Fig. 45.—First upper molar, left side, of a fossil horse (Equus sp.). On the right, external side. On the left, the grinding surface, showing two stages of wear. Heavy black line, enamel; white, dentine; shaded, cement. Hypsodont, roots not yet formed.
The structure of mammalian teeth varies greatly, from the simplest slender cones to enormous and highly complicated apparatus, and, in order to comprehend the significance of these differences, we must look a little more closely into the materials of which the teeth are constructed and the manner in which those materials are combined. In all primitive mammals and in many of the higher and more advanced ones (including Man) a tooth is composed of the crown, or portion which is exposed above the gum, and the roots, one or more in number, by means of which the tooth is firmly inserted in the jaw-bone; the roots are at least partly formed before the tooth comes into use. Such a tooth is said to be short or low-crowned, or brachyodont. In many plant-feeders, such as horses, cattle, elephants, beavers, etc., the teeth continue to grow in height for a long time and do not form roots until late in life, or perhaps not at all. Such teeth are said to be long- or high-crowned, or hypsodont, and in very many instances the development of brachyodont into hypsodont teeth may be followed through every step of the change. The advantage of the change is obvious in lengthening the animal’s life, especially in those which feed upon abrasive substances, like grass, for the growth of the teeth long continues to make up for the loss through wear. Serious trouble has often been caused for captive elephants by giving them too soft food, so that the growth of the teeth is not properly balanced by abrasion. Still another category of teeth is the rootless, which are of simple form, like those of an armadillo, and grow throughout life, never forming roots. The chisel-like, or scalpriform incisors of the rodents do not cease to grow while the animal lives; they are kept of constant length by continual use, and the arrangement of harder and softer tissue is such that the sharp edge is maintained; through accident or malformation it sometimes happens that the upper and lower teeth fail to meet, then the continued growth causes them to form curved hoops in the mouth, locking the jaws and bringing death by starvation to the unfortunate animal.
Fig. 46.—Dentition of Beaver (Castor canadensis). m. 3, last molar. p. 4, last premolar. i., scalpriform incisors; enamel face black, dentine in vertical lines.
The typical mammalian tooth is composed of three kinds of tissue, all differing in structure and hardness and called respectively (1) dentine, (2) enamel, (3) cement. (1) The dentine, or ivory, is the indispensable tissue of the tooth; the other kinds may be absent, but never the dentine. Chemically, it is like bone, but the microscope shows that its structure is quite different from that of true bone, being composed of an immense number of fine tubules, which radiate from the “pulp-cavity,” or chamber which contains the blood-vessels and nerves, these entering the tooth through the canals of the roots. The tubules of the dentine lodge excessively fine fibrillæ of the nerve and that is why the cutting into a live tooth is so painful an operation. (2) The enamel, which is the hardest of all animal tissues, has a polished and shining appearance and is arranged in a mosaic of microscopic prisms, closely packed together, which in most mammals are solid, but in the marsupials, with some exceptions, are tubular. The enamel normally covers the entire crown of the tooth, but does not extend upon the roots, where its superior hardness would be of no advantage. In several instances, always as a secondary specialization, the enamel does not cover the whole crown, but is arranged in vertical bands, it may be on one side only, or at intervals around the tooth. The scalpriform incisors of the rodents, already alluded to, have the enamel band on the front face of the tooth; the softer dentine behind wears away more rapidly, keeping the cutting surface bevelled, like the edge of a chisel, while the hard enamel forms the sharp edge. In some instances the enamel is absent altogether and the teeth are composed entirely of dentine, as in the elephant tusk. In all the Edentata, such as sloths and armadillos, both living and extinct, that have any teeth at all, the teeth have no enamel, but in some of the fossil forms the place of the missing enamel is taken by a harder dentine and thus the effect of differential hardness is secured.
Fig. 47.—Section through a lower molar of the Indian Elephant (Elephas maximus). Enamel, heavy black; dentine, white; cement, horizontal lines.
(3) The cement is simply bone, both chemically and in microscopic structure; it is not quite so hard as dentine, but it is less affected by the fluids of the mouth and the juices of the food. In the brachyodont or low-crowned tooth, such as a human molar, the cement merely forms a sheath over the roots and does not appear upon the crown, but in many hypsodont teeth, those of horses and elephants, for example, the cement completely encases the entire tooth in a thick layer, filling up all the depressions and irregularities of the enamel surface and making a freshly erupted and unworn tooth look like a shapeless lump. When the cement and the enamel covering are partially worn through, the masticating surface is made up of three distinct substances, each having a different degree of hardness and thus, through unequal wear, the grinding surface is always kept rough and therefore efficient. Not all hypsodont teeth have the cement covering, but in such teeth the differing degrees of hardness of enamel and dentine suffice to keep a rough surface, though not so effectively.