THE TEETH. 635 



Charles S. Tomes* says: " Secondary dentine occurs in the teeth of aged 

 persons, in which the pulp-cavity is much contracted in size, and also is very 

 frequently formed as a protection to the pulp, when threatened by the 

 approach of dental caries, or by the thinning of the walls of the pulp- 

 cavity through excessive wear." Tomes illustrates secondary dentine 

 filling up one of the cornua of the pulp-cavity from a human molar affected 

 by caries, which figure closely resembles the formation I have described 

 above. Tomes remarks: "It would be impossible to attempt to give any 

 description of the almost endless minor modifications of the dentine 

 structure." 



From the facts recorded in dental literature, it is evident that there are 

 several causes universally agreed upon as to formations of secondary dentine. 

 These causes are mainly : First, advanced age ; second, caries of the primary 

 dentine ; and, third, injuries on the external surface of the tooth. 



C. Heitzmann t first drew attention to the fact that in old dogs and cats 

 a number of Haversian canals in the compact bone become obliterated. This 

 investigator observed that the capillary blood-vessel, which represents the last 

 remnant of the medullary tissue within the Haversian canals, is finally trans- 

 formed into a solid mass, which immediately assumes the character of the 

 bony basis-substance. The bone-corpuscles, which under these circumstances 

 are visible in the center of a Haversian system, are as a rule larger than those 

 scattered within the lamellse of an earlier formation. 



If we consider the pulp-cavity as a medullary space containing blood- 

 vessels, nerves, and medullary elements, around which are arranged the layers 

 of dentine, enamel, and cementum, we find a coincidence of the formation of 

 bone on the one hand, and of secondary dentine on the other, in advancing 

 age. In both instances the medullary elements are transformed into basis- 

 substance ; the nerves, probably, after having been reduced to medullary 

 elements, also assisting in the formation of secondary dentine ; and, lastly, 

 the blood-vessels are solidified. On an average the pulp-cavity is the smaller 

 the older the person, until at last hardly any trace of the pulp-tissue is left,, 

 and the tooth represents an almost completely solid mass. 



Opposed to carious destruction of the crown I have repeatedly met with 

 formations of secondary dentine, as described by Salter and Wedl. This, 

 occurred, however, only in those forms of caries which have been described 

 by Frank Abbott as chronic. 



As to the irritation from without, first stated by Salter to be the cause of 

 the formation of secondary dentine, I would add chronic pericementitis, which, 

 when limited to one root or to a portion of the root, leads to the formation of 

 secondary dentine in the pulp-canal of the affected root. This fact strongly 

 supports my assertion that the tooth in its normal condition is living, and 

 irritation of the external surface may result in a new production on the corre- 

 sponding inner surface of the dentine. There are instances, however, in 

 which neither age nor an external injury accounts for the formation of 

 secondary dentine ; and such an instance is that of the tooth above described. 



The coarser anatomical relations of secondary dentine in general are 

 accurately described by C. Wedl, with whom I fully agree. In analyzing 

 the manifold formations of this kind, I would divide them as follows : 



* " Manual of Dental Anatomy," Philadelphia, 1876. 



t " Ueber Ruck- uiid Neubilduug von Blutgelassen in Knot-pel und Knochen." Wiener 

 Medicinische Jahrbiicher. 1872. 



