-a. 



THE DIGESTIVE TRACT. 



pany the larger blood-vessels as medullated fibres ; these give off 

 filaments which pass to the layers of odontoblasts, among which 

 they extend as pale fibres. The ulti- 

 mate distribution of these latter is still 

 unsettled ; the assertion that fine fibrillae 

 accompany the dentinal fibres into the 

 tubules lacks confirmation. 



Distinct lymphatic vessels have 

 not been demonstrated within the pulp, 

 although the clefts within the matrix 

 between the connective-tissue fibres 

 represent the lymph-spaces and are in 

 close relation with the adjacent lym- 

 phatic channels. 



DEVELOPMENT OF THE TEETH. 



The teeth of man and the higher 

 animals are really exaggerated papillae, 

 the peripheral parts of which have 

 become specialized and have under- 

 gone calcification. The ectoderm con- 

 tributes the enamel, while the dentine, 

 cementum, and pulp are derived from 

 the mesoderm. 



A linear thickening of the primitive 

 oral epithelium marks the earliest indication of the formation of the 

 teeth ; in man this band appears before the end of the sixth week 

 (Rose), and is adherent to the under surface 

 of the epithelial layer.// Following the ex- 

 pansion of this ectodermic thickening a con- 

 tinuous lateral projection, the dental ridge, 

 grows obliquely into the mesodermic tissue. 

 The dental ridge continues to grow back- 

 ward towards the mandibular articulation, 

 forming an unbroken arch of ectodermic 

 tissue connected with the under side of the 

 oral epithelium. The line of this attach- 

 ment is later marked on the oral surface by 

 a longitudinal furrow, the dental groove, 

 which has been long known, and which was formerly regarded as the 

 initial step in the dental development. 



While the dental ridge constitutes a shelf-like common epithelial 

 invagination, the position and further development of the individual 

 teeth are marked by local thickenings along the under surface of 



Section of young tooth of child, show- 

 ing peripheral portion of pulp and ad- 

 joining dentine : b, pulp-cells, some of 

 which send processes (a) within dentinal 

 tubules ; c, stroma of delicate connective 

 tissue ; d, blood-vessels. 



FIG. 179. 



Section of jaw of rabbit embryo, 

 showing thickening of ectodermic 

 epithelium (ec) from which dental 

 ridge (t) begins its growth into 

 mesoderm (m). 



