a 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 sugar, by means of Fehling's
solution or tablets.[21] _A_ and _B_ give no evidence of sugar, while
_C_ reduces the Fehling, giving a yellow or red deposit of cuprous
oxide. Therefore, starch is converted into a reducing sugar by the
saliva. This is done by the ferment ptyal