ORAL CAVITY 



225 



sides of the dentine. By continued additions to the deeper portions 

 of the dentine the tooth is gradually forced up through the epithelium 

 so that its tip or crown comes into position for use (eruption of the 



tooth). 



Fig. 234. — Section of developing tooth germ of Amblystoma. e, epidermis; eo, enamel 

 organ; m, Malpighian layer; me, mesenchyme; p, pulp of tooth. 



In the lower vertebrates there may be a separate invagination of the ectoderm 

 for each tooth, but in the mammals there is a continuous ingrowth, the dental 

 ridge (fig. 235) along the margin of the jaw. Later this becomes differentiated 

 into separate enamel organs and dental papillae, the separate teeth developing 

 much as in other groups. From the posterior side of this dental ridge there 



Fig. 235. — Model of ectodermal parts of jaw of human embryo 40 mm. long, after ROse, 

 showing the dental ridge with the germs of the teeth. 



arises a continuous projection, the dental shelf (fig. 236) which later gives rise 

 to the enamel organs for the second or permanent dentition (infra). 



The dental papilla persists throughout life as the pulp of the tooth, 



continuing to occupy the space (pulp cavity) in which it first appeared. 



Nerves (branches of the trigeminal) and blood-vessels enter the 



cavity through the base of the tooth. Usually, when the tooth is 



15 



