The Chemical Constituents of tin- Organism. 15 



glucose and levulose, lactose or milk sugar forming galac- 

 tose, and maltose forming glucose. 



Maltose is an important substance, as it is the chief sugar 

 formed in the body by the action of the diastatic ferment on 

 starch. The inverting ferment of the intestine, as with 

 lactose and saccharose, can convert it into glucose. 



The Amylose or starchy group represents a large class 

 of substances furnished by the vegetable kingdom, and the 

 function of which as food is to afford the animal the fuel by 

 which muscular energy is alone rendered possible. Starch 

 does not exist in the animal body as a natural constituent ; 

 animal starch, or glycogen, is derived from vegetable starch, 

 but glycogen is not a constituent of the body, only a some- 

 thing stored up in it. 



Starch is insoluble in cold water, but under ihe influence 

 of moisture the starch granules swell considerably ; by boil- 

 ing these burst and form a solution. If starch be heated it 

 undergoes change, first into a substance termed dextrin, or 

 British gum, and secondly into glucose : it may also be 

 converted into glucose by heating with mineral acids ; by 

 the diastatic ferments of the saliva and pancreas starch is 

 converted into maltose : in each case, viz., whether by 

 mineral acids or ferments, dextrin is an intermediate 

 product. 



Starch gives the well-known blue reaction with iodine, 

 but if it be acted upon so as to become converted into 

 dextrin, a red colour, and not a blue, is obtained on the 

 addition of iodine ; if the process of conversion is carried 

 still further to the stage of maltose or glucose, no colour 

 is developed with iodine, but the presence of sugar is abun- 

 dantly shown. 



Dextrin was first discovered in the muscles of the horse ; 

 it is found in most plants, and its preparation from starch 

 is alluded to above. In muscles it occurs as an intermediate 

 stage in the conversion of glycogen into sugar. 



Glycogen, or animal starch, is a carbo-hydrate found in 

 the liver, muscles, blood-cells, etc., it is derived from starch, 

 and stored up in the tissues for future use in a manner 



