178 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



boundaries of the original cells of the dentinal pulp ( 213) that have not been 

 obliterated by the process of calcification.* These are particularly evident 

 in the teeth of the Dugong, and of the extinct Mylodon; but they occasionally 

 present themselves in the Dentine of Man (Fig. 81). In certain Mammals 

 and Reptiles, and in a large number of Fishes, the Dentine is traversed by 

 canals, which are prolonged into it from the central pulp-cavity, and which 

 are lined (like the pulp-cavity itself) by a highly-vascular membrane ; and it 

 is then distinguished as Vascular Dentine. These canals are obviously ana- 

 logous to the medullary or Haversian canals of bone ; and the tubuli usually 

 radiate from them, rather than from the central cavity. In some instances, 

 there is no central cavity whatever ; but the whole tooth is traversed by an 

 irregular network of these medullary canals, which become continuous with 

 the Haversian canals of the subjacent bone. A substance still more resem- 

 bling bone, but formed from the dentinal pulp, is found in the interior of the 

 teeth of certain Reptiles and Mammalia, and occasionally in the teeth of 

 Man, especially at the later periods of life. This substance possesses not 

 only vascular or medullary canals, but also the stellate lacunae and radiating 

 canaliculi of true bone. It sometimes occupies the whole of the cavity of the 

 pulp, and is formed by the ossification of its cellular parenchyma ; but in 

 other cases, it forms merely a thin shell upon the interior of the ordinary 

 Dentine. 



211. The Cementum or Crusta Petrosa corresponds in all essential parti- 

 culars with Bone ; possessing its characteristic lacuna? ; and being also tra- 

 versed by vascular medullary canals, wherever itoc 4 curs of sufficient thickness, 

 as in the exterior of the tooth of the extinct Megatherium, and in the thick 

 plates interposed within the islands of Enamel in the teeth of Ruminants, Ro- 

 dents, &c. The varieties of microscopic structure presented by the Cemen- 

 tum in different classes of animals, correspond with the modifications of the 

 osseous tissue, which exist in the skeletons of those animals respectively. 

 The Cementum was formerly supposed to be restricted to the compound 

 teeth of Herbivorous animals ; and its presence in the simple teeth of Man 

 and the Carnivora can be shown only by the application of the Microscope. 

 In the latter it forms a layer, which invests the fang, and which decreases in 

 thickness as it approaches the crown of the tooth; at the time of the first 

 emersion of the tooth, it covers the crown with a very thin lamina ; but this 

 is speedily worn away by use ; on the other hand, its thickness around the 

 apex of the fang often undergoes a subsequent increase, especially when chro- 

 nic inflammation and thickening take place in the membranous contents of the 

 socket. 



212. The following are the results of the most recent Chemical Analyses 

 of the component structures of Human Teeth : t 



Incisors of Mult Man. 



Dentine. Enamel. Cementum. 



Organic matter . . . 28-70 3-59 29-27 



Earthy matter . . 71-30 96-41 70-73 



100-00 100-00 100-00 



The proportion of these two components varies considerably in different species; thus the 

 i r-auic basis of the Elephant's tusk forms as much as 43 per cent, of the whole. It would 

 M-em even to vaiy considerably in different individuals of the same species: thus in the 

 n.nlar teeth of one man,Bibra found the organir matter to constitute as little as 21 per cent., 



* See Prof. Owen's Odontography, Introduction. 



t Op. Cit.; and Bibra's "Chemische Untersuchungen iibcr die Knochcn und Ziihne." 



