PULPS AND SACS OF THE HUMAN TEETH. 41 



or gi-eat cavities of resei-ve, and the final effects of develop- 

 ment in the secuiulary dental groove* 



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

 a deep narrow f,Toove, situated between the lip and a semi- 

 circidar lobe. This groove, instead of temiinating 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 veiy 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 primilive 

 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 l)y 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 cun^es inwards between the two lips posteriorly, imder 

 a form wliich is evidently a development of the original 

 sigmoidal gi-oove. 



Near the posterior extremity of the groove there is an 

 elevation of a small portion of its lloor, which speedily 

 becomes the germ or papilla of the inferior anterior milk- 

 molar tooth — the second tooth which appeare in the primitive 



* It is prolxiblt' that the successive dentitions of tlie elephant are coniiuct«Hl 

 in a cavity of reserve, wliich must consetiuently exist even in the adult animal, 

 till a late {w^ricMl of its lifi-. If sm li Iw the case, the molar dentition of the ele- 

 phant, iiiitl the forinutinn of the human adult molars, an> analogous processes. 



