538 UNGULATES. 



pheral third part of the dentine are reduced to less than half the 

 diameter of their beginnings : the primary curve of the terminal 

 portions of the tubes in the crown is convex towards the upper 

 margin; in the basal portion of the tooth it is concave towards the upper 

 margin and sends off, as Retzius rightly describes in the Sheep, the 

 most numerous ramuli from the concavity and directed towards the 

 upper surface of the tooth. The white colour of the dentinal 

 tubes, when viewed by reflected light, is well displayed in the 

 figures of Dr. Erdl:(l) though they give an idea of the solidity of 

 the tubes which is not accurate : the opacity is due to the particles 

 of the lime salts which the tubes contain in a disgregated state ; 

 but the more important functions of the tubes are perforated by 

 that part of the area which is left unoccupied by such salts, and 

 through which the vital and nutritious plasmatic fluid meanders. In 

 the portion of dentine of a Calf's molar, taken from the bottom 

 of 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 relatively 

 smaller than in the dentine of carnivorous and mixed feeding 

 Mammals : those in the peripheral third part of the Deer's incisor 

 are jith of an inch in diameter, of a subcircular figure, and with an 

 overlapping or imbricated arrangement when viewed in a section 

 taken in the axis of the tooth : they increase in size and lose their 

 regularity of form nearer the pulp-cavity. In an antero-posterior 

 longitudinal section of the incisor of the Ox, Sheep, and Deer, a linear 

 tract of the orifices of medullary canals is seen continued from the 

 summit of the pulp-cavity to the grinding surface : these canals extend 

 towards the lateral borders of the crown and form loops at a short 

 distance from the enamel. 



In old teeth, especially the molars, the residue of the pulp is 

 converted into osteo-dentine, forming the yellow-coloured nodules 

 mentioned by Retzius, around which the dentinal tubes irregularly 

 wind. The peripheral terminations of the tubes are resolved into 

 rich plexuses of minute branches interspersed, close to the enamel, 

 with minute opake cells. 



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



