PULPS AND SACS OF THE HUMAN TEETH. 41 



or great cavities of reserve, and the final effects of develop- 

 ment in the secondary dental groove.* 



In the lower jaw, as in the upper, dentition commences in 

 a deep narrow groove, situated between the lip and a semi- 

 circular lobe. This groove, instead of terminating in a simple 

 curve posteriorly, as in the upper jaw, becomes shallow, and 

 assumes a sigmoidal form upon the surface of the posterior 

 bulbous ovoidal portion of the lobe. 



About the seventh week the lip becomes very loose, and 

 separates widely from the lobe, between which and the former 

 a ridge appears, growing from behind forwards, and dividing 

 the original groove into two, an outer one the labial dupli- 

 cature of mucous membrane, and an inner the primitive 

 dental groove. This ridge, which, as in the upper, does not yet 

 extend to the incisive portion of the jaw, is flat, or in the 

 same continuous plane with the bottom of the dental groove, 

 and its lip is turned out, or overhangs the labial mucous 

 membrane. The inner lip of the groove is formed by the 

 semicircular lobe, which has become thin, and arched over 

 the groove, particularly anteriorly, where it is cut into four 

 festoons, two on each side of the median line ; and posteriorly, 

 where it still retains the appearance of an oval lobe, from 

 under which the outer lip or ridge appears to proceed. The 

 groove curves inwards between the two lips posteriorly, under 

 a form which is evidently a development of the original 

 sigmoidal groove. 



Near the posterior extremity of the groove there is an 

 elevation of a small portion of its floor, which speedily 

 becomes the germ or papilla of the inferior anterior milk- 

 molar tooth the second tooth which appears in the primitive 



* It is probable that the successive dentitions of the elephant are conducted 

 in a cavity of reserve, which must consequently exist even in the adult animal, 

 till a late period of its life. If such be the case, the molar dentition of the ele- 

 phant, and the formation of the human adult molars, are analogous processes. 



