60 INTRODUCTION. 



gen in the organism. The term calorific elements, which 

 is sometimes applied to them, cannot therefore be accepted. 

 When we endeavor to substitute for this theory a definite ex- 

 planation of the uses of sugar in the economy, we find our- 

 selves at a loss ; but it must be remembered that we are yet far 

 from having a complete knowledge of the functions of the 

 body, particularly those connected with the intimate pro- 

 cesses of nutrition. 



In the present state of science, we are only justified in 

 saying that sugar is important in the process of development 

 and nutrition, at all periods of life. The precise way in 

 which it influences these processes is not fully understood. 



Sugar disappears from the blood in its passage through 

 the lungs, in great part, probably, by conversion into lactic 

 acid. This change has been demonstrated in the blood of a 

 diabetic patient ; all the sugar contained in the blood being 

 thus changed in less than twenty minutes. 1 



Sugar is never discharged from the body in health, with 

 the single exception of the sugar of milk in the female during 

 lactation. Under certain diseased conditions of the system 

 its production by the liver is exaggerated, so that a certain 

 quantity passes through the lungs, exists in the arterial blood, 

 and appears in the urine, constituting the very serious affec- 

 tion called diabetes mellitus. 



Fats. 



Fatty or oily matters exist in both the animal and 

 vegetable kingdoms. Those which are most interesting to 

 us as physiologists are the varieties found in animals, which 

 constitute an important group of proximate principles. Both 

 vegetable and animal fats are important elements of food. 



In the animal economy fat exists in three varieties, which 

 are called, respectively, Oleine, Margarine, and Stearine. 

 In certain situations are found some of the fatty acids and 



1 ROBIN and VERDEIL, Chimle Anatomique, tome ii., p. 553. 



