The disposition of the dentine is still more complicated than that of the cement. It con sists of a slender, central, conical column, excavated by a conical pulp-cavity for a cer twin distance from the base of the tooth ; and this column sends radiating outwards, from its circumference, a series of vertical plates, which divide into two once or twice before they terminate at the periphery of the tooth.
Each of these diverging and dichotomising plates gives off throughout its course smaller processes, which stand at right angles, or nearly so, to the main plate ; they are gene rally opposite, but sometimes alternate; many of the secondary plates or processes, which are given off near the centre of the tooth, also divide into two before they terminate; and their contour is seen, in the transverse section, to partake of all the undulations of the folds of cement which invest and divide the den tinal plates and processes from each other.
The dental pulp-cavity is reduced to a mere line about the upper third of the tooth, but throughout its whole extent fissures radiate from it, corresponding in number with the radiating plates of dentine. Each fissure is continued along the middle of each plate, dividing where this divides, and extending along the middle of each bifurcation and pro cess to within a short distance of the line of cement. The pulp-fissure commonly dilates into a canal at the origin of the lateral pro cesses of the radiating plates, before it divides to accompany and penetrate those processes.
The main fissures or radiations of the pulp cavity extend to within a line or half a line of the periphery of the tooth, and suddenly dilate at their terminations into spaces, which, in transverse section, are subcircular, oval, or rally smaller spaces. All these spaces, or canals, in the living tooth, must have been occupied by corresponding processes of the vascular pulp : they constitute so many cen tres of radiation of the fine calcigerous tubes, which, with their uniting clear substance, constitute the dentine.f An analogous complexity is produced by numerous fissures radiating from a central mass of vaso-dentine, which more or less fills up the pulp-cavity of the seemingly simple conical teeth of the extinct family of fishes which I have called " Dcndrodonts."t Fig. 553. is one of these fossil teeth, of the natu
ral size ; a a transverse section; andfig. 554 a reduced view of a portion of the same section, enlarged twenty diameters.
Thus magnified, a central pulp-cavity, of pyriform, p: the branches of the radiating lines, which are continued into the lateral secondary plates or processes of the dentinal lamella;, likewise dilate into similar, and gene relatively small size, and of an irregular lated form, is discerned, a portion of which is shown at p; this is immediately surrounded by the transverse sections of large cylindrical medullary, or pulp-canals of different sizes; and, beyond these, there are smaller and more numerous medullary canals, which are pro cesses of the central pulp-cavity. In the transverse section these processes are seen to be connected together by a net-work of smaller medullary canals belonging to a coarse. osseous texture into which the pulp has been converted, and this structure occupies the middle half of the section. All the medullary canals were filled by the opaque matrix. From the circumference of the central net work, straight medullary fissures radiate at pretty regular intervals to the periphery of the tooth : most of these canals divide once, rarely twice, in their course ; the division taking place sometimes at their origin, in others at different distances from their termi nations, and the branches diverge slightly as they proceed. Each of the above medullary fissures is continued from a short process of the central structure, which is connected by a concave line with the adjoining process, so that the whole periphery of the transverse section of the central coarse reticulo-medul lary body of the tooth presents a crenate out• line. From each ray and its primary dicho tomous divisions, short branches are sent off at brief intervals, generally at right angles with the trunk, or slightly inclined towards the periphery of the tooth. These subdivide into a few short ramifications, like the branches of a shrub, and terminate in irregular and somewhat angular dilatations, simulating leaves, but which resolve themselves into radiating fasciculi of calcigerous tubes. There are from fifteen to twenty-five or thirty.six of these short and small lateral branches on each side of the medullary rays.