While this dentine is developing the tissue around the tooth-germ is forming a capsule or wall called the tooth-sac, the inner layer of which ossifies, forming a bony coat, the crusta petrosa or cement, on the external surface of the tooth. It is often found only as a thin layer on the surface of the root, which develops downward as the tooth grows, forcing the tooth upward, through and above the gum; but some times, as in the molar teeth of the horse, ele phant and mastodon, it is a structure which plays a very important part covering and filling in the interstices between the folds of the enamel. It is much like true bone in its con stitution. Meanwhile the epithelial cells of the superficial layer of the skin over the tooth germ are depositing upon the dentine of the upper part of the tooth, which is to be exposed, an extremely hard bluish white, translucent, protective layer, composed of about 96 per cent of mineral matter, called the enamel, and destined to resist the wear to whidi teeth are subjected in their work. It is the hardest organic substance known.
Teeth are of various forms, but reducible to two types, of which the simpler and more pnmitive is represented on a large scald by the tuslc (incisor) of the elephant or beaver, in which the pulp retains its conical form and activity, or is °persistent? and continues to supply dentine at the base of the tooth, which thus grows throughout life to conipensate for its wearing away at the tip. These teeth are said to be °rootless? The other type, ex emplified in man, is called °rooted?' and in this case, after the °crown? or exposed part of the tooth has been fully formed, the pulp within the °neck? or that part just beneath the sur face of the gum, begins to fill with dentine, and to form a downward growing pointed mass, the °root'' or °fang? which at last is solid ex cept for a narrow central canal in which the contracted remainder of the pulp persi,sts, largely supplied with nervous filaments from the pair of cranial nerves. Various intermedi ate conditions between these two types exist.
In form teeth vary from a simple spine-like or conical shape, to many- chisel-like or massive and complicated forms, all of which are de termined in the germ, and before the tooth makes its appearance above the gum or is °cut? which in mammals never normally occurs be fore birth. This appearance in some, as the seals, may talce place all at once; but in most cases occurs at intervals, the front teeth usually showing before the back ones, which sometimes are not cut (for example, the °wisdom teeth° in man) until several years later. In man and higher mammals two sets of teeth are de veloped: rthe early, milk, or deciduous teeth, and the permanent set. Such forms are, there fore, named diphyodont; while those in which one set only is developed are named mono phyodont. When more than one set occurs those of the second are developed in precisely the same place and manner as the first, except as to certain details of the enamel germs. The milk or temporary teeth are gradually displaced from below by the upward growth of the permanent teeth, the fangs of the milk teeth being ab sorbed, and the latter falling out as their successors are more fully developed. This ar rangement is adaptive to the growth of the animal's jaws, among other advantages. The
milk set in man consists of 20 teeth; and num bers four incisors, two canines, and four pre molars or bicuspids in each jaw. The per manent set includes, in addition to the forego ing teeth, six true tnolars in each jaw— the latter being thus unrepresented in the millc set. The milk-teeth begin to appear about the sixth month of life, and continue to be developed till about the end of the second year. The permanent teeth begin to appear about the sixth year of life, the first being the front molar of each side, while the last molars or wisdom teeth are not usually developed until adult life has been attained. Man has thus 32 teeth in his permanent set-16 in each jaw.
The incisors or front teeth are single-fanged, and have chisel-shaped crowns, suitable for bit ing. To the incisors succeeds on each side the single canine tooth, which has also a single root and a more pointed crown. This is a piercing, holding or tearing instrument, most highly de veloped in the dog (canis) and other beasts of prey. The fourth and fifth teeth, premolars or bicuspids, derive their latter name from the presence of two pointed cusps or tubercles on their crowns. The fourth tooth, or first pre molar, shows a tendency to become double fanged, while the fifth, or second premolar, is double rooted, and the crowns of both, are broad. The molars— three on each side of each jaw—have the broadest crowns of all, and possess each two or three fangs, each growing from a separate pulp and rooted in its own socket. The sixth upper molar is the largest tooth of the whole sei These massive teeth crush or grind the food, and vary greatly in the character of the surface of the crown according to the nature of the food. Thus in the insectivorous mannnals, as the shrews, the crowns are covered with numerous sharp edges and points which, working against one another, like shears, as the lower and upper molars are closed together in chewing, cut up the hard shells of Insects into little pieces fit for swal lowing and digestion. Such a type of crown is called secodont. Another type, the butiodont, is seen in the molars of omnivorous animals, such as man, monkeys, pigs, etc., where the sur face of the crown is broad, flattened, and ele vated into rounded tubercles. In the herbivores the crown is crossed by parallel ridges, which are greatly varied and complicated, up to the huge molars of the elephant family; in these cases the ridges are formed by partition-like infoldings of the enamel and the interspaces are filled with cement. When such a tooth wears away at the surface, the different density of the layers of the substances of which it is composed — enamel, cement and dentine causes them to wear unequally, the hard enamel rides projecting beyond the others and thus, as Flower says, "giving rise to a grinding sur face of great mechanical advantage.° The pat terns of these ridges are characteristic of species; and by the changes of pattern which occur as they wear down, the age of the animal may often be closely estimated, a fact con stantly utilized in the case of horses. This in folding of the enamel reached its highest com plication in the curious "labyrinthodont° teeth of the ancient stegocephalian reptiles.
The dentition of any animal is expressed by a dental formula. That of man runs thus: 2 — 2 — 2-2 3 — 3