Ixxii INTRODUCTION. 



structural resemblance to bone and is almost identical in chemical 

 composition : its modifications, which I have called ' vaso- 

 dentine' and * osteo-dentine,'(l) forming intermediate gradations 

 between the hard dentine and true bone. True enamel is a tissue 

 yer se ; but in the teeth of Fishes there are several intermediate 

 stages of gradation which link enamel to dentine, as the dentine 

 itself, in most Fishes, passes gradually into bone. 



Heusinger admits that the relation of the teeth to the corneous 

 tissue (Horngewebes) is not clearly elucidated in Human Anatomy, 

 but he affirms that it is most conclusively established in that 

 of the lower classes of animals. (2) No doubt in tracing the mo- 

 difications of the dental system through the Animal Kingdom, 

 we find true horny productions substituted for teeth in certain 

 Vertebrate Species, as the Ornithorhynchus, Whale, Tortoise, &c. 



(1) These tissues are respectively defined, as follows, in the * Report of the British 

 Association, 1838.' "With respect to the component structures of a tooth, Professor Owen 



' commenced by observing, that in addition to those usually described and admitted, there were 

 other substances entering into composition of teeth, and presenting microscopic characters 

 equally distinct both from ivory, enamel, and cement, and from true bone, and as easily recog- 

 nisable. One of these substances was characterized by being traversed throughout by 

 numerous coarse canals, fiUed with a highly vascular medulla or pulp, sometimes anastomosing 

 reticularly, — sometimes diverging, and frequently branching, — sometimes disposed nearly 

 parallel with one another, and presenting more or fewer dichotomous divisions. The canals in 

 many cases are surrounded by concentric lamellae, and thus resemble very closely the Haver- 

 sian canals of true bone ; but the calcigerous tubes which everywhere radiate from them are 

 relatively much larger. The highly-organized tooth-substance just described differs from true 

 osseous substance, and from the csementum in the absence of the Purkingian corpuscles or cells. 

 This structure is exemplified in the teeth of many fishes and some Edentate Mammalia. 

 Another component substance of tooth more closely resembles true bone and cement, inasmuch 

 as the Purkingian cells are abundantly scattered through it ; it difiers, however, in the greater 

 number and close parallel arrangement of the medullary canals. This structure is exhibited in 

 the teeth of the Megatherium, Mylodon, and other extinct Edentata," p. 137. 



(2) System der Histologie, 4to. 1823, p. 160. 



