24 THE SURGICAL ANATOMY OF THE HORSE 



prolongation of the protoplasmic contents of the central excavation. 

 The tubules are too small to admit blood-vessels or nerves. Chemically, 

 dentine is composed of 28 per cent, organic matter, the remainder being 

 inorganic. The latter is made up principally of phosphate of calcium. 



The enamel is the shiny white layer w^hich extends over the exposed 

 portion of tooth, and w^hich is seated upon the underlying dentine. At 

 the tables of some teeth, the enamel dips into the tooth to a varying 

 depth. It is destitute of blood-vessels, and is made up of a peculiarly 

 arranged series of prisms. Its inorganic constituents make up 96.5 

 per cent, of the whole, and it is the densest structure in the body. 



The cement is simply connective tissue converted into bone and it is 



spread over parts of the exterior of the tooth. It differs from true 



bone inasmuch as its lacunas have no definite arrangement, hut seem 



to be scattered indiscriminately, and moreover, they are much fewer 



in number. 



Development of Teeth 



The first stage in the development of a tooth is the appearance in a 

 groove in one of the maxillary bones of a little closed sac, which is called 

 the dental follicle. The membranous wall of the follicle encloses a 

 papilla which is at a later stage termed the dental pulp, and from which 

 the dentine is secreted. The enamel is developed from a special layer of 

 epithelium which covers the upper aspect of the pulp, and which is 

 called the enamel organ. As the dentine is formed from the super- 

 ficial cells of the pulp it becomes deposited between the latter and 

 the follicular wall. 



From the superficial cells the tubular processes of the dentine are 

 thrown out, but the inter-tubular substance is secreted by the deeper 

 layer of cells. This latter substance contains the earthy salts. 



The cells which form the enamel are at first cylindrical in shape, but 

 they become transformed into the prisms of which the enamel is made 

 up as already stated, and the enamel thus becomes deposited as a layer 

 covering the dentine. 



