178 DIGESTION 



occasion in nearly every part. In mastication its office is to gather up 

 the portions of the food being chewed and to keep them sufficiently long 

 between the teeth for complete mastication. 



This directing, guiding, and restraining function of the versatile tongue 

 would be impossible of its accomplishment, even with the muscles of the 

 cheeks to aid, were it not for the saliva, with its sticky mucin, to fasten 

 the crumbs together. The saliva is poured out into all portions of the 

 mouth in a certain amount, but principally where it will be most useful 

 to the grinding mechanism, behind the molar teeth, under the tongue, 

 and on the floor of the mouth. Besides reducing the bite of food to a 

 mass of fine particles easy of access by the digestive juices, mastication 

 serves to mix thoroughly the bolus of food with this important amylo- 

 lytic and alkaline digestant. 



The saliva is, then, an essential substance. It has the power of chemi- 

 cally dissolving an important order of foodstuffs, and it alone makes 

 possible adequate mastication and deglutition (passage of the food into 

 the stomach from the mouth). Of these two sorts of function, the former, 

 chemical solution, is apparently of the lesser importance, for saliva in 

 this respect has perhaps a more efficient substitute in the pancreatic juice 

 employed farther down the gut. Without the mechanical services of 

 saliva, however, the swallowing of food is inconceivable, if not in liquid 

 form or taken with a large quantity of liquid. 



Saliva, then, (a) lubricates the tongue and keeps the mucosa of the 

 mouth in normal condition. (6) It softens the food by its fluidity, 

 makes it chewable, and renders it easily moved about by the tongue 

 and cheeks, (c) It sticks together the food-particles by its mucin, so that 

 a definite bolus suitable in size and shape to be swallowed is easily made 

 by the tongue, (d) It lubricates this bolus by the serous element of its 

 composition, so that it may be quickly passed into the esophagus and then 

 painlessly down that narrow tube, (e) It starts at least the hydrolysis 

 of starchy carbohydrates. (/) It dissolves such food as is soluble in a 

 faintly alkaline liquid, and thus makes possible the important sense of 

 taste (see page 357). 



One of these uses of the saliva demands further description namely, its 

 solvent or digestive action on carbohydrates. This effect is brought about 

 by a member of a class of bodies as yet in themselves little understood, 

 called enzymes, or ferments, this particular enzyme (probably identical 

 with the animal diastase or amylopsin of the pancreas) being called 

 ptyalin. What ptyalin is, chemically speaking, is still unknown. It has 

 been supposed that it is protein in its composition or a close derivative of 

 a proteid. Recent researches, however, have made it necessary to deny 

 that one enzyme, at least, is proteid substance, for an eminent chemist 

 (Pekelharring) is sure that he has removed by repeated precipitation all 

 traces of proteid from pepsin, a typical enzyme. Whatever the details 

 of the actions set going by enzymes, the process, reduced to its simplest 

 terms, seems to be one of hydrolysis. This term means the absorption of 

 water by a molecule of the material acted upon and the latter's immediate 



