Salivary Glands.—We are all familiar with the substance called saliva which acts as a lubricant in the mouth. Saliva is manufactured in the cells of three pairs of glands which empty into the mouth, and which are called, according to their position, the parotid (beside the ear), the submaxillary (under the jawbone), and the sublingual (under the tongue).
Digestion of Starch.—If we collect some saliva in a test tube, add to it a little starch paste, place the tube containing the mixture for a few minutes in tepid water, and then test with Fehling's solution, we shall find grape sugar present. Careful tests of the starch paste and of the saliva made separately will usually show no grape sugar in either.
Experiment showing non-osmosis of starch in tube A, and osmosis of sugar in tube B.
If another test be made for grape sugar, in a test tube containing starch paste, saliva, and a few drops of any weak acid, the starch will be found not to have changed. The digestion or change of starch to grape sugar is caused by the presence in the saliva of an enzyme, or digestive ferment. You will remember that starch in the growing corn grain was changed to grape sugar by an enzyme called diastase. Here a similar action is caused by an enzyme called ptyalin. This ferment acts only in an alkaline medium at about the temperature of the body.
The mouth cavity of man. e, Eustachian tube; hp, hard palate; sp, soft palate; ut, upper teeth; bc, buccal cavity; lt, lower teeth; t, tongue; ph, pharynx; ep, epiglottis; lx, voice box; oe, gullet; tr, trachea.
Mouth Cavity in Man.—In our study of a frog we find that the mouth cavity has two unpaired and four paired tubes leading from it. These are (a) the gullet or food tube, (b) the windpipe (in the frog opening through the glottis), (c) the paired nostril holes (posterior nares), (d) the paired Eustachian tubes, leading to the ear. All of these openings are found in man.
In man the mouth cavity, and all internal surfaces of the food tube, are lined with a mucous membrane. The mucus secreted from gland cells in this lining makes a slippery surface so that the food may slip down easily. The roof of the mouth is formed in front by a plate of bone called the hard palate, and a softer continuation to the back of the mouth, the soft palate. These separate the nose cavity from that of the mouth proper. The part of the space back of the soft palate is called the pharynx, or throat cavity. From the pharynx lead off the gullet and windpipe, the former back of the latter. The lower part of the mouth cavity is occupied by a muscular tongue. Examination of its surface with a looking-glass shows it to be almost covered in places by tiny projections called papillæ. These papillæ contain organs known as taste buds, the sensory endings of which determine the taste of substances. The tongue is used in moving food about in the mouth, and in starting it on its way to the gullet; it also plays an important part in speaking.
