But they rarely form an unbroken series.82 The teeth implanted in the premaxillary bone, and in the corresponding part of the lower jaw, whatever their number, are incisors. The first tooth behind the premaxillary, if sharp and projecting, is a canine.

Teeth of the right lower jaw of adult male Chimpanzee (Anthropopithecus troglodytes), natural size. The molar series does not form a curve, as in Man.

FIG. 233.

Each tooth has its particular bony socket.83 The molars may be still further strengthened by having two or more diverging fangs, or roots, a feature peculiar to this class. The incisors and canines have but one fang; and those that are perpetually growing, as the incisors of rodents and elephants, have none at all. The teeth of flesh-eating mammals usually consist of hard dentine, surrounded on the root with cement and capped with enamel. In the herbivorous tribes, they are very complex, the enamel and cement being inflected into the dentine, forming folds, as in the molar of the ox, or plates, as in the compound tooth of the elephant. This arrangement of these tissues, which differ in hardness, secures a surface with prominent ridges, well adapted for grinding. The cutting teeth of the rodents consist of dentine, with a plate of enamel on the anterior surface, and the unequal wear preserves a chisel-like edge.

Enamel is sometimes wanting, as in the molars of the sloth and the tusks of the elephant.

In fishes and reptiles, there is an almost unlimited succession of teeth; but mammalian teeth are cast and renewed but once in life.

Vertebrates use their teeth for the prehension of food, as weapons of offense or defense, as aids in locomotion, and as instruments for uprooting or cutting down trees. But in the higher class they are principally adapted for dividing or grinding the food. While in nearly all

FIG. 234.-Upper Molar Tooth of Indian Elephant (Elephas indicus), showing transverse arrangement of dentine, d, with festooned border of enamel plates, e; c, cement; one-third natural size.

other vertebrates the food is bolted entire, mammals masticate it before swallowing. Mastication is more essential in the digestion of vegetable than of animal food; and hence we find the dental apparatus most efficient in the herbivorous quadrupeds. The food is most perfectly reduced by the rodents.

Teeth, as we shall see, are appendages of the skin, not of the skeleton, and, like other superficial organs, are especially liable to be modified in accordance with the habits of the creature. They are, therefore, of great zoological value; for such is the harmony between them and their uses, the naturalist can predict the food and general structure of an animal from a sight of the teeth alone. For the same reason, they form important

DODGE'S GEN. ZOÖL. - 18

guides in the classification of animals; while their durability renders them available to the paleontologist in the determination of the nature and affinities of extinct species, of which they are often the sole remains. Even the structure is so peculiar that a fragment will sometimes suffice.

4. Deglutition, or how Animals Swallow. In the lowest forms of life, the mouth is but an aperture opening immediately into the body substance, and the food is drawn in by ciliary currents (Figs. 9, 11). But in the majority of animals, a muscular tube, called the gullet, or esophagus, intervenes between the mouth and stomach, the circular fibers of which contract, in a wavelike manner, from above downward, propelling the morsel into the stomach.85 In the higher mollusks, arthropods, and vertebrates, deglutition is generally assisted by the tongue, which presses the food backward, and by a glairy juice, called saliva, which facilitates its passage through the gullet.86 Vertebrates have a cavity behind the mouth, called the throat, or pharynx, which may be considered as a funnel to the esophagus.87 In air breathers, it has openings leading to the windpipe, nose, and ears. In man, as in mammals generally, the process of deglutition is in this wise: the food, masticated by the teeth and lubricated by the saliva, is forced by the tongue and cheeks into the pharynx, the soft palate keeping it out of the nasal aperture, and the valvelike epiglottis falling down to form a bridge over the opening to the windpipe. The moment the pharynx receives the food, it is firmly grasped, and, the muscular fibers contracting above it and left lax below it, it is rapidly thrust into the esophagus. Here, a similar movement (the peristaltic) strips the food into the stomach.88 The rapidity of these contractions transmitted along the esophagus may be observed in the neck of a horse while drinking.

Deglutition in the serpents is painfully slow, and somewhat peculiar. For how is an animal, without limbs or molars, to swallow its prey, which is often much larger than its own body? The boa constrictor, e.g., seizes the head of its victim with its sharp, recurving teeth, and crushes the body with its overlapping coils. Then, slowly uncoiling, and covering the carcass

FIG. 235.1 Skull of Boa Constrictor: 1, frontal; 2, prefrontal; 4, postfrontal; 5, basioccipital; 6, sphenoid; 7, parietal; 12, squamosal; 13, proötic; 17, premaxillary; 18, maxillary; 20, nasal; 24, transverse; 25, internal pterygoid; 34, dentary, lower jaw; 35, angular; 36, articular; a, quadrate; s, prenasal; v, petrosal.

with a slimy mucus, it thrusts the head into its mouth by main force, the mouth stretching marvelously, the skull being loosely put together. One jaw is then unfixed, and the teeth withdrawn by being pushed forward, when they are again fastened farther back upon the animal. The other jaw is then protruded and refastened; and thus, by successive movements, the prey is slowly and spirally drawn into the wide gullet.

CHAPTER XII

THE ALIMENTARY CANAL

The Alimentary Canal is the great route by which nutritive matter reaches the interior of the body. It is the most universal organ in the animal kingdom, and the rest are secondary or subservient to it. In the higher animals, it consists of a mouth, pharynx, gullet, stomach, and intestine.

It is a general law, that food can be introduced into the living system only in a fluid state. While plants send forth their roots to seek nourishment from without, animals, which may be likened to plants turned outside in, have their roots (called absorbents) directed inward along the walls of a central tube or cavity. This cavity is for the reception and preparation of the food, so that animals may be said to carry their soil about with them. The necessity for such a cavity arises not only from the fact that the food, which is usually solid, must be dissolved, so as to make its way through the delicate walls of the cavity into the system, but also from the occurrence of intervals between the periods of eating, and the consequent need of a reservoir. For animals, unlike plants, are thrown upon their own wits to procure food.

The Protozoa, as the amoeba and Infusoria, can not be said to have a digestive canal. The animal is here composed of a single cell, in which the food is digested. The jellylike amoeba passes the food through the firmer outer layer (ectosarc) into the more fluid inner part (endosarc), where it is digested (Fig. 1). The Infusoria,