66 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



grow continuously in Eodents. If from any accident to their 

 teeth the normal wear does not take place, the incisors grow 

 into long tusks which may ultimately prevent mastication. 



(3) The Pulp. — The pulp is the remnant of the dental papilla 

 enclosed by the dentine. It is made up of a matrix of branching 

 cells and is said to have no lymphatics. Thus, like the tissue 

 of the umbilical cord and vitreous humour of the eye, it retains 

 the embryonic form of the mesoblast (Berry Hart). It contains 

 the ramifications of the artery, vein and nerve of the tooth. 



(4) The Dental Sac. — The foetal tooth, as may be seen from 

 Fig. 49, lies imbedded in the alveolus within the dental sac. 

 When the enamel bud is inyaginated on the dental papilla, the 

 invaginated layer forms the enamel, while the invaginating or 

 parietal layer becomes surrounded by a dense layer of mesoblast 

 and forms the dental sac. Between the enamel (invaginated) and 

 parietal (invaginating) layers, filling the cavity of the sac, lies a 

 mass of epithelium corresponding to the corneous epithelium of 

 the skin. 



(5) The Peridental Membrane. — The peridental membrane 

 (Kg. 47) is formed by that part of the dental sac which sur- 

 rounds the fang of the tooth. The part of the dental sac 

 which surrounds the crown is destroyed by the eruption of 

 the tooth (Fig. 49). 



(6) The Crusta Petrosa. — The peridental membrane is of the 

 nature of periosteum, and contains osteoblasts which deposit the 

 crusta petrosa (bone) on that part of the dentine which forms the 

 fang and also on the inner wall of the alveolus. It may 

 inflame and give rise to an abscess or keep on discharging pus ; 

 the tooth then becomes loosened in its socket and drops out. 



Origin of the Permanent Teeth. — From the dental shelf, 

 besides the buds for the milk teeth, there grow inwards, so as to 

 lie on the lingual aspect of the milk buds, processes of epiblast 

 which form later the enamel of the ten teeth which replace the 

 milk teeth (Figs. 48 and 49). The three permanent molars of 

 each side arise from a process which prolongs the dental shelf 

 backwards behind the part from which the enamel buds of the milk 

 teeth arise. The first molar is the earliest of all the permanent 

 teeth to undergo development. The permanent teeth are formed 

 in exactly the same manner as the milk set. They develop on 



