[Sidenote: Starch digestion in the mouth]
The saliva is the digestive juice of the mouth. It is secreted by three
pairs of salivary glands. The secretions from these three glands
are slightly different in composition, but for our purpose may be
considered as one secretion. The saliva is an alkaline fluid, and the
principal enzym that it contains is a starch-digesting enzym known as
ptyalin, which can act only in an alkaline solution. As the gastric
juice is strongly acid, the digestive action of the saliva is stopped
soon after the food has entered the stomach, and the enzym is of no
further use. The action of the saliva is very weak, and the amount of
starch digestion which is accomplished in the mouth is comparatively
insignificant.
[Sidenote: Saliva and mastication]
The chief function of the saliva is to moisten food and to facilitate
swallowing. From these statements one might first infer that the
emphasis given to thorough mastication is unwarranted. In fact, the
mastication of food has a much more important function than the
digestion of starch by saliva. This subject will be referred to again
when the physical condition of food as a factor in digestion, and
the nervous control or co-ordination of the various functions of the
digestive system are considered. (See "Composition of Gastric Juice," p.
147.)