32 MANUAL OF HISTOLOGY. 



swells up in cold water, and is dissolved in warm, forming an opalescent 

 fluid, manifesting strong right-handed polarisation. Glycogen becomes 

 of a wine-red colour on the addition of iodine, or also brown and violet. 

 It occurs in the tissue of the liver, and also temporarily in different 

 embryonic tissues, as well as in the muscles of phytophagous animals. 

 It may be converted in various ways into grape sugar, as, for instance, 

 by boiling with dilute acids, by admixture with diastase, saliva, pan- 

 creatic juice, or blood. Glycogen is of great importance in the formation 

 of hepatic sugar through the action of a ferment. Its hypothetic origin 

 may be referred to the decomposition of some albuminous substance. 



Dextrin, C 6 H 10 5 . 



This compound is soluble in water, and viscous when in concentrated 

 solution. A polarised ray is deflected strongly to the right by its watery 

 solution, which is coloured reddish- violet by the addition to it of iodine 

 dissolved in iodide of potassium. It is converted readily into grape 

 sugar by the action of dilute sulphuric acid, diastase, and saliva. 



It is found in the contents of the intestine after starchy food, in the 

 blood of phytophagous animals, in the liver of horses after feeding on 

 oats, as well as in the muscles of the latter (Limpricht). 



Grape Sugar, C 6 H 12 6 + H 2 . 



Grape sugar (fig. 7) usually crystallizes indistinctly in crumb-like or 

 warty masses ; rarely in tables, which probably belong to the clinorhom- 

 bic system. It is soluble in water, and in this form polarises light tc 

 the right. Grape sugar reduces sulphate of copper with a solution of 

 potash, .and, on being heated slightly, to the condition of suboxide, and 

 forms a combination with chloride of sodium, which crystallizes in 

 large four or six-sided pyramids. In the presence of other nitrogenous 

 substances, as albumen and casein, and also of bases, it undergoes lactic, 

 and later, butyric acid fermentation. 



Grape sugar occurring in the animal kingdom, and springing from 

 other carbohydrates in various ways, is formed from the latter, more par- 

 ticularly from arnylum, by the fermenting action of many glandular 

 secretions, particularly those of the mouth, pancreas, and intestines 



within the body. It is absorbed in the 

 digestive tract, and appears in the chyle and 

 blood. It is generally supposed, from the 

 fact of its disappearing rapidly in the latter, 

 that it undergoes combustion with the for- 

 mation of carbonic acid and water ; but the 

 intermediate products are unknown. 



Besides this, the grape sugar found in 

 dead hepatic tissue has a second significance 



O already alluded to in speaking of glycogen. 



Normal human urine is probably not en- 

 tirely free from grape sugar ; but the latter 



FIR. 7. Tabular crystals of crape sugar may be made to appear in considerable 

 obtained from honey. quantities in the urine of animals, by irri- 



tation of the medulla elongata (Bernard) : irritation of other portions 

 of the nervous centres, however, produces the same remarkable pheno- 

 menon. This substance occurs pathologically, and often in considerable 



