24 



ON THE ORIGIN AND DEVELOPMENT OF THE) 



of one of the central incisives of the same fcetus exhibited 

 externally nothing peculiar. After a transverse section, it was 

 found to be composed of two, the temporary and permanent 

 combined. 



The walls of the temporaiy sac (b, Fig. 23) were composed 

 of an external membrane, which was rather thick and con- 

 densed ; the inner could be separated from it, and had the 

 appearance, as in the molar sacs, of an injected villous 

 membrane. The little permanent sac was situated in the 

 substance of the outer membrane of the temporary sac, as if 

 the latter had been split to receive it. It was lined by a 

 membrane similar to that of the temporary, and exhibited 

 near the lower end of its posterior wall the incipient pulp, 

 which was evidently a development of the fold observed in 

 that situation at the fifth month. It terminated towards the 

 gum by an indistinct pointed extremity, from which a short 

 opaque impervious line proceeded, near to which the anterior 

 and posterior folds, observed at the fifth month, were seen. 



13. The lower jaw of an infant about eight or nine months 

 old, in which the central incisives had cut the gum, was pre- 

 pared by removing a section from its external posterior lateral 

 aspect, so as to expose the sacs of the posterior milk-molar, 



and of the anterior permanent 

 molar (x, Fig. 24). The latter (6), 

 instead of being buried in the base 

 of the coronoid process, was situated 

 further forward, and the cavity (ft) 

 having been displayed by a longi- 

 tudinal section of the former, was 

 found, comparatively speaking, to 

 have recovered its original small extent, being attached in- 

 feriorly to the top of the sac (6), and superiorly to the an- 

 terior edge of the base of the coronoid process. 



