270 ITS BEAEING ON MEDICAL PRACTICE. 



result of the waste of the organs, wlietlier muscles or braiu, on the 

 exercise of which that work depends.* 



The ancients, who derived all their knowledge from observation, and 

 not from experiment, were well aware of the double duty imposed upon 

 food in early life — of producing both the secular and the periodic varia- 

 tions of the body, or, in other words, of promoting growth, and of 

 developing work. Their practical knowledge is summed up by Hiiipo- 

 crates in the aphorism, "Old men bear want of food best; next those 

 that are adults. Youths bear it least, more especially children, and of 

 these the most lively are the least capable of enduring it." 



The food consumed in twenty-four hours, including air and water, 

 undergoes a series of changes of a chemical character before leaving the 

 body, in the form of one or other of its excretions. Some of these changes 

 develop force, and others expend force, but the algebraic sum of all the 

 gains and losses of force represents the quantity available for work. 

 This work must be expended as follows : 



1. The work of growth, [secular.) 



2. The work of maintaining heat, {jjeriodiG.) 



3. Mechanical work, [periodic.) 



4. Vital work, [periodic.) 



During childhood and youth the work of growth is positive, for a cer- 

 tain proportion of the food used is employed in building up the tissues 

 of the body instead of being expended in actual work ; it is, in fact, 

 "stored up" in the body, as i^is tn'ya is stored up by the fly-wheel of 

 machinery, and constitutes a reservoir of force that may be called uj)on 

 at ail emergency requiring sudden exiienditure of force, as in case of 

 illness, or to supply- the gradual wasting of old age. In adult life and 

 in old age, the work of growth ceases completely, except so far as is 

 necessary to repair, from day to day, the small wastes of the organs 

 employed in work; so that nearly the whole of the food employed is 

 expended on the periodic work of the body. Hence We can readily see 

 the reason for the aphorism which asserts that food is more necessary 

 for the young than for the old, and more required by those of a lively 

 disposition, either of mind or body, than by others. 



* The very skill with wliich provisiou is made for tlie repair of the waste of the organ 

 used as the iustrument of work may mislead the observer iuto supposiug that the work 

 itself may be measured by the waste of its instrument. Thus, it has been shown by 

 Mr. A. Macalister, of Dublin, that the heart, which has imposed upon it the necessity 

 of workiug day and night without ceasing, during life, is furnished with double the 

 usual supply of blood through the coronary arteries, which are injected twice for every 

 single beat of the lieart. If, indeed, it were possible to assume that all muscles wasted 

 equally for equal quantities of work, and also to measure separately the products of 

 that waste, we might then assume the waste of the organ as the measure of its work. 

 Neither of these assumptions, however, can be admitted, for it can be shown that dif- 

 ferent muscles act under diiferent conditions more or less advautageoiislj'^, so that equal 

 "wastea would represent uuequal works; and, also, it is impossible to separate in prac- 

 tice the lu'oducts of waste of muscles from those of the general changes of the blood. 



