FATS AND CARBO-HYDRATES. 405 



that is to say, the relatively best kinds of food must contain fat as 

 well as carbo-hydrates ; under favourable relations the animal body 

 is certainly able to elaborate from the carbo-hydrates the fat 

 which it requires ; but independently of the fact, that this produc- 

 tion of fat would appear from all our positive experiments to be 

 tolerably limited, the production of sugar in the animal organism 

 shows, that fat and sugar have very different and not unimportant 

 objects to fulfil in it (see p. 220). If it be true that the dictates of 

 animal instinct ought in general to be followed, this is more espe- 

 cially the case in reference to the selection of food. The general 

 disposition to combine highly amylaceous food with fats, and 

 fatty matters with amylaceous substances, and the undoubted 

 greater digestibility of such mixtures, prove no less than the 

 simultaneous occurrence of fat and sugar in the milk of animals, 

 which is generally recognised as a normal type of food, that both 

 substances are necessary to the completion of an article of nourish- 

 ment which perfectly satisfies the requirements of the animal 

 organism. If, therefore, any one of these substances may serve 

 in certain general relations as a substitute for another, especially 

 in reference to the development of heat, this does not in any way 

 militate against the special utility of either. But when Liebig 

 included such different substances as fats and carbo-hydrates under 

 the general designation of respiratory elements, he was far from 

 holding the opinion that, independently of the difference in therr 

 capacity for generating heat or their so-called respiratory value, they 

 were of equal importance in the metamorphosis of animal matters 

 a fact of which we might readily convince ourselves by an attentive 

 study of his most recent deductions regarding the forms of metamor^ 

 phosis which the fats and sugar undergo. Although Liebig compares 

 animals to "moving furnances" in respect to the development of 

 heat and its causes in the animal body, it requires a strong faith to 

 interpret this expression in the broadest sense of the words, or to 

 regard his somewhat overstrained physical view as calling for 

 serious refutation. Liebig ranks the fats with the carbo-hydrates in 

 his consideration of the different articles of food, on the one hand, 

 because both serve to compensate for the carbon and hydrogen 

 which are lost through the lungs, and on the other hand, because 

 however much might be advanced in favour of a systematic sepa- 

 ration of these groups, their specific functions in the metamorphosis 

 of animal matter have not been determined with sufficient strict- 

 ness either by decided experiments or direct observation. The 

 time, however, will come, and is assuredly not far distant, when we 



