236 SALIVA. [BOOK n. 



of iodine frequently gives rise to a red or violet colour instead of 

 a pure blue, but when the conversion is complete no colouration 

 at all is observed. The appearance of this red or violet colour 

 indicates the presence of dextrin. 



The temporary appearance of dextrin shews 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 dextrose or 

 maltose. There are probably several other bodies formed out of 

 the starch besides these, 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, dextrin appearing tempo- 

 rarily in the process, and 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 treat- 

 ment 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 granulose by traversing 

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

 much hindered and delayed. 



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

 the amylolytic action of saliva, will go on at the ordinary tempera- 

 ture 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, favours 

 the change, and the greatest activity is said to be witnessed at 

 about 40. Much beyond this, 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 per- 

 manently destroyed. 



The action of saliva on starch needs for its development a 

 slightly alkaline or at least a neutral reaction of the mixture ; 

 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. 



