10 STRUCTURE. 



In the brown upper pharyngeal tooth of the carp, Stromeyer(l) 

 detected a small proportion of magnesia. 



8. Structure. — The tubular structure common to the dentine of 

 all classes of animals, though not first discovered in the teeth of 

 fishes, has been most frequently recognised, because most conspicuous, 

 in them : and, as in several fishes, the coarser features of this 

 structure are obvious to the naked eye, it was admitted as applicable 

 to a greater or less proportion of that class by some comparative 

 anatomists, before the researches of Purkinje and Retzius had established 

 its existence in the teeth of the higher vertebrate animals. Leeuwen- 

 hoek, indeed, in his account of the minutely tubular structure of the 

 teeth of man and of the ox, (2) attributes the same structure to the 

 tooth of the haddock, in which he states that the dental tubes are smaller 

 than in the ox. Mr. Andre (3) detected the ramified canals which 

 pervade the substance of the tooth of the Acanthurus. Cuvier(4) first 

 described the coarser tubes composing the teeth of the rays and of 

 the wolf-fish; and Von Born (5) ascribes to the teeth of a greater 

 number and variety of fishes the same structure, which was regarded 

 by both these anatomists as analogous to the tubular structure of the 

 teeth of the ornithorhynchus and orycteropus, and also compared with 

 that of whalebone and of the horn of the rhinoceros. Such comparisons, 

 however, are wanting in accuracy when applied in that loose and 

 general manner. 



In the following pages there will be shown to be, at least, 

 four principal modifications of the tubular structure in the teeth 

 of fishes. Premising that the essential character of this structure 

 is a cavitas pulpi, or medullary canal, from which the calcigerous 

 tubes radiate, the first modification which may be noticed is 

 where the tooth is traversed by a number of equidistant and 

 parallel medullary canals, each canal and its system of medul- 

 lary tubes representing a cylindrical or prismatic denticle, and being 

 separated from the contiguous denticles by a thin coat of bone or 

 cement. This modification is exemplified in the rostral teeth of 



(1) Gilbert's Annalen, Bd. vii, 1811. 



(2) Philos. Trans., 1678, p. 1003. 



(3) lb. vol. 74, Description of the teeth of Chaetodon (Acanthurus) nigricans, 



(4) Le9ons d'Anat. Comparee, tome iii, p. 113 (1805). 

 (o) Heusinger's Zeitschrift, Bd. i, 1827» 



