[Illustration: Fig. 47.--Diagram of the Structure of Secreting Glands.

A, simple tubular gland;
B, gland with mouth shut and sac formed;
C, gland with a coiled tube;
D, plan of part of a racemose gland
]

The digestive apparatus of the human body consists of the alimentary canal
and tributary organs which, although outside of this canal, communicate
with it by ducts. The alimentary canal consists of the mouth, the pharynx,
the oesophagus, the stomach, and the intestines. Other digestive organs
which are tributary to this canal, and discharge their secretions into it,
are the salivary glands,[20] the liver, and the pancreas.

The digestive process is subdivided into three steps, which take place in
the mouth, in the stomach, and in the intestines.

131. The Mouth. The mouth is the cavity formed by the lips, the
cheeks, the palate, and the tongue. Its bony roof is made up of the upper
jawbone on each side, and the palate bones behind. This is the _hard
palate_, and forms only the front portion of the roof. The continuation of
the roof is called the _soft palate_, and is made up of muscular tissue
covered with mucous membrane.

The mouth continues behind into the throat, the separation between the two
being marked by fleshy pillars which arch up from the sides to form the
soft palate. In the middle of this arch there hangs from its free edge a
little lobe called the uvula. On each side where the pillars begin to
arch is an almond-shaped body known as the tonsil. When we take cold,
one or both of the tonsils may become inflamed, and so swollen as to
obstruct the passage into the throat. The mouth is lined with mucous
membrane, which is continuous with that of the throat, oesophagus,
stomach, and intestines (Fig. 51).

132. Mastication, or Chewing. The first step of the process of
digestion is mastication, the cutting and grinding of the food by the
teeth, effected by the vertical and lateral movements of the lower jaw.
While the food is thus being crushed, it is moved to and fro by the varied
movements of the tongue, that every part of it may be acted upon by the
teeth. The advantage of this is obvious. The more finely the food is
divided, the more easily will the digestive fluids reach every part of it,
and the more thoroughly and speedily will digestion ensue.

The act of chewing is simple and yet important, for if hurriedly or
imperfectly done, the food is in a condition to cause disturbance in the
dig