DIGESTION. 61 



of the bolus swallowed. It is influenced only by the reflex act and 

 the summation of other acts. It also has an inhibitory influence upon 

 the respiration. This is very evident during rapid drinking in an 

 animal with a tracheotomy tube. For increasing the activity of the 

 heart's action a tablespoonful of water taken in a large number of 

 swallows is more beneficial than a glass of wine taken in one swallow. 



THE CHEMICAL CHANGES OCCURRING IN THE MOUTH 

 DURING DIGESTION. 



As before stated, the chief aim of digestion in the animal econ- 

 omy is the reduction of the alimentary substances into a soluble and 

 absorbable condition, so they can pass through the various animal 

 membranes and become components of the tissues and blood of the 

 body. No matter how soft, through the influence of insalivation, or 

 finely divided and triturated by reason of mastication, the food may 

 be, it cannot become a constituent of the body until it has been acted 

 upon chemically and dissolved by the various ferments present in the 

 different digestive fluids. 



When food enters the mouth, the commencement of the digestive 

 tract, the first digestive fluid that it comes in contact with is the 

 saliva. Besides its mechanical functions of moistening and softening 

 the food to render easier the task of swallowing it in the form of boli, 

 it performs other duties of a chemical nature. 



First, by reason of its watery base, it has the power to dissolve 

 saline substances, the organic acids, alcohols, sugars, and a few other 

 substances soluble in water. 



Secondly, it has the power to transform certain materials, as 

 starches, into maltose, a form of sugar. The starch must have its 

 cellulose coat dissolved by boiling, however, for the ferment in saliva 

 will not act readily upon cellulose. The active, transforming prin- 

 ciple in saliva is an unorganized ferment, or enzyme, to which the 

 name piyalin has been given. The conversion of starch into dextrin 

 and maltose by it is known as the amylolytic action of saliva. Its 

 action is by mere contact, for no appreciable change in quantity or 

 character is noted in it after its functions are performed, and so active 

 is it that it is able to convert two thousand times its own weight of 

 starch into dextrin and maltose. 



The word "ase" is given to an enzyme, and this is preceded by 

 the name, or its root, of the substance upon which it acts. Thus, 

 ptyalin, an amylase (amylum, starch), breaks up starch into maltose. 

 Thus, saccharase or invertase of the small intestines acts on saccha- 



