Page:First course in biology (IA firstcourseinbio00bailrich).pdf/566

 having become semi-liquid, and the proteids having been partly digested by the gastric juice, an acid secretion from the small glands in the stomach walls.

(4) The small intestine, a narrow tube more than twenty feet long, where the fats are acted upon for the first time, and where the starches and proteids are also acted upon, and where, after about ten hours, the digestion of the three classes of foods is completed by pancreatic juice from the pancreas, and bile from the liver.

(5) The large intestine, about five feet long, where the last remnant of nutriment is absorbed, and the indigestible materials in the food are gathered together (Exp. 9).

The Teeth.—The main body of the tooth consists of bone-*like dentine, or ivory. Hard, shining enamel protects the crown, or visible portion. The part of the tooth beneath the gum is called the neck, and the part in the bony socket, is called the root. The enamel ends just beneath the gum, where it is overlapped by cement of the root. There is a pulp cavity in every tooth (Fig. 91); it contains pulp made up of connective tissue, with nerves and blood vessels which enter at the tip of the root (Exp. 6).

The temporary set of teeth is completed at about two years of age and consists of twenty teeth. The teeth cannot grow as the jaw grows, and soon a larger and permanent set starts to growing deeper in the