80 HANDBOOK OF PHYSIOLOGY. 



meanwhile, until when the tooth breaks through the gum it is covered 

 by an uncalcified layer of the keratin-like substance which is called 

 Nasmyth's membrane. At this time the other layers of the enamel cap 

 have disappeared. 



The cement or crusta petrosa is formed from the internal tissue of 

 the tooth sac, the structure and function of which are identical with 

 those of the osteogenetic layer of the periosteum, or, in other words, os- 

 sification in membrane occurs in it. 



The outer layer or portion of the membrane of the tooth sac forms, 

 the fibrous dental periosteum. 



This periosteum, when the tooth is fully formed, is not only a means 

 of attachment of the tooth to its socket, but also in conjunction with 

 the pulp a source of nourishment to it. Additional laminae of cement 

 are added to the root from time to time during the life of the tooth, as 

 especially well seen in the abnormal condition called exostosis, by the 

 process of calcification taking place in the periosteum. On the other 

 hand absorption of the root may equally occur through the same mem- 

 brane. 



In this manner the first set of teeth, or the milk-teeth, are formed; 

 and each tooth, by degrees developing, presses at length on the w;,ll of 

 the sac inclosing it, and, causing its absorption, is cut, to use a familiar 

 phrase. 



The temporary or milJc-teeth are speedily replaced by the growth of 

 the permanent teeth, which push their way up from beneath them. 



Each temporary tooth is replaced by a tooth of the permanent set 

 which is developed from a small sac set by, so to speak, from the sac of 

 the temporary tooth which precedes it, and called the cavity of reserve 

 (fig. 78, c, fp). Thus the temporary incisors and canines are succeeded 

 by the corresponding permanent ones, the temporary first molar by the 

 first bicuspid, the temporary second molar develops two offshoots, one 

 for the second bicuspid, the other for the permanent first molar. The 

 permanent second molar is budded off from the first permanent molar 

 and the wisdom from the permanent second molar. 



The development of the temporary teeth is said to commence about 

 the sixth week of intra-uterine life, after the laying down of the bony 

 structure of the jaws. Their permanent successors begin to form about 

 the sixteenth week of intra-uterine life. 



The second permanent molars are believed to originate about the 

 third month after birth, and the wisdom teeth about the third year. 



