274 ELEMENTARY PHYSIOLOGY less. 



food-stuffs they contain, so that it is possible to draw up 

 many such tables all satisfying the condition of covering 

 the daily loss of 20 grammes of nitrogen and about 270 or 

 300 granimes of carljon. 



In drawing up such a table of diet the question i>f cost 

 must also not be forgotten, as in the case of fixing the 

 diet for soldiers or prisoners. Thus, for instance, it costs 

 more to obtain the requisite amount of carbon from fat 

 than from sugar or starch. Moreover, the value of a diet 

 depends also on the ease with which its constituents can 

 be digested and utilised. Mere chemical analysis is 

 by itself a very insufficient guide as to the useful- 

 ness and nutritive value of an article of food. A 

 substance to be nutritious must not only contain 

 some or other of the various food-stuffs, but contain 

 them in an available, that is a digestible, form. A 

 piece of beef -steak is far more nourishing than a quantity 

 of pease pudding containing even a lai'ger proportion of 

 proteid material, because the former is far more digestible 

 than the latter ; and a small piece of dry hard cheese, 

 though of high nutritive value as judged by mere chemical 

 analysis, will not satisfy the more subtle criticism of the 

 stomach. 



3. The Economy of a Mixed Diet.— The body, as we 

 have repeatedly pointed out, cannot obtfiin the nitrogen 

 it requires from any source other than the ready-made 

 proteids. Hence in the absence of these from the food of 

 an animal it must sooner or later die from what is known 

 as nitrogen starvation. 



In this case, and still more in that of an animal deprived 

 of vital food altogether, the organism, so long as it con- 

 tinues to live, feeds upon itself. In the former case, all 

 the processes involving a loss of nitrogen, in the latter, 

 all the processes leading to the appearance of all the 

 several waste products, are necessarily carried on at the 

 expense of its own body ; whence it has been rightly 

 enough ob.served that a starving sheep is as much a car- 

 nivore as a lion. Protein is thus the essential element of 



