[Illustration:  Fig. 50.—­Section of Face. (Showing the parotid and submaxillary glands.)]

The crown of the tooth is not covered by cement, but by the hard enamel, which forms a strong protection for the exposed part.  When the teeth are first “cut,” the surface of the enamel is coated with a delicate membrane which answers to the Scriptural phrase “the skin of the teeth.”  This is worn off in adult life.

136.  Insalivation.  The thorough mixture of the saliva with the food is called insalivation.  While the food is being chewed, it is moistened with a fluid called saliva, which flows into the mouth from six little glands.  There are on each side of the mouth three salivary glands, which secrete the saliva from the blood.  The parotid is situated on the side of the face in front of the ear.  The disease, common in childhood, during which this gland becomes inflamed and swollen, is known as the “mumps.”  The submaxillary gland is placed below and to the inner side of the lower jaw, and the sublingual is on the floor of the mouth, between the tongue and the gums.  Each gland opens into the mouth by a little duct.  These glands somewhat resemble a bunch of grapes with a tube for a stalk.

The saliva is a colorless liquid without taste or smell.  Its principal element, besides water, is a ferment called ptyalin, which has the remarkable property of being able to change starch into a form of cane-sugar, known as maltose.

Thus, while the food is being chewed, another process is going on by which starch is changed into sugar.  The saliva also moistens the food into a mass for swallowing, and aids in speech by keeping the mouth moist.

The activity of the salivary glands is largely regulated by their abundant supply of nerves.  Thus, the saliva flows into the mouth, even at the sight, smell, or thought of food.  This is popularly known as “making the mouth water.”  The flow of saliva may be checked by nervous influences, as sudden terror and undue anxiety.

Experiment 56. To show the action of saliva on starch.  Saliva for experiment may be obtained by chewing a piece of India rubber and collecting the saliva in a test tube.  Observe that it is colorless and either transparent or translucent, and when poured from one vessel to another is glairy and more or less adhesive.  Its reaction is alkaline to litmus paper.

Experiment 57.  Make a thin paste from pure starch or arrowroot.  Dilute a little of the saliva with five volumes of water, and filter it.  This is best done through a filter perforated at its apex by a pin-hole.  In this way all air-bubbles are avoided.  Label three test tubes A, B, and C.  In A, place starch paste; in B, saliva; and in C one volume of saliva and three volumes of starch paste.  Place them for ten minutes in a water bath at about 104 degrees Fahrenheit.

Test portions of all three for a reducing