900 SPECIAL PHYSIOLOGY. 



of fat is 5J parts for every 100 in the body ; that of salts, less than J 

 a part ; and of water, 6 parts only for every 100. On the other hand, 

 the proportion of the carbhydrates in the food, as compared with the 

 small quantity of substances of similar composition in the body, is as 

 much as 86 per cent. This obviously suggests that the amyloid and 

 saccharine substances are not largely employed for conversion into tis- 

 sue, but have some other function in the economy, one of which, there 

 is reason to believe, is to supply fuel for the purposes of generating 

 chemical force, to be transformed into animal motion and heat. These, 

 indeed, are the so-called calorific substances, heat-givers, or respiratory 

 food, as distinguished from the albuminoid substances, plastic food, or 

 flesh-formers. From the small percentage of these latter bodies, daily 

 supplied to the system, it is evident that not more than y^th part of 

 such substance in the body can, on an average, be replaced by nutri- 

 tive metamorphoses in one day. Hence, we arrive at the conclusion, 

 that 100 days, at least, are necessary, supposing waste and supply to 

 be equal, for the complete transformation of all the albuminoid, and 

 their derived constituents, in the living body. But the actual rate of 

 metamorphosis, is so different in the several albuminoid tissues, as e. g. 

 in muscular, as compared with tendinous, tissues, and moreover, so 

 inconstant, that no safe conclusions can be arrived at upon such gen- 

 eral data. The fatty matters of the body are possibly changed in 

 much less time. 



It is difficult to estimate the ordinary daily waste of the human body. 

 It has been shown, however, that the daily quantity of food necessary 

 to maintain an animal at its normal weight, is more than twice the 

 weight of the daily loss which it undergoes, when deprived of all food. 

 When the weight of the food is only equal to the loss during tempo- 

 rary starvation, the animal continues to lose weight, and the egesta 

 given off by the alimentary canal, the kidneys, the skin, and the lungs, 

 weigh more than the quantity of food taken. This has been attrib- 

 uted to the requirements of the processes of digestion, which demand 

 the formation of copious secretions containing much solid matter ; but 

 as most of them are reabsorbed, it is more probably to be explained 

 by the fact that, in a starving animal, the waste is reduced to a mini- 

 mum, and that the effect of insufficient food is to excite the system to 

 an unaccustomed activity, and to loss by metamorphosis. Neverthe- 

 less, during health, with sufficient food, and in a sufficiently long period, 

 there must be an actual balance between the loss and the supply. 



Destination of the Food in the Living Economy. 



This subject includes two points of investigation viz., the interme- 

 diate, and the ultimate, chemical changes or metamorphoses of the dif- 

 ferent proximate constituents of the food. The latter point may be 

 first examined. 



In order to arrive at the ultimate destination of the proximate con- 

 stituents of the food, after these have compensated for the waste of 

 tissue, or have been consumed in furnishing force for imparting motion 

 and heat, it is necessary to determine the intrinsic composition of those 



