CHEMICAL BASIS OF THE ANIMAL BODY. 91 



CARBOHYDRATES l . 



Certain members only of this extensive class have been found in 

 the human body ; of these, the most important and wide-spread are 

 glycogen, grape-sugar or dextrose (glucose), with which diabetic sugar 

 seems to be identical 2 , maltose, and milk-sugar. Inosit which has 

 the same percentage composition as a sugar (C 6 H 12 6 ) and possesses a 

 distinctly sweet taste has hence been usually classed with the carbo- 

 hydrates. This is incorrect since it is now known to belong to the 

 benzol series (see below, p. 108). 



Although the above-mentioned carbohydrates may be detected in 

 various tissues and secretions of the animal body, their presence in the 

 several cases is not so much due to their introduction into the body in 

 the form in which they there occur as to their production from other 

 members of the carbohydrate group existing in food. The chief of 

 these is starch, and it will perhaps conduce to completeness to deal 

 first very briefly with this parent-substance and some of the products 

 of its decomposition. 



THE STARCH GROUP. 



1. Starch (C 6 H 10 B ) B . 



Starch occurs characteristically in plants and is formed in their 

 green parts under the determinant influence of the chlorophyll- 

 corpuscles. The exact mode of its formation is however as yet un- 

 decided. It exists in plant-tissues in the form of grains which vary in 

 size and shape according to the plant, but which possess the common 

 characteristic of exhibiting a stratified structure, which is much more 

 marked in some cases (potato-starch) than in others, and the phenomena 

 of double-refraction when examined in polarised light. Considered as 

 a whole the grains appear to be composed of two substances of which 

 the chief both in quantity and importance is called granulose and the 

 other cellulose. The former which yields the blue colour characteristic 

 of starch on the addition of iodine, may be dissolved out by the action 

 of saliva or malt-extract leaving a cellulosic skeleton of the original 

 grain. This so-called cellulose is not identical with ordinary cellulose 

 as shown by its ready solubility in several reagents which do not dissolve 

 the latter 3 . When treated with boiling water the grains swell up and 



1 The carbohydrates are very fully treated in Tollens' Hdbch. d. Kohlenhydrate, 

 Breslau, 1888. See also Miller's Chemistry, Pt. in. Sec. 1 (1880), p. 567 et seq. 



2 There is perhaps some slight doubt as to this identity, based chiefly upon a 

 slight apparent difference in the specific rotatory power of true dextrose and that 

 obtained from diabetic urine. (See Miller's Chemistry, p. 583. ) 



3 Brown and Heron, JL Ch. Soc. Vol. xxxv. (1879), p. 611. Liebig's Ann. Bd. 

 cxcix. S. 165. 



