644 UNGULATES. 



as it approaches the osteo-dentine, hecomes modified by the presence 

 of vascular canals, and assumes the character of vaso-dentine ; and 

 the canals frequently form loops directed towards the osteo-dentine. 

 In this substance the medullary canals diverge, leaving clear inter- 

 spaces into which numerous irregular branches proceed from the 

 canals, partially uniting to form a net-work, and finally breaking 

 up into distinct cells, of larger size than the purkingian cells of the 

 external cement. In the small detached fusiform nodules of osteo- 

 dentine, one or more vascular canals are present with concentric 

 coats of the clear basal substance, studded by purkingian cells of 

 the size of those in the cement, but with richer radiating systems 

 of tortuous calcigerous tubes. Some of the larger-sized cells are 

 likewise frequently here present. From both the medullary canals, 

 their sub-divisions and the cells, minute calcigerous tubes diverge, 



numerous interesting examples given in his Memoir in the ' Edinburgh Philosophical Transac- 

 tions for 1841, where he states " One circumstance was at once detected in all these specimens, 

 and its importance was evident, as affording a clue to the explanation of the mode of inclosure. 

 The circumstance to which I allude is, that in none of the specimens are the bullets of foreign 

 bodies surrounded by regular ivory. They are in every instance inclosed in masses, more or 

 less bulky, of a substance which, although abnormal in the tusk of the Elephant is nevertheless 

 well known to the Comparative Anatomist, as occupying the interior of the teeth of some of the 

 other Mammals, and usually considered to be the ossified pulp." p. 93. Mr. Goodsir cites 

 (p. 97) a paper read by his countryman Dr. Knox to the Royal Society of Edinburgh, five years 

 before the date of his own Memoir (January 1841) in which the osteo-dentine (tissue like pud- 

 dingstone of Cuvier) which fills the pulp -cavities of the tusks of the Walrus and the teeth of the 

 Cetacea, was "first announced as a distinct species of dental tissue." I have not found any 

 distinctive characters, microscopic or otherwise, of such tissue, pointed out in the publications 

 of the Society referred to by Mr. Goodsir. The first clear definition of substances entering into 

 the composition of teeth and presenting microscopic characters equally distinct from ivory, 

 enamel and cement, and from true bone, is that which I communicated to the British Associa- 

 tion in 1838. (See the Volume of Reports, 1838, p. 137). Of the two kinds there defined, 

 " The first, or ' vaso-dentine', characterized by being traversed throughout by numerous coarse 

 canals, filled with a highly vascular medulla or pulp," but ' differing from true osseous sub- 

 stance, and from the coementum in the absence of the purkingian corpuscles,' (loc. cit. p. 137), 

 is not abnormal in the Elephant's tusk, but is a constant constituent of its central part ; this 

 fact I demonstrated by microscopic sections exhibited in my Hunterian Lectures on the 

 Comparative Anatomy of the Digestive System, in the Theatre of the College of Surgeons in 

 1838, and at the same time showed the identity of the irregular ivory of Cuvier which surrounds 

 the foreign bodies, with "the second distinct component substance of tooth, which more 

 closely resembles true bone and cement, inasmuch as the purkingian cells are abundantly 

 scattered through it," loc. cit. 



