ON THE TEETH OF MAMMALIA. Ill 



When the tooth, as in the Cachalot, is invested with cortical substance 

 only, this membrane, it may be presumed, takes the place of the enamel 

 membrane ; and, from the mass of matter it has to supply, must be of 

 great extent and thickness. By the evolution of the teeth it becomes 

 obliterated in its centre ; but its marginal parts amalgamate with the 

 gum, originally spreading over the whole. Thus, then, it is not until 

 the internal pulp and the enamel membrane have formed the crown 

 (where the crown is composed of ivory and enamel), that the cor- 

 tical membrane, or external pulp, assumes its functional office. One 

 part, however, of the process only is as yet finished ; the root has still to 

 be formed, in order that the tooth may rise. The enamel membrane 

 having disappeared, the bulb secretes the root ; and, where the cement 

 enters into the substance of the tooth, this membrane also contri- 

 butes its share. It must not be forgotten, however, that the root is 

 either real, or a mere continuation of the crown. In the first case, 

 it would seem that, having duly completed its task, the pulp loses its 

 productive energy, having become, during the process, enclosed within 

 the ivory of the crown, and reduced to a small space ; and hence the roots 

 are pierced at their points, for the transmission of the vessels and nerves 

 of this pulp, which sustain its vitality, and endow it with the highest sen- 

 sibility. In the second case, the elaborating apparatus continues active 

 during life, or, at least, till a very late period, ever carrying on a work of 

 ceaseless renovation, so as to raise up the tooth in proportion as it is 

 worn away. This process is beautifully exemplified in the incisor teeth 

 of rodents. 



The dental capsules exist at an early stage of existence ; and the 

 teeth, though as yet undeveloped, are, to a great extent, formed at the 

 period of birth. This observation chiefly applies to the deciduous, or 

 milk teeth, which, in a few years, fall out, and are succeeded by a per- 

 manent series. 



The terms, deciduous and permanent, founded upon the dental pheno- 

 mena observed in the human species, and some other animals, must not 

 be taken in a rigorous sense as applicable to the teeth of all Mammalia ; 

 for, among them, it has been proved that, on the one hand, there are 

 instances, in which the milk teeth fall before birth ; and, on the contrary, 

 that there are others, in which they are retained to a period long after 

 maturity. A statement of the progress of dentition, as presented in the 

 human species, and in one or two of such Mammalia as differ the most in 

 the routine of their dentition from that standard, will serve to give a 

 clearer idea of the matter than any other mode of explanation. 



To begin, then, with the human species. The first dentition here 

 takes place from the sixteenth to the twenty-fourth, or thirtieth, month 

 after birth, and usually commences in the lower jaw : the incisors are the 



