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 digestive process.  Thorough mastication is a necessary introduction to the more complicated changes which occur in the later digestion.

133.  The Teeth.  The teeth are attached to the upper and lower maxillary bones by roots which sink into the sockets of the jaws.  Each tooth consists of a crown, the visible part, and one or more fangs, buried in the sockets.  There are in adults 32 teeth, 16 in each jaw.

Teeth differ in name according to their form and the uses to which they are specially adapted.  Thus, at the front of the jaws, the incisors, or cutting teeth, number eight, two on each side.  They have a single root and the crown is beveled behind, presenting a chisel-like edge.  The incisors divide the food, and are well developed in rodents, as squirrels, rats, and beavers.

Next come the canine teeth, or cuspids, two in each jaw, so called from their resemblance to the teeth of dogs and other flesh-eating animals.  These teeth have single roots, but their crowns are more pointed than in the incisors.  The upper two are often called eye teeth, and the lower two, stomach teeth.  Next behind the canines follow, on each side, two bicuspids.  Their crowns are broad, and they have two roots.  The three hindmost teeth in each jaw are the molars, or grinders.  These are broad teeth with four or five points on each, and usually each molar has three roots.