on._--These are attached to the lower jaw and
bring about its different movements. The _masseter_ muscles, which are the
heavy muscles in the cheeks, and the _temporal_ muscles, located in the
region of the temples, raise the lower jaw and supply the force for
grinding the food. Small muscles situated below the chin depress the jaw
and open the mouth.

[Fig. 67]

Fig. 67--*Salivary glands* and the ducts connecting them with the mouth.

4. _The Salivary Glands._--These glands are situated in the tissues
surrounding the mouth, and communicate with it by means of ducts (Fig.
67). They secrete the saliva. The salivary glands are six in number and
are arranged in three pairs. The largest, called the _parotid_ glands,
lie, one on either side, in front of and below the ears. A duct from each
gland passes forward along the cheek until it opens in the interior of the
mouth, opposite the second molar tooth in the upper jaw. Next in size to
the parotids are the _submaxillary_ glands. These are located, one on
either side, just below and in front of the triangular bend in the lower
jaw. The smallest of the salivary glands are the _sublingual_. They are
situated in the floor of the mouth, on either side, at the front and base
of the tongue. Ducts from the submaxillary and sublingual glands open into
the mouth below the tip of the tongue.

*The Saliva and its Uses.*--The saliva is a transparent and somewhat slimy
liquid which is slightly alkaline. It consists chiefly of water (about 99
per cent), but in this are dissolved certain salts and an active chemical
agent, or enzyme, called _ptyalin_, which acts on the starch. The ptyalin
changes starch into a form of sugar (maltose), while the water in the
saliva dissolves the soluble portions of the food. In addition to this the
saliva moistens and lubricates the food which it does not dissolve, and
prepares it in this way for its passage to the stomach. The last is
considered the most important use of the saliva, and dry substances, such
as crackers, which require a considerable amount of this liquid, cannot be
eaten rapidly without choking. Slow mastication favors the secretion and
action of the saliva.

*Deglutition.*--Deglutition, or swallowing, is the process by which food is
transferred from the mouth to the stomach. Though this is not, strictly
speaking, a digestive process, it is, nevertheless, necessary for the
further digestion of the food