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). [Illustration: Experiment showing non-osmosis of starch in tube _A_, and osmosis of sugar in tube _B_.] 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. 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. [Illustration: 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.