THE USE AND ABUSE OF FOOD. 331 



change our diet, too often, without any consideration, or 

 from false considerations, of the wants of the body. When 

 we have derived benefit from some change of diet, we are 

 apt to continue the new diet after the necessity for it has 

 passed away. As to quantity, also, we seldom follow well- 

 judged rules. Some take less nutriment (or less of some 

 particular form of nutriment) than is needed to supply the 

 absolute requirements of the system ; others persistently 

 overload the system, despite all the warnings which their 

 own experience and that of others should afford of the mis- 

 chief likely to follow that course. 



It is only of late years that systematic efforts have been 

 made to throw light on the subject of the proper use of food, 

 to distinguish between its various forms, and to analyze the 

 special office of each form. I propose to exhibit, in a 

 popular manner, some of the more important practical con- 

 clusions to which men of science have been led by their 

 investigations into these questions. 



The human body has been compared to a lamp in which 

 a flame is burning. In some respects the comparison is a 

 most apt one, as we shall see presently. But man does 

 more than live; he works, with his brain or with his 

 muscles. And therefore the human frame may be more 

 justly compared to a steam-engine than to the flame of a 

 lamp. Of mere life, the latter illustration is sufficiently apt, 

 but it leaves unillustrated man's capacity for work; and 

 since food is taken with two principal objects the main- 

 tenance of life and the renewal of material used up in brain 

 work and muscular work we shall find that the comparison 

 of man to a machine affords a far better illustration of our 

 subject than the more common comparisons of the life of man 

 to a burning flame, and of food to the fuel which serves to 

 maintain combustion. 



There is, however, one class of food, and, perhaps, on 

 the whole, the most important, the operation of which is 

 equally well illustrated by either comparison. The sort of 

 food to which I reier may be termed teat-maintaining food. 



