412 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXYIII. 



Phenyl-hydrazine Test. The three important reducing sugars 

 with which we have to deal in physiology are dextrose, lactose, and 

 maltose. They may be distinguished by their relative reducing 

 powers on Fehling's solution, or by the characters of their osazones. 

 The osazone is formed in each case by adding phenyl hydrazine hydro- 

 chloride, and sodium acetate, and boiling the mixture for about half an 

 hour. In each case the osazone is deposited in the form of bright 

 canary-coloured, needle -like crystals, usually in bunches, which differ 

 in their crystalline form, melting-point, and solubilities. Cane sugar 

 does not yield an osazone. 



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 

 FIG. 3i2.-Grams of potato 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 enzymes, 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 2? 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 intermediate products in the 

 hydrolysis of starch or glycogen, and two chief varieties are distin- 

 guished : erythro-dextrin, which gives a reddish-brown colour with 

 iodine ; and achroo -dextrin, which does not. 



It is readily soluble in water, but insoluble in alcohol and ether. 

 It is gummy and amorphous. It does not give Trommer's test, nor 

 does it ferment with yeast. It is dextro-rotatory. By hydrolysing 

 agencies it is converted into glucose. 



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

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



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

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



