TRANSACTIONS OF THE SECTIONS. 137 



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 the composi- 

 tion of teeth, and presenting microscopic characters equally distinct 

 both from ivory, enamel, and cement, and from true bone, and as easily 

 recognisable. 



One of these substances was characterized by being traversed through- 

 out by numerous coarse canals, filled with a highly vascular medulla or 

 pulp, sometimes anastomosing reticularly, — sometimes diverging, and 

 frequently branching, — sometimes disposed nearly parallel with one an- 

 other, and presenting more or fewer dichotomous divisions. The canals 

 in many cases are surrounded by concentric lamellae, and thus resemble 

 very closely the Haversian 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 caementum, in the absence of the 

 Purkingian corpuscles or cells. This structure is exemplified in the 

 teeth of many fishes and of some of the 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 differs, however, in the greater number and 

 close parallel arrangement of the medullary canals. This structure is 

 exhibited in the teeth of the Megatherium, 3Iylodon, and other extinct 

 Edentata. 



Mr. Owen then proceeded to describe the modifications of the above- 

 mentioned dental substances in the teeth of different classes of the ver- 

 tebrate animals, of Avhich the following examples are selected. 



1st. Teeth of Fishes. — With respect to this class, although the low- 

 est of the vertebrate series, their teeth present in general the most 

 highly organized condition, approximating most closely to the vascular 

 character of true bone, and being in many species fixed by anchylosis 

 or continuity of substance Avith the bones supporting them. 



It was in the teeth of fishes that, in recent times, the tubular struc- 

 ture had been first recognised. Cuvier*, e. g. describes them as jsre- 

 senting three different structures, of which one kind (Jes composees) 

 are formed of an infinity of tubes, all united and terminated by a com- 

 mon covering of enamel ; of this kind he instances the tesselated teeth 

 (^clents en forme de pave), as those of Rays. 



Dr. Born also describes what he terms the " fibrous teeth of fishes," 

 as being composed of hollow fibresf, and he compares these hollow 

 filires, or tubes, to those Avhich enter into the composition of the teeth 

 of the Orycteropus, &'C. 



The tubes here spoken of, as well as those mentioned by Cuvier, are 

 sufficiently large to be distinguished by the naked eye ; they do not, 

 however, form the constituent texture of the teeth instanced, but only 

 the coarser part of that texture. They contain a vascular medulla, 



* Legons d'Anat. Comp. 2d ed. torn. iii. p. 209. 



t Heusinger's Zeitsclirift, B.i. p. 184. " Die Faserzilhne besteheii in ihrem Inuern 

 aus hohlen rasern," &c. 



