360 PRACTICAL ANATOMY. 



the articulating surfaces, and most of all over the cusps ; 

 thinnest toward the neck, where it is overlapped by the 

 cementum. 



The cementum, or crusta petrosa, is the thin layer of 

 material covering the roots and extending from the apex to 

 neck. In structure and resemblance, it is the most anal- 

 ogous of all tooth-substance to bone, containing sparingly 

 lacunae and canaliculi, and about 30 per cent, of organic 

 matter. In single-rooted teeth the cementum is thickest 

 at the apex, in multi-rooted teeth at the bifurcation. 



The pericementum is that membrane which envel- 

 ops the root of the tooth and fills the space intervening 

 between it and the wall of the alveoli. It is analogous 

 to the periosteum of the bone, and performs the double 

 function of nourishing the cementum on the one side 

 and bone on the other. 



Tooth-development begins about the sixth week of 

 fcetal life. Along the whole length of the foetal gum 

 is a projecting ridge, termed the " dental ridge," due 

 to the excessive activity of the Malpighian layer of 

 epithelial cells, which, for want of space, necessarily 

 crowd up the corneous layer. This same activity pro- 

 duces, in like manner, the dental groove in the gum, also 

 filled with epithelial cells, and running along under the 

 ridge. This groove gradually becomes deeper, extending 

 into the deeper layer, and a hollow sack is formed, which 

 gradually closes at the top and the groove becomes 

 obliterated by the closure of the Malpighian layer. This 

 sack has the appearance of a pear hanging by a stem, 

 with the blossom end downward, and is pressed in to 

 accommodate the advancing papillae, which are the makers 

 of the future pulp and dentine. The stem becomes the 

 gubernaculum, or the future enamel organ of the accom- 

 panying permanent tooth, by a similar dipping down on 



