The mechanism of mastication has already been discussed, so far as its triturating power is concerned. In its final object of forming the subdivided food into a bolus which can be easily swallowed, it is much aided by insalivation, particularly in chewing dry food; and in this latter, the moistening of the particles, so as to make them adhere together, is the most necessary act of mouth digestion, and is next in importance to the subdivision accomplished by the teeth. The saliva, also, covers the bolus with a coating of viscid fluid, so that it can be more easily propelled down the oesophagus. Deglutition of solids is difficult without an adequate supply of saliva.
While in the mouth the saliva dissolves a great quantity of the more readily soluble materials, such as sugar and salt, which may be either mingled with the insoluble substances, and swallowed together with the bolus, or separately in a fluid form. Solution, then, is an important item in mouth digestion.
In many carnivorous animals the use of the mouth fluid is chiefly mechanical, dissolving some insignificant part of the food, and aiding mastication and deglutition. In man, however, and other animals that make use of much vegetable food, it has a chemical function, and acts on the insoluble starch, converting it into soluble sugar.
The active principle which brings about this change is Ptyalin. This is one of a series of ferments to which most of the chemical changes in digestion are due.
As a group they are remarkable for the following characters in which they differ from most chemical agents: (i) They effect alterations in the substances on which they act, while they themselves do not undergo any perceptible change or diminution. (2) They exist in such small quantities that as a rule their presence can only be shown by the effects they produce. (3) They are most active at the body temperature, but are "killed" by that at which albumin coagulates.
Ptyalin acts on starch, and hence is spoken of as an amylolytic ferment; its action consists in causing the starch to unite chemically with one molecule of water, thus: - -
C6H10O5 + H2O = C6H12O6.
During this process, which takes at the least a few minutes to complete, various stages can be detected: first, two substances are formed which together are commonly spoken of as dextrin; one, erythro-dextrin, which give a red color with iodine, and easily passes into soluble sugar; and the other, achroo-dextrin, gives no color with iodine, and is with difficulty converted into sugar. As it gives no color with the ordinary test, its presence is often overlooked.
The sugar thus formed has been called Ptyalose, which can be converted into ordinary grape sugar (glucose) by the action of sulphuric acid. Some say the product is maltose.
The presence of starch, either in its soluble or insoluble form, is easily recognized by the blue color given by free iodine, which color disappears on heating to about 1oo° C, but reappears on cooling.
Very many tests have been recommended for the detection of sugar. The most generally applicable one is Trommer's. The liquid is made strongly alkaline with potash, and a few drops of a dilute solution of cupric sulphate is added, a clear blue solution results, which, on being raised to the boiling point, deposits an orange precipitate of cuprous oxide. Fehling's and Pavy's solutions are modifications of the above test adapted for quantitative analysis.
When yeast is added to a solution of grape sugar, the sugar is converted into alcohol and carbon dioxide. This may be seen in an inverted test tube. The C02 rises to the top, and can be used as an indication of the quantity of sugar present. Experiments may be carried out with saliva obtained directly from any of the glands, but the mixture of the secretion of all is found to be more efficacious than that of any single one. The ordinary mouth fluid, filtered, serves well for ordinary experiments.
An effective glycerine solution of ptyalin may be obtained by steeping chopped salivary glands in alcohol, and then extracting for some days with glycerine and water.
The following facts must be borne in mind concerning the amylolytic action of ptyalin:
1. The extremely small amount of the ferment required to make the fluid effective.
2. There is no appreciable diminution in the amount of ferment, so that it cannot be said to be used up in the process.
3. The action takes place most readily in alkaline solutions, such as the saliva, slowly in neutral solution, and not at all in acids of the strength of.2 per cent, of hydrochloric acid.
4. Temperature has a marked effect on the process. Cold (o° C.) quite checks the action; heat (750 C.) destroys the power of the ferment, which is most active at the body temperature (35 - 40° C).
5. Strong acids or alkalies destroy the amylolytic power of ptyalin.
6. The ferment has but little effect on raw starch, its cellulose coating protecting it; but it acts rapidly on well-boiled starch.
7. Ptyalin is more active in weak starch solutions, and is much impeded in its action by an accumulation of sugar. To recapitulate, we find that the following changes take place in the mouth:
(1) Solid food is, or should be, finely subdivided; (2) dry food is moistened, (3) rolled into a bolus, (4) and lubricated; (5) the soluble part is dissolved, and rendered capable of being tasted; (6) and part of the indififusible starch is converted into soluble diffusible sugar by the action of a ferment called ptyalin. In the short time occupied by the passage of food through the oesophagus no special change takes place in it, so we may pass at once to the gastric digestion, which will occupy the next chapter.