76 PHYSIOLOGY 



group is then changed into maltose by the further assimilation of two molecules 

 of water. The central dextrin-like group is attacked with great difficulty 

 by the ferment, and therefore remains at the end of the reaction as achroo- 

 dextrin. The malto-dextrin, the penultimate stage in the action of diastase, 

 can be regarded as formed by the condensation of three molecules of maltose 

 attached by the oxygen of two CHO groups, so that one CHO group remains 

 free and determines the reducing power of the malto-dextrin molecule. Its 

 formula may therefore be represented as follows : 



/C/i 2 H 20 O 9 



\ 



C 12 H 2 iO 10 



s 

 / 



the sign - being used to denote the open terminal CHO group. 



They further found that the stable dextrin remaining at the end of the 

 diastatic hydrolysis of starch probably had the formula of 40C 6 H 10 5 H 2 O, and 

 might be regarded as a condensation of forty glucose molecules with the elimi- 

 nation of thirty-nine molecules of water. The starch molecule cannot be less 

 than five times that of the stable achroodextrin. Since the latter has a molecular 

 weight of 6498, the molecular weight of starch cannot be less than 32,400, and 

 its empirical formula can be represented by : 



Oio, or (80C 12 H 20 O 1 o.40C 6 H 1 oO5). 



INULIN. Another kind of starch, known as inulin, occurs in dahlia 

 tubers. It is easily hydrolysed by weak acids, and is entirely con- 

 verted into d- fructose, or levulose. 



GLYCOGEN, or animal starch, is found in the liver, muscles, and 

 other tissues of the body, and occurs in large quantities in all foetal 

 tissues. It is a white powder, soluble in water, forming an opalescent 

 solution. It is precipitated from its solution on the addition of 

 alcohol to 60 per cent., or by saturation with solid ammonium sul- 

 phate. On boiling with acids, it is entirely converted into glucose. It 

 is affected by the ferments diastase and amylase, in the same way as 

 vegetable starch, giving first dextrins and finally a mixture of maltose 

 and dextrin. With iodine it gives a mahogany-red colour, which, like 

 the blue colour produced in starch, is destroyed by boiling, to return 

 again on cooling. We shall have occasion to consider its properties 

 more fully when we are dealing with the functions of the liver. 



THE CELLULOSES. Cellulose (C 6 H 10 5 ) X is a colourless, in- 

 soluble material, or mixture of materials, which compose the cell walls 

 of the younger parts of plants, and therefore forms a constituent of 

 most of our vegetable foods. It is insoluble in water or dilute acids 

 or alkalies, its only solvent being an ammoniacal cupric oxide solution. 

 On boiling with strong acids, it gradually undergoes hydrolysis and 

 yields sugar, the nature of which varies according to the source of the 

 cellulose. In herbivorous animals cellulose undergoes digestive 



