112 GENERAL PHYSIOLOGY 



which they share with all soluble carbohydrates viz., the power of 

 rotating the plane of polarised light. As their names indicate, 

 dextrose rotates it to the right ; Isevulose, to the left. 



The disaccharids may be regarded as having arisen from the 

 monosaccharids by the combination of two molecules of the latter 

 and the loss of a molecule of water ; this would yield the formula 



C 12 H 22 O n . 



Among the disaccharids are to be noted especially cane-sugar 

 (saccharose), which is contained in large quantities in the cell-sap 

 of the sugar-cane ; and milk-swgar (lactose), the carbohydrate of 

 milk. By certain methods, as by boiling with dilute inorganic 

 acids or by the action of certain bacteria, the disaccharids can be 

 made to undergo hydrolytic cleavage, so that they pass over 

 into the monosaccharids. This change is termed inversion. In 

 contact with certain fermentation-agents, especially Bacterium lac- 

 ticum, the disaccharids are induced not to ferment directly, but to 

 pass over into monosaccharids, which are themselves subject to the 

 fermentative action of these organisms. If Bacterium lacticum be 

 employed as the fermentation-agent, lactic acid results 



C 6 H 12 6 =2C 3 H 6 3 



a process which, in contrast to alcoholic fermentation by the yeast, 

 plant, is termed lactic acid fermentation ; to it is due the souring 

 of milk exposed to the air. Finally, under the influence of another 

 fermentation agent, Bacillus butyricus, lactic acid can be still further 

 decomposed into butyric acid, carbonic acid and hydrogen 



2G 3 H 6 3 = C 4 H 8 2 + 2C0 2 + 4H ; 



thus a butyric acid fermentation is recognised. 



The polysaccharids are anhydride stages of the monosaccharids. 

 still further removed ; in them several monosaccharid molecules 

 combine with the loss of a molecule of water, so that their 

 formula is a multiple of C 6 H 10 O 5 . Among the polysaccharids 

 occurs a series of bodies that play an important role and 

 are wide- spread, some in the life of the plant-cell, others in many 

 animal-cells. They are, first, starch, which occurs in all green cells 

 of plants in the form of granules, in which the layers are arranged 

 concentrically (Fig. 40) ; secondly, glycogen, which occurs as flakes 

 and irregular particles, especially in the cells of the liver, but in 

 smaller quantities in many other tissue-cells; thirdly, cellulose, 

 which constitutes the cell-membranes of all plant-cells, and has 

 been demonstrated also in the leathery mantle of the Tunicates. 

 These members of the group of polysaccharids may be distin- 

 guished from one another in a very characteristic manner by 

 their behaviour towards solutions of , iodine : by iodine starch is 

 coloured an intense blue, glycogen a mahogany brown, and cellulose 



