Fig. 65—*The teeth.* _A._ Section of a single molar. 1. Pulp. 2. Dentine. 3. Enamel. 4. Crown. 5. Neck. 6. Root. _B._ Teeth in position in lower jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. _C._ Upper and lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4. Molars. 5. Wisdom. _D._ Upper and lower incisor, to show gliding contact. *Accessory Organs of the Mouth.*—The work of mastication and insalivation is accomplished through organs situated in and around the mouth cavity. These comprise: 1. _The Teeth._—The teeth are set in the upper and lower jaws, one row directly over the other, with their hardened surfaces facing. In reducing the food, the teeth of the lower jaw move against those of the upper, while the food is held by the tongue and cheeks between the grinding surfaces. The front teeth are thin and chisel-shaped. They do not meet so squarely as do the back ones, but their edges glide over each other, like the blades of scissors—a condition that adapts them to cutting off and separating the food (_D_, Fig. 65). The back teeth are broad and irregular, having surfaces that are adapted to crushing and grinding. Each tooth is composed mainly of a bone-like substance, called _dentine_, which surrounds a central space, containing blood vessels and nerves, known as the _pulp cavity_. It is set in a depression in the jaw where it is held firmly in place by a bony substance, known as _cement_. The part of the tooth exposed above the gum is the _crown_, the part surrounded by the gum is the _neck_, and the part which penetrates into the jaw is the _root_ (_A_, Fig. 65). A hard, protective material, called _enamel_, covers the exposed surface of the tooth. The teeth which first appear are known as the _temporary_, or milk, teeth and are twenty in number, ten in each jaw. They usually begin to appear about the sixth month, and they disappear from the mouth at intervals from the sixth to the thirteenth year. As they leave, teeth of the second, or _permanent_, set take their place. This set has thirty-two teeth of four different kinds arranged in the two jaws as follows: In front, above and below, are four chisel-shaped teeth, known as the _incisors_. Next to these on either side is a tooth longer and thicker than the incisors, called the _canine_. Back of these are two short, rounded and double pointed teeth, the _bicuspids_, and back of the bicuspids are three heavy teeth with irregular grinding surfaces, called the _molars_ (_B_ and _C_, Fig. 65). Since the molar farthest back in each jaw is usually not cut until maturity, it is called a _wisdom_ tooth. The molars are known as the superadded permanent teeth because they do not take the place of milk teeth, but form farther back as the jaw grows in length. [Fig. 66] Fig. 66—*Diagram* showing directions of muscular fibers in tongue.