298 A MANUAL OF PHYSIOLOGY 



hydrazine (p. 431). Traces of dextrose, a sugar which rotates 

 the plane of polarization less than maltose, but has greater 

 reducing power, are produced by the further action of the 

 saliva on maltose itself. When a small quantity of ferment 

 acts for a short time, the production of isomaltose is favoured. 

 The production of maltose and dextrose is favoured by the 

 prolonged action of a large quantity of ferment. 



The red colour indicates the presence of a kind of dextrin 

 called erythrodextrin ; the violet colour shows that at first 

 this is still mixed with some unchanged starch. Soon the 

 erythrodextrin disappears, and is succeeded by another 

 dextrin, which gives no colour with iodine, and is therefore 

 called achroodextrin. This is partly, but in artificial 

 digestion never completely, converted into maltose, and can 

 always at the end be precipitated in greater or less amount 

 by the addition of alcohol to the liquid. It is probable that 

 a whole series of dextrins is formed during the digestion of 

 starch. Some of these may appear as forerunners of the 

 sugar, others merely as concomitants of its production. The 

 latter may never pass into sugar; and it is certain that sugar 

 may appear before all the starch has been converted into 

 achroodextrin. When the sugar is removed as it is formed, 

 as is approximately the case when the digestion is performed 

 in a dialyser, the residue of unchanged dextrin is less than 

 when the sugar is allowed to accumulate (Lea). In ordinary 

 artificial digestion, for instance, under the most favourable 

 circumstances at least 12 to 15 per cent, of the starch is left 

 as dextrin ; in dialyser digestions the residue of dextrin may 

 be little more than 4 per cent. This goes far to explain the 

 complete digestion of starch which takes place in the ali- 

 mentary canal, a digestion so exhaustive that, although solu- 

 ble starch and dextrin may be found in the stomach after a 

 starchy meal, they do not occur in the intestine, or only in 

 minute traces. Here the amylolytic ferment of the pancreatic 

 juice, which is essentially the same in its action as ptyalin, 

 only more powerful, must effect a very complete conversion 

 of the starch molecules accessible to its attack. It is not in- 

 consistent with this, that unchanged starch granules may 

 sometimes be excreted in the faeces, especially when imbedded 

 in raw vegetable structures. 



