This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The first solvent poured upon the food is the saliva, which, when it meets the food, is a mixture of pure saliva with the mucus secreted by the membrane lining the mouth. This mixed saliva is a colorless, cloudy, feebly alkaline liquid.
The Uses Of Saliva are mainly physical and mechanical. It keeps the mouth moist and allows us to speak with comfort; most young orators know the distress occasioned by the suppression of the salivary secretion through nervousness, and the imperfect efficacy under such circumstances of the traditional glass of water placed beside public speakers. The saliva also enables us to swallow dry food; such a thing as a cracker when chewed would give rise merely to a heap of dust, impossible to swallow, were not the mouth cavity kept moist.* The saliva also dissolves such bodies as salt and sugar, when taken into the mouth in a solid form, and enables us to taste them; undissolved substances are not tasted, a fact which any one can verify for himself by wiping his tongue dry and placing a fragment of sugar upon it. No sweetness will be felt until a little moisture has exuded and dissolved part of the sugar.
Explain the object of digestion.
What is the first digestive liquid which the food meets with? How does it differ from pure saliva? Describe mixed saliva.
In addition to such actions the saliva, however, exerts a chemical one on an important foodstuff. Starch (although it swells up greatly in hot water) is insoluble and could not be absorbed from the alimentary canal. The saliva has the power of turning starch into the readily soluble and absorbable grape sugar, the sugar of most fruits.† The starch is made to combine with the elements of water, and the final result is grape sugar.
What foodstuff does saliva act upon chemically? What change is produced by its action ?
* This fact used to be taken advantage of in the East Indian rice ordeal for the detection of criminals. The guilty person believing firmly that he cannot swallow the parched rice given him, and sure of detection, is apt to have his salivary glands paralyzed by fear, and so does actually become unable to swallow the rice; while in those with clear consciences the nervous system, acting normally, excites the usual reflex secretion, and the dry food causes no difficulty of deglutition.
† Grape sugar or glucose is now an extensively produced article of commerce, being made for this purpose by the prolonged action of dilute acids upon starchy substances.
C18H30O15 + 3H2O = 3C6H12O6 Starch. Water. Grape Sugar.
So far as chemical changes are concerned the saliva is but of secondary importance in digestion: its main use is to facilitate swallowing. It only changes starch into grape sugar (at least rapidly) when no acid is present, and food passes from the mouth to the stomach where it is mixed with the acid gastric juice, before the saliva has time to do much. Indirectly, however, the saliva promotes digestion in the stomach. Weak alkalies stimulate the gastric glands to pour forth more abundant secretion,* and the saliva, being alkaline, acts in this way. This is one reason why food should be well chewed before being swallowed; its taste, and the movements of the jaws, excite a more abundant salivary secretion, and this alkaline saliva, when swallowed, helps to stir the stomach up to work.