ON THE TEETH OF MAMMALIA. 107 



searches have led. He observes, " that, until a very recent period, the 

 analogy of tooth to bone was supposed to extend no farther than related 

 to the chemical composition of the hardening material ; while the arrange- 

 ment of this earthy constituent, as well as its mode of deposition, during 

 the growth of the entire tooth, were considered to be wholly different from 

 that of bone, and to agree with the mode of growth of hair, and other so- 

 called extravascular parts, with which teeth in general closely correspond 

 in their vital properties." To this he adds, " that the supposed proofs of 

 the laminated structure of teeth (in favour of their distinction from bone), 

 derived from the appearances presented by the teeth of growing animals 

 fed alternately with madder, and ordinary food, and by those which 

 often occur during the decomposition of certain teeth, which are then 

 resolved into a series of concentric or superimposed laminae, were equally 

 applicable to true bone, and quite unavailable in illustrating the point 

 under consideration ; and that the appearances presented by the 

 superficies of vertical sections of teeth, viewed with the naked eye, or a 

 low magnifying power, were due, not to the intervals of separate and 

 superimposed lamellae, but to the different refractions of light caused by 

 the parallel undulations or alternations of structure of minute tubes, 

 proceeding in a contrary direction to the supposed lamellae. This 

 apparently lamellated structure, however, is not constant, nor equally 

 plain in different teeth : on the contrary, the fractured surface, or the 

 polished surface of the human, and many other teeth, presents a silky or 

 iridescent lustre, which has attracted the attention of several anatomists." 

 Hence Malpighi conceived, that the teeth were composed of minute fibres, 

 reticulately interwoven ; and Leuwenhoeck, in 1683, discovered, that these 

 apparent fibres, were, in reality, minute tubes. In 1835, this tubular 

 structure of ivory was re-discovered by Purkinje and Fraenkel, 

 who, moreover, " added to dental anatomy several new and interest-, 

 ing facts relating to the structure of the enamel ; pointing out, more 

 especially, the form and characteristic transverse striae of the component 

 crystals ; and, farther, they determined the true osseous nature of that 

 distinct layer of substance, which had been previously known to surround 

 the fang in the teeth of Man, and which they once observed to be 

 continued upon the enamel of a human incisor." This, Professor Owen 

 has discovered to be identical in its structure with the cement (crusta 

 petrosa), which enters more abundantly into the composition of the 

 compound teeth of the Herbivora. After adverting to the experiments of 

 Professor Miiller, on the nature and contents of these dental tubuli, 

 Professor Owen refers to Professor Retzius of Sweden, and states, that, 

 " besides confirming the fact that the ivory or bony constituent of 

 a human tooth consists of minute tubes, lodged in a transparent me- 

 dium, disposed in a radiated arrangement, with the lines proceeding 



