16 PHYSIOLOGY AT THE FARM. 



ing branches, the interstices between these branches being 

 subsequently filled up with enamel and cement, while the 

 stem consisting of dentine, covered externally with cement, 

 represents the fang. The minute study of the original forma- 

 tion of the teeth is of great physiological interest, but hardly 

 falls within the scope of this treatise. 



The pulp, with its encircling membrane, consists originally 

 of cells freely interspersed with capillary blood-vessels ; and 

 while these cells are developed into the matured forms of the 

 pulp and the capsule, the hard substances become deposited 

 upon the soft from the adjacent blood-vessels. The hard 

 matter constituting the dentine appears first on the outside of 

 the pulp, and successive shells from within that first produced, 

 so that, finally, a minute cavity remains in the interior of the 

 pulp, to which blood-vessels and nerves have access by a small 

 aperture at the extremity of the fang. This minute cavity is 

 hereafter the only seat of vital activity and sensibility in the 

 system of the teeth. 



All the substances composing the teeth consist of earthy 

 matter disposed in a fixed arrangement on an organic matrix. 

 Even in the enamel, the hardest of substances in the living 

 animal, the earthy matter is mainly contained in canals pro- 

 duced from the original cells, which, in animals like the horse, 

 being comparatively large with extremely thin walls, are com- 

 pletely filled therewith. 



The chemical bodies found in the teeth are not unknown 

 elsewhere in the living system. They are phosphate of lime, 

 with a trace of fluate of lime, carbonate of lime, phosphate of 

 magnesia, some other saline matter, chondrine, and fat. 



After these several bodies have once been 'deposited in the 

 teeth, there is no reason to suppose that any part of them is 

 subsequently removed therefrom, so as to require renewal the 

 teeth, from their extravascular character, being in this respect 



