THE CHEMISTRY OF THE DIGESTIVE JUICES 341 



passes into colloidal solution, yielding an opalescent liquid. If a 

 little saliva be added to some boiled starch solution which is free 

 from sugar, and the mixture be set to digest at a suitable temperature 

 (say 40 C), the solution in a very short time loses its opalescence 

 and becomes clear. It still, however, gives the blue reaction with 

 iodine; and Trommer's test (p. 10) shows that no sugar has as yet 

 been formed. Later on the iodine reaction passes gradually through 

 violet into red; and finally iodine causes no colour change at alf 

 while maltose is found in large amount, along with some isomaltose 

 (p. 332), a sugar having the same formula as maltose, but differing 

 from it in the melting-point of the crystalline compound formed by 

 it with phenyl-hydrazine (p. 517). Traces -of dextrose, a sugar 

 which rotates the plane of polarization less than maltose, but has 

 greater reducing power, may be found among the end-products 

 when the digestion is conducted in vitro. It is probable that this 

 is produced from the maltose by another enzyme (maltase), present 

 in small amount in saliva. In any case it is well known that mal- 

 tose may be a stage in the hydrolysis of starch to dextrose, and can 

 be detected among the intermediate products when the starch is 

 acted on by dilute acid under conditions which permit a gradual 

 decomposition of the fragments into which the polysaccharide 

 molecule is successively split. But the observation has also been 

 made that the saliva itself (in the cat) may contain a trace of dex- 

 trose (Carlson). 



The red colour indicates the presence of a kind, or a group, of 

 dextrins sometimes called erythrodextrin, because of this colour 

 reaction; the violet colour shows that at first this is still mixed with 

 some unchanged starch. Soon the dextrins which give the red colour 

 disappear, and are succeeded by other dextrins, which give no colour 

 with iodine, and are therefore called achrodextrins. These are 

 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. But little is known of their chemical nature. Recently, 

 however, some of these intermediate bodies have been obtained 

 in crystalline form. One of these appears to be a hexa-amylose 

 i.e., it consists of six C 6 H 10 O 5 groups, and would therefore be 

 capable of yielding, on the assumption of water, three molecules 

 of maltose or six of dextrose. This is accordingly a relatively 

 small fragment of the big original molecule of starch. 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 favour- 

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



