tongue and mastication.

The mouth, in most animals, is armed with very hard substances,
which, by the motion of the lower jaw, are brought strongly into
contact. Those parts of the teeth which are above the sockets, are
not simply bony, they are much harder than the bones, and possess the
property of resisting putrefaction, as long as this hard crust
continues to cover them. The teeth are divided into three classes:
1st. The cutting teeth, which are sharp and thin, and which serve to
cut or divide the food: 2nd. The canine teeth, which serve to tear it
into pieces still smaller: 3rd. The grinders, which present large and
uneven surfaces, and actually grind the substance already broken down
by the other teeth. Birds, whom nature has deprived of teeth, have a
strong muscular stomach, called the gizzard, which serves the
purposes of teeth, and they even take into the stomach small pieces
of grit, to assist in grinding to a powder the grain that they have
swallowed.

Among those parts of the mouth which contribute to the preparation of
the food, we must reckon the numerous glands which secrete saliva,
and which have therefore been called salivary glands. The saliva is a
saponaceous liquor, destitute of taste or smell, which is squeezed
out from these glands, and mixed with the food during mastication. In
the mouth, therefore, the food becomes first broken down by the
teeth, impregnated with saliva, and reduced to a soft pasty
substance, capable of passing with these, through the oesophagus,
into the stomach. It is here that it undergoes the change, which is
particularly termed digestion.

Digestion comprehends two classes of phenomena, distinct from each
other: 1st. Physical and chemical: 2nd. Organic and vital. The object
of the first, is to bring the alimentary substances to such a state
as is necessary, that they may be capable of the new combinations
into which they are to enter, to obtain the animal character. The
object of the second is, to produce those combinations which some
have thought to be very different from those produced by simple
chemical attractions.

The physical and chemical phenomena of digestion, relate chiefly,
1st. To the action of heat; 2ndly. To the dissolution of the
alimentary substances. The heat of the animal is such, as is well
fitted to promote solution.

That digestion is performed by solution, is evident, from several
experiments, particularly those made by Dr. Stevens, who enclosed