392 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV. 



Starch is widely diffused through the vegetable kingdom. It 

 occurs in nature in the form of microscopic grains, varying in size and 

 appearance, according to their source. Each consists of a central spot, 

 round which more or less concentric envelopes of starch proper or 

 granulose alternate with layers of cellulose. Cellulose has very little 

 digestive value, but starch is a most important food. 



Starch is insoluble in cold water : it forms an opalescent solution 

 in boiling water, which if concentrated gelatinises on cooling. Its 

 most characteristic reaction is the blue colour it gives with iodine. 



On heating starch with mineral acids, dextrose is formed. By the 

 action of diastatic ferments, maltose is the chief end product. In 

 both cases dextrin is an intermediate stage in the process. 



Before the formation of dextrin the starch solution loses its opal- 

 escence, a substance called soluble starch being formed. This, like 

 native starch, gives a blue colour with iodine. Although the mole- 

 cular weight of starch is unknown, the formula for soluble starch is 

 probably 5(C 12 H 20 10 ) 20 . Equations that represent the formation of 

 sugars and dextrins from this are very complex, 

 and are at present only hypothetical. 



Dextrin is the name given to the inter- 

 mediate products in the hydration of starch or 

 glycogen, and two chief varieties are distin- 

 guished : erythro-dextrin, which gives a reddish- 

 brown colour with iodine ; and achroo -dextrin, 



FIG. 340. Grains of potato which d06S not. 



It is readily soluble in water, but insoluble 



in alcohol and ether. It is gummy and amorphous. Ft does not 

 give Trommer's test, nor does it ferment with yeast. It is dextro- 

 rotatory. By hydrating agencies it is converted into glucose. 



Glycogen, or animal starch, is found in liver, muscle, and white 

 blood corpuscles. It is also abundant in all embryonic tissues. 



Glycogen is a white tasteless powder, soluble in water, but it 

 forms, like starch, an opalescent solution. It is insoluble in alcohol 

 and ether. It is dextro-rotatory. With Trommer's test it gives a 

 blue solution, but no reduction occurs on boiling. 



With iodine it gives a reddish or port-wine colour, very similar to 

 that given by erythro-dextrin. Dextrin may be distinguished from 

 glycogen by (1) the fact that it gives a clear, not an opalescent, solu- 

 tion with water ; and (2) it is not precipitated by basic lead acetate 

 as glycogen is. It is, however, precipitated by basic lead acetate and 

 ammonia. (3) Glycogen is precipitated by 55 per cent, of alcohol ; 

 the dextrins require 85 per cent, or more. 



Cellulose. This is the colourless material of which the cell-walls 

 and woody fibres of plants are composed. By treatment with strong 

 mineral acids it is, like starch, converted into glucose, but with much 



