62 ON THE MODE IN WHICH MUSKET-BULLETS BECOME 



ivory in the interior ceases. The peripheral surface of the 

 irregular ivory is now, in reference to the general pulp, in the 

 same relation as the whole internal surface of the irregular 

 ivory of the tusk. The pulp, therefore, becomes converted 

 into ivory, not only on the whole internal surface of the tusk, 

 but also on the surface of the newly-formed mass. The cause 

 of the formation of the irregular ivory to a limited extent only, 

 when it exists as an abnormal structure, I have not been able 

 to ascertain ; but its mode of development and limitation is 

 highly interesting, and forms a leading distinction between a 

 tooth and a true bone under similar circumstances. 



From this description it is evident that the abnormal ivory 

 in the elephant's tusk strongly resembles, if it be not identical 

 with, the peculiar substance which fills the pulp-cavities of 

 the tusks of the walrus and the teeth of the cetacea, first an- 

 nounced as a distinct species of dental tissue in a paper read 

 before this society five years ago by Dr. Knox, and since 

 minutely described by Ketzius, Owen, and Alexander 

 Nasmyth. * 



This identity of a diseased structure in one animal with a 

 normal structure in another is remarkable, and must be looked 



* Cuvier described this species of dental tissue in the tusk of the walrus, 

 and compared it to pudding-stone. Dr. Kiiox, in the paper to which I have 

 referred in the text, affirmed that, in addition to the cement, enamel, and 

 ivory, a fourth substance namely, the substance described by Cuvier entered 

 into the formation of many teeth. He stated that, in the teeth of certain 

 fishes, this substance, or a tissue closely allied to it, constituted the greater 

 part of their mass ; the other three elements having disappeared or become 

 greatly diminished in bulk or importance. Ketzius has accurately described 

 the microscopic structure of this class of dental substances as existing in dif- 

 ferent animals. Mr. Owen has extended and confirmed the observations of 

 Eetzius. Lastly, to Mr. A. Nasmyth belongs the merit of having pointed out 

 the resemblance which this kind of substance (which he denominates ossified 

 pulp) bears to diseased ivory in the tusks of the elephant, and still more closely 

 to the substance which fills the pulp-cavity of the aged human tooth. In 

 ignorance of Dr. Knox's previous observations, he announced this kind of ivory 

 as a fourth dental substance. 



