an order that those
preceding are preparatory to those that follow. These processes are known
as _mastication, insalivation, deglutition, stomach digestion_, and
_intestinal_ digestion. As the different materials become liquefied they
are transferred to the blood, and substances not reduced to the liquid
state are passed on through the canal as waste. The first two of the
digestive processes occur in

*The Mouth.*--This is an oval-shaped cavity situated at the very beginning
of the canal. It is surrounded by the lips in front, by the cheeks on the
sides, by the hard palate above and the soft palate behind, and by the
tissues of the lower jaw below. The mucous membrane lining the mouth is,
soft and smooth, being covered with flat epithelial cells. The external
opening of the mouth is guarded by the lips, and the soft palate forms a
_movable_ partition between the mouth and the pharynx. In a condition of
repose the mouth space is practically filled by the teeth and the tongue,
but the cavity may be enlarged and room provided for food by depressing
the lower jaw.

The mouth by its construction is well adapted to carrying on the processes
of mastication and insalivation. By the first process the solid food is
reduced, by the cutting and grinding action of the teeth, to a finely
divided condition. By the second, the saliva becomes mixed with the food
and is made to act upon it.

[Fig. 65]

Fig. 65--*The teeth.* _A._ Section of a single molar. 1. Pulp. 2. Dentine.
3. Enamel. 4. Crown. 5. Neck. 6. Root. _B._ Teeth in position in lower
jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. _C._ Upper and
lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4.
Molars. 5. Wisdom. _D._ Upper and lower incisor, to show gliding contact.

*Accessory Organs of the Mouth.*--The work of mastication and insalivation
is accomplished through organs situated in and around the mouth cavity.
These comprise:

1. _The Teeth._--The teeth are set in the upper and lower jaws, one row
directly over the other, with their hardened surfaces facing. In reducing
the food, the teeth of the lower jaw move against those of the upper,
while the food is held by the tongue and cheeks between the grinding
surfaces. The front teeth are thin and chisel-shaped. They do not meet so
squarely as do the back ones, but their edges glide over each other, like
the blades of scissors--a condition that adapts th