SALIVA AND GASTRIC JUICE. 249 



While the conversion of the starch by the saliva is going on the addition 

 of iodine, frequently gives rise to a red or violet color instead of a pure blue, 

 but when the conversion is complete no coloration at all is observed. The 

 appearance of this red color indicates the presence of dextrin (erythrodex- 

 trin) ; the violet color is due to the red being mixed with the blue of still 

 unchanged starch. 



The appearance of dextrin shows that the action of the saliva on the 

 starch is somewhat complex ; and this is still further proved by the fact 

 that even when the saliva has completed its work the whole of the starch 

 does not reappear as maltose or dextrose. A considerable quantity of the 

 other dextrin (achroodextrin) always appears and remains unchanged to the 

 end ; and there are probably several other bodies also formed out of the 

 starch, the relative proportions varying according to circumstances. The 

 change, therefore, though perhaps we may speak of it in a general way as 

 one of hydration, cannot be exhibited under a simple formula, and we may 

 rest content for the present with the statement that starch when subjected 

 to the action of saliva is converted chiefly into the sugar known as maltose 

 with a comparatively small quantity of dextrose and to some extent into 

 achroodextrin (erythrodextrin appearing temporarily only in the pro- 

 cess), other bodies on which we need not dwell being formed at the 

 same time. 



Raw unboiled starch undergoes a similar change but at a much slower 

 rate. This is due to the fact that in the curiously formed starch grain the 

 true starch, or granulose, is invested with coats of cellulose. This latter 

 material, which requires previous treatment with sulphuric acid before it 

 will give the blue reaction on the addition of iodine, is apparently not acted 

 upon by saliva. Hence the saliva can only get at the grauulose by travers- 

 ing the coats of cellulose, and the conversion of the former is thereby much 

 hindered and delayed. 



185. The conversion of starch into sugar, and this we may speak of as 

 the amylolytic action of saliva, will go on at the ordinary temperature of the 

 atmosphere. The lower the temperature the slower the change, and at about 

 C. the conversion is indefinitely prolonged. After exposure to this cold 

 for even a considerable time the action recommences when the temperature 

 is again raised. Increase of temperature up to about 35-40, or even a 

 little higher, favors the change, the greatest activity being said to be 

 manifested at about 40. Much beyond this point, however, increase of 

 temperature becomes injurious, markedly so at 60 or 70 ; and saliva 

 which has been boiled for a few minutes not only has no action on 

 starch while at that temperature, but does not regain its powers on 

 cooling. By being boiled, the amylolytic activity of saliva is permanently 

 destroyed. 



The action of saliva on starch is most rapid when the reaction of the 

 mixture is neutral or nearly so ; it is hindered or arrested by a distinctly 

 acid reaction. Indeed, the presence of even a very small quantity of free 

 acid, at all events of hydrochloric acid, at the temperature of the body, not 

 only suspends the action but speedily leads to permanent abolition of the 

 activity of the juice. The bearing of this will be seen later on. 



The action of saliva is hampered by the presence in a concentrated state 

 of the product of its own action that is, of sugar If a small quantity of 

 saliva be added to a thick mass of boiled starch, the action will after a while 

 slacken, and eventually come to almost a standstill, long before all the starch 

 has been converted. On diluting the mixture with water, the action will 

 recommence. If the products of action be removed as soon as they are 

 formed, dialysis for example, a small quantity of saliva will, if sufficient time 



