DENTAL SYSTEM 17 



the second set are developed in a precisely similar manner to the 

 first, bnt the enamel-germ, instead of being derived directly from an 

 independent part of the oral epithelium, is formed from a budding 

 out of the neck of the germ of the tooth succeeded. In the case of 

 the true molars, which have no predecessors, the germ of the first 

 has an independent origin, but that of the others is derived from the 

 neck of the germ of the tooth preceding it in the series. The 

 foundations of the permanent teeth are thus laid as it were almost 

 simultaneously with those of their predecessors, although they 

 remain in many cases for years before they are developed into 

 functional activity. 



Although the commencement of their formation takes place 

 at an early period of embryonic life, teeth are in nearly all mam- 

 mals still concealed beneath the gum at the time of birth. The 

 period of eruption, or " cutting " of the teeth as it is called, that is, 

 their piercing through and rising above the surface of the mucous 

 membrane, varies much in different species. In some, as Seals, the 

 whole series of teeth appears almost simultaneously; but more often 

 there are considerable intervals between the appearance of the 

 individual teeth, the front ones usually coming into place first, and 

 those at the back of the mouth at a later period. 



Farms <>f Teeth. — The simplest form of tooth may be exemplified 

 on a large scale by the tusk of the Elephant (Fig. 1, I.) It is a 

 hard mass almost entirely composed of dentine, of a conical shape 

 at first, but during growth becoming more and more cylindrical or 

 uniform in width. The enamel -covering, present on the apex in 

 its earliest condition, soon disappears, but a thin layer of cement 

 covers the circumference of the tooth throughout life. In section 

 it will be seen that the basal portion is hollow, and contains a large 

 conical pulp, as broad at the base as the tooth itself, and deeply 

 imbedded in the bottom of a recess, or socket, in the maxillary 

 bone. This pulp continues to grow during the lifetime of the 

 animal, and at the same time is converted at its surface into dentine. 

 The tooth therefore continually elongates, but the use to which the 

 animal subjects it in its natural state causes the apex to wear away, 

 at a rate generally proportionate to the growth at the base, other- 

 wise it would become of inconvenient length and weight. Such 

 teeth of indefinite growth are said to be "rootless," or to have 

 "persistent pulps." 



One of the corresponding front teeth of man (Fig. 2, II. and III.) 

 may be taken as an example of a very different condition. After its 

 crown is fully formed by calcification of the germ, the pulp, though 

 continuing to elongate, begins to contract in diameter ; a neck or 

 slight constriction is formed ; and the remainder of the pulp is con- 

 verted into the root (often, but incorrectly, called "fang"), a taper- 

 ing conical process imbedded in the alveolar cavity of the bone, and 



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