DIGESTION 321 



vitro. It is possible that this is produced from the maltose by 

 maltase, which some writers assert to be present in small amount 

 in saliva. But the observation has also been made that the saliva 

 itself (in the cat) may contain a trace of dextrose (Carlson). 



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 erythro- 

 dextrin 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, con- 

 verted 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 achroo- 

 dextrin. 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 alimentary canal, a digestion so 

 exhaustive that, although soluble starch and dextrin may be 

 iound 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 inconsistent with this, that unchanged starch 

 granules may sometimes be excreted in the faeces, especially when 

 imbedded in raw vegetable structures. 



It is a notable fact that amylolytic or starch-splitting ferments, 

 also called diastases or amylases, are not confined to the animal 

 body, but are widely distributed in plants. A diastase, which 

 is present in all sprouting seeds, and may be readily extracted 

 by water from malt, forms dextrin and maltose from starch. 

 The optimum temperature of malt diastase, however, is about 

 55 C., while that of pytalin is about 40 C. 



While a neutral or weakly alkaline reaction is not unfavourable 

 to salivary digestion, it goes on best in a slightly acid medium. 

 It has been shown that the activity of ptyalin on starch, both 

 having been previously dialysed to get rid as far as possible 



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