26 TEXT-BOOK OF PHYSIOLOGY. 



starch paste. On the addition of iodin, starch strikes a characteristic 

 deep blue color; the compound formed iodid of starch is weak, 

 the color disappearing on heating, but reappearing on cooling. 



Boiling starch with dilute sulphuric acid (twenty-five per cent.) 

 converts it into dextrose. In the presence of vegetable diastase or 

 animal ferments, starch is converted into maltose and dextrose, two 

 forms of sugar. 



Dextrin is a substance formed as an intermediate product in 

 the transformation of starch intosugar. There are at least two 

 principal varieties erythrodexWtfi^which strikes a red color with 

 iodin, and achroodextrin, which is without color when treated with 

 this reagent. In the pure state dextrin is a yellow-white powder, 

 soluble in water. In the presence of vegetable ferments erythro- 

 dextrin is converted into maltose. 



Glycogen is a constituent of the animal liver, and, to a slight 

 extent, of muscles and of tissues generally. In the tissues of the 

 embryo it is especially abundant. When obtained in a pure state 

 it is an amorphous, white powder. It is soluble in water, forming 

 an opalescent solution. With iodin it strikes a port-wine color. In 

 some respects it resembles starch, in others dextrin. Like vegetable 

 starch, glycogen or animal starch can be converted by dilute acids 

 and ferments into sugar (dextrose). 



Cellulose is the basic material of the more or less solid framework 

 of plants. It is soluble in ammoniacal solution of cupric oxid, from 

 which it can be precipitated by acids. It is an amorphous powder; 

 dilute acids can convert it into dextrose. 



2. DEXTROSES, C 6 H 12 O 6 . 



Dextrose, glucose, or grape-sugar is found in grapes, most 

 sweet fruits, and honey, and as a normal constituent of liver, blood, 

 muscles, and other animal tissues. In the disease diabetes mellitus 

 it is found also in the urine. 



When obtained from any source, it is soluble in water and in hot 

 alcohol, from which it crystallizes in six-sided tables or prisms. As 

 usually met with, it is in the form of irregular, warty masses. It is 

 sweet to the taste. When examined with the polariscope, dextrose 

 turns the plane of polarized light to the right. It is therefore termed 

 dextro-rotatory. It has for a long time been known that when sugar, 

 cupric hydroxid, and an alkali e. g., sodium or potassium are 

 present in solution, the sugar will abstract from the cupric hydroxid 

 a portion of its oxygen, thus reducing it to a lower stage of oxidation 

 giving rise to cuprous oxid. Sugar has a similar action on both 

 silver and bismuth. On this property of sugar a standard solution 

 of cupric hydroxid was suggested by Fehling which may be employed 

 for both qualitative and quantitative tests for the presence of sugar 

 in solution. 



