, that the body 

 while within the calorimeter is placed in abnormal conditions, 

 which produce an abnormal metabolism. Hence results arrived at 

 by this method are of little value unless they be accompanied by a 

 comparison of the egesta and ingesta, so that the rate and nature 

 of the metabolism going on may be known. Many attempts have 

 been made to calculate the amount in an indirect manner. As 

 trustworthy as any is the plan of simply subtracting the normal 

 daily mechanical expenditure from the normal daily income. Thus, 

 150,000 k.-m. subtracted from one million k.-m. gives 850,000 k.-m. 

 as the daily expenditure in the form of heat ; i. e. between one-fifth 

 and one-sixth of the total income is expended as mechanical labour, 

 the remaining four-fifths or five-sixths leaving the body in the 

 form of heat. 



The Sources of Muscular Energy. Liebig, satisfied with 

 having proved that the animal body was constructive as far as the 

 formation of fat was concerned, still held to the distinction between 

 nitrogenous or plastic and non-nitrogenous or respiratory food. 

 Put broadly, his view was that all the nitrogenous food went to 

 build up the proteid tissues, the muscular flesh, and other forms 

 of protoplasm, and that the nitrogenous egesta arose solely from 

 the functional metabolism of these tissues, while the non- 

 nitrogenous food was used with equal exclusiveness for respiratory 

 or calorific purposes, being either directly oxidized in the blood or, 

 if present in excess, stored up as fatty tissue. According to him 

 the two classes of income corresponded exactly to the two forms of 

 expenditure. "We have already urged several objections against 

 this view. We have seen that in the blood itself very little 

 oxidation takes place, that it is the active tissue, and not the 

 passive blood-plasma, which is the seat of oxidation. We have 

 further seen that proteid food may undoubtedly be in Liebig's 

 sense respiratory, and incidentally give rise to the storing-up of 

 fat. One division of Liebig's view is thereby overthrown. We 

 have now to inquire whether the other division holds good, whether 

 muscle or other protoplasm is fed exclusively on the proteid 

 material of food, and whether muscular energy comes exclusively 



