216. The four front teeth in each jaw are called _in-ci´sors_, (cutting teeth;) the next tooth in each side, the _cus´pid_, (eye tooth;) the next two, _bi-cus´pids_, (small grinders;) the next two, _mo´lars_, (grinders.) The last one on each side of the jaw is called a _wisdom tooth_, because it does not appear until a person is about twenty years old. The incisors, cuspids, and bicuspids, have each but one root. The molars of the upper jaw have three roots, while those of the lower jaw have but two. [Illustration: Fig. 55. The permanent teeth of the upper and lower jaw. _a_, _b_, The incisors. _c_, The cuspids. _d_, _e_, The bicuspids. _f_, _g_, The molars, (double teeth.) _h_, The wisdom teeth.] _Observation._ The shape of the teeth in different species of animals is adapted to the kind of food on which they subsist. Those animals that feed exclusively on flesh, as the lion, have the cuspids, or canine teeth, largely developed, and the molars have sharp cutting points. Those animals that feed on grass and grain, as the horse and the sheep, have their molar teeth more rounded and flat on the crown. The human teeth are adapted to feed on fruits, grain, or flesh, as they are less pointed than those of the cat, and more pointed than those of the sheep. -=-=-=-=-=-=-=-=-=-=-=-= 216. Give the names of the permanent teeth. What teeth have but one root, or "fang"? How many roots have the molars of the upper jaw? Of the lower jaw? What is said of the shape of the teeth in different species of animals? -=-=-=-=-=-=-=-=-=-=-=-= 217. The teeth are composed principally of two substances--the _i´vo-ry_ and the _en-am´el_. The internal part of the tooth or the ivory, is harder and more enduring than bone, and forms the body of the tooth. The enamel is remarkable for its hardness, and varies somewhat in color with the age, temperament, habits, and manner of living of different individuals. When any part of the enamel is destroyed, it is never regenerated. [Illustration: Fig. 56. A side view of the body and enamel of a front tooth.] [Illustration: Fig. 57. A side view of a molar tooth. 1, The enamel. 2, The body of the tooth. 3, The cavity in the crown of the tooth that contains the pulp. 4, A nerve that spreads in the pulp of the tooth. 5, An artery that ramifies in the pulp of the tooth.] 218. Each tooth is divided into two parts, namely, _crown_ and _root_. The crown is that part which protrudes from the jaw-bone and gum, and is covered by the highly polished enamel. The root, or "fang," is placed in the sockets of the jaw, and consists of bony matter. Through this bony substance several small vessels pass, to aid in the growth and also in the removal of the tooth. There are, beside these vessels, small white cords passing to each tooth, called _nerves_. (See fig. 57.) When these nerves are diseased, we have the toothache. -=-=-=-=-=-=-=-=-=-=-=-=