538

UNGULATES.

pheral third part of the dentine are reduced to less than half thediameter of their beginnings : the primary curve of the terminalportions of the tubes in the crown is convex towards the uppermargin; in the basal portion of the tooth it is concave towards the upper !margin and sends off, as Retzius rightly describes in the Sheep , themost numerous ramuli from the concavity and directed towards theupper surface of the tooth. The white colour of the dentinaltubes, when viewed by reflected light, is well displayed ih thefigures of Dr. Erdl :(1) though they give an idea of the solidity ofthe tubes which is not accurate: the opacity is due to the particlesof the lime salts which the tubes contain in a disgregated state;but the more important functions of the tubes are perforated bythat part of the area which is left unoccupied by such salts, andthrough which the vital and nutritious plasmatic fluid meanders. Inthe portion of dentine of a Calf’s molar, taken from the bottomof the pulp-cavity (PI. 109, fig. 3) the undulations of the tubes,the stronger flexuosities of some of the terminal branches, and ]the occasional anastomotic loops which they form are shown.

The clear compartments of the basal substance are relativelysmaller than in the dentine of carnivorous and mixed feedingMammals: those in the peripheral third part of the Deer ’s incisorare ^th of an inch in diameter, of a subcircular figure, and with anoverlapping or imbricated arrangement when viewed in a sectiontaken in the axis of the tooth : they increase in size and lose theirregularity of form nearer the pulp-cavity. In an antero-posteriorlongitudinal section of the incisor of the Ox, Sheep , and Deer , a linear !tract of the orifices of medullary canals is seen continued from thesummit of the pulp-cavity to the grinding surface : these canals extendtowards the lateral borders of the crown and form loops at a shortdistance from the enamel.

In old teeth, especially the molars, the residue of the pulp isconverted into osteo-dentine, forming the yellow-coloured nodulesmentioned by Retzius, around which the dentinal tubes irregularlywind. The peripheral terminations of the tubes are resolved intorich plexuses of minute branches interspersed, close to the enamel,with minute opake cells.

(1) Abhandlungen der Akadernie der Wissenschaften, Bd. iii. tab. ii.