CESTRACIONTS. 55 



of the tooth consists of groups of short, branched and frequently 

 irregularly wavy medullary canals imbedded in a clear matrix. 

 These canals are surrounded by concentric layers, and closely 

 resemble the Haversian canals in true bone. The calcigerous 

 tubes, which radiate from the medullary canals, have a grace- 

 ful undulatory course, and are much branched ; but towards the 

 periphery of the tooth, the ramified tubes are all directed, as in or- 

 dinary dentine, at right angles to the superficies, and thus constitute a 

 regular layer of hollow columns disposed so as to offer the greatest 

 resistance to external pressure. This layer is equal in thickness to 

 about one-fifteenth part of the vertical diameter of the thickest part 

 of the tooth. 



The finest or terminal branches of this peripheral layer of 

 tubes, I have traced in various places into what, at first sight, appears 

 to be the enamel. Under a magnifying power of 400 diameters, how- 

 ever, this outermost layer is seen to be composed of extremely minute 

 tubes, -njWth of a line in diameter (PL 16, fig. 2,) ; they are branched 

 like the coarser tubes of the body of the tooth ; irregularly wav^y in 

 their course, having a general tendency to an arrangement at right 

 angles to the superficies, but inextricably interwoven, and connected 

 anastomotically together, so as to require a strong light to penetrate 

 even the thinnest section, and render their structure and arrangement 

 visible. The continuation of these finer superficial tubes with the 

 coarser tubes of the body of the tooth, is best observed by slightly 

 changing the focus of the glass with which they are viewed, which 

 brings the transitional tubes at different depths in the section into 

 view. In some parts of the section, a medullary or Haversian canal 

 is displayed longitudinally ; and the parallel lines of the surrounding 

 concentric strata on each side are exhibited. The canal maintains a 

 general uniform diameter, but slightly dilates where it divides or 

 sends off" a cross branch to communicate with the adjoining canals. 

 These canals commence from the large cells of the bony base, and 

 pass into the substance of the tooth towards its periphery ; they 

 communicate by transverse canals, but all ultimately terminate in 

 bundles of the wavy ramified calcigerous tubes of the body of the 



