THE CHEMICAL BASIS OF THE BODY 761 



Lactose, or milk-sugar, is found solely in milk. It reduces Fehling's 

 solution, and has the same rotatory power as dextrose, but it does 

 not reduce Barfoed's reagent, nor does it undergo direct alcoholic 

 fermentation with yeast. If boiled with dilute mineral acids, it 

 is converted into equal parts of dextrose and galactose. Lactose 

 readily undergoes lactic fermentation, as, for instance, in souring 

 milk. The cause of this is a micro-organism ; but there are reasons 

 for believing that an enzyme may also bring it about. In spite of 

 the fact that isolated lactose is unable to ferment in the presence 

 of yeast, yet an alcoholic fermentation is capable of occurring in 

 milk, such, for instance, as in the koumiss from mare's milk and 

 kephir from cow's milk. It is probable that the changes which 

 bring this about are very complex, and due to several organisms. 



Lactose, like saccharose and maltose, is non-assimilable as such, 

 and it is probable that it is changed into dextrose before absorption, 

 not necessarily as the result of the action of any digestive secretion, 

 but during its passage through the intestinal wall. Like maltose, 

 lactose yields an osazone, phenyl-lactosazone, which crystallises 

 in characteristic rounded clumps of yellow crystals. These crystals 

 melt at 200 C, and are soluble in 80 to 90 parts of boiling water. 



Monosaccharides (C 6 H 12 6 ) . — When the members of the preceding 

 group of sugars, the disaccharides, are boiled with dilute acids or 

 otherwise hydrolysed, they take up a molecule of water and split 

 into two molecules of a new sugar. Thus cane-sugar yields dextrose 

 and laevulose, maltose gives two molecules of dextrose, and lactose 

 yields dextrose and galactose. Of these the most important is — 



Dextrose, Glucose, or Grape-Sugar. — This is probably the form to 

 which all sugars must be reduced in the alimentary canal, whether 

 before or during absorption, in order that they may be assimilable 

 by the tissues. In its ordinary reactions dextrose resembles maltose, 

 but may be easily distinguished from it by the following differences 

 in behaviour. Its specific rotatory power is only 4- 5 2°. It reduces 

 Barfoed's reagent (see Maltose). The osazone it forms, phenyl- 

 glucosazone, crystallises in fine yellow needles ; these melt at 205 C., 

 and, unlike the corresponding compound of maltose, are almost 

 insoluble in water. Dextrose is capable of undergoing three fer- 

 mentations — i.e., alcoholic, lactic, and butyric ; the latter two are 

 probably always present in the intestinal canals of animals, especi- 

 ally after a carbohydrate diet. Dextrose is found in the blood and 

 many organs of the body. It is also the form in which sugar escapes 

 by the kidneys in diabetes. 



Laevulose. — This occurs in fruits and honey, mixed with glucose ; 

 It may also be prepared by acting upon cane-sugar with sulphuric 

 acid, by which means the cane-sugar is converted into equal parts 

 of dextrose and laevulose. 



Inosite. — This is a cryst alii sable substance, found among the 

 1 extractives ' of many tissues, usually in very minute quantities, 

 though it is markedly present in heart-muscle and in the muscles of 

 the horse, which may contain as much as 0003 per cent. It occurs 

 also in semen. Inosite is found abundantly in vegetable tissues, 

 especially in unripe beans, which thus provide a convenient source for 

 its preparation. Possessed of a sweet taste, and as being originally 

 found in muscles, inosite has at times been called ' muscle sugar.' 

 But although its empirical formula is the same as that of a mono- 

 saccharide, it is not a sugar : its solutions exert no rotatory power on 

 polarised light, they do not reduce metallic salts, form no osazone 



