TOOTH-DEVELOPMENT. 137 



both medullated and nonmedullated fibres, the latter being sympathetic 

 fibres destined for the walls of the blood-vessels. On reaching the crown- 

 pulp, the larger twigs are replaced, by finer branches that subdivide into 

 many fibres. These, on reaching the periphery of the pulp, form a plexus 

 beneath the layer of odontoblasts from which nonmedullated axis-cylinders 

 are given off. Some of these end beneath the odontoblasts in minute nodular 

 swellings; others penetrate between the odontoblasts and terminate in pointed 

 or bulbous free endings. Delicate fibres have been traced from the super- 

 ficial pulp-nerves seemingly into the dentinal tubules. 



DEVELOPMENT OF THE TEETH. 



About the beginning of the seventh week of foetal life, the ectodermic 

 epithelium thickens along the margins of the oral opening and forms a ridge 

 of proliferated epithelium, the labio-dental strand. This grows into the 

 surrounding mesoderm and divides into two plates which, while continuous 

 at the surface, diverge almost at right-angles at the deeper plane. The 

 lateral or outer plate is vertical and corresponds to the plane of separation, 

 effected by the labial furrow, that later occurs in differentiating the lips 

 from the tissues forming the jaw. The median or inner plate grows more 

 horizontally into the mesoderm and is the one directly concerned in the 

 tooth-development;, for this reason it is termed the dental ledge. 



The anlages or embryonic rudiments of the milk-teeth are indicated 

 by club-shaped epithelial outgrowths, which grow from the deeper surface 

 of the dental ledge to form the enamel- organs, as well as to meet, and 

 later cap, the mesodermic elevations, the dental papilla. The bulbous 

 end of the epithelial outgrowth increases rapidly and differentiates into 

 the typical three-layered enamel-organ. The latter is attached for a 

 time to the dental ledge by a broad strand of cells, which becomes more 

 and more reduced until, finally, it is broken and the enamel-organ is 

 isolated from the oral epithelium. 



The Dental Papilla. This structure appears, shortly after the 

 club-shaped enamel-organ begins to expand (Fig. 177, C), as a condensa- 

 tion of the mesoderm beneath the epithelial outgrowth. At first the 

 papilla consists of a close aggregation of small round proliferating cells, 

 but later, coincidently with the differentiation of the three layers of the 

 enamel-organ, the peripheral cells of the dental papilla become elongated 

 and arranged as a continuous row of cylindrical cells, which cover the 

 apical portion of the papilla and lie beneath the enamel-organ. These 

 cylindrical mesodermic cells are the odontoblasts, the active agents in 

 the production of the dentine. Where engaged in this process, particularly 

 over the summit of the papilla, the cells measure from 35-50 /* in length 

 and from 5-10 /* in breadth, but over the sides of the papilla they gradually 

 become lower and less characteristic, until, at the base, they resemble the 

 general cells of the papilla. So long as the tooth grows, odontoblasts are 

 differentiated in the vicinity of the last-formed part of the root; after such 

 differentiation and the odontoblasts engage in producing dentine, mitosis 

 is no longer observed in these cells. 



The formation of the dentine is accomplished through the agency 

 of the odontoblasts in the same manner as the osteoblasts produce the 

 matrix of bone. The earliest dentine appears as a thin homogeneous 

 stratum, the prodentine, which overlies the tip of the papilla and underlies 

 the enamel-organ. This layer is resolvable into collagenous fibrillae similar 



