_Permanent teeth:_ F, central incisors; H, lateral incisors; K, canines; L, first bicuspids; M, second biscuspids; N, first molars The plan of a gradual succession of teeth is a beautiful provision of nature, permitting the jaws to increase in size, and preserving the relative position and regularity of the successive teeth. Illustration: Fig. 49.—Showing the Principal Organs of the Thorax and Abdomen _in situ_. (The principal muscles are seen on the left, and superficial veins on the right.) 135. Structure of the Teeth. If we should saw a tooth down through its center we would find in the interior a cavity. This is the pulp cavity, which is filled with the dental pulp, a delicate substance richly supplied with nerves and blood-vessels, which enter the tooth by small openings at the point of the root. The teeth are thus nourished like other parts of the body. The exposure of the delicate pulp to the air, due to the decay of the dentine, gives rise to the pain of toothache. Surrounding the cavity on all sides is the hard substance known as the dentine, or tooth ivory. Outside the dentine of the root is a substance closely resembling bone, called cement. In fact, it is true bone, but lacks the Haversian canals. The root is held in its socket by a dense fibrous membrane which surrounds the cement as the periosteum does bone. Illustration: Fig. 50.—Section of Face. (Showing the parotid and submaxillary glands.) The crown of the tooth is not covered by cement, but by the hard enamel, which forms a strong protection for the exposed part. When the teeth are first “cut,” the surface of the enamel is coated with a delicate membrane which answers to the Scriptural phrase “the skin of the teeth.” This is worn off in adult life. 136. Insalivation. The thorough mixture of the saliva with the food is called insalivation. While the food is being chewed, it is moistened with a fluid called saliva, which flows into the mouth from six little glands. There are on each side of the mouth three salivary glands, which secrete the saliva from the blood. The parotid is situated on the side of the face in front of the ear. The disease, common in childhood, during which this gland becomes inflamed and swollen, is known as the “mumps.” The submaxillary gland is placed below and to the inner side of the lower jaw, and the sublingual is on the floor of the mouth, between the tongue and the gums. Each gland opens into the mouth by a little duct. These glands somewhat resemble a bunch of grapes with a tube for a stalk. The saliva is a colorless liquid without taste or smell. Its principal element, besides water, is a ferment called _ptyalin_, which has the remarkable property of being able to change starch into a form of cane-sugar, known as maltose.