272 MANUAL OF HISTOLOGY. 



seen here, the latter reminding us of Sharpey's fibres ( 142) in the 

 way in which they ossify. According to this description, both parts 

 have a similar or identical nature to that of osseous tissue. Dentine 

 represents a modified bony substance, and the cement is deposited upon 

 it in the same manner as a younger periosteal layer upon an older, while 

 the communications between the canaliculi of the tooth and those of the 

 bone occur in a way analogous to that taking place in concentric growth 

 of bone. 



Just as the cement is formed around the root, so is the enamel laid 

 down upon the crown as a coating closely adherent to the subjacent 

 mass. The elongated tooth then presses gradually against the enamel 

 organ and roof of the dental sac until these disappear with the super- 

 imposed gum. Thus the eruption of the twenty milk teeth comes to pass, 

 which begins in the sixth or seventh month of an infant's life, terminating 

 at about the commencement of the second, or sometimes in the middle 

 of the third year. The residue of the dental follicle persists as the 

 periosteum of the alveolus. Around the milk teeth, which have been 

 already protruded, it forms a system of obliquely ascending fibres passing 

 from the edge of the alveolus to the neck of the tooth (ligamentum 

 circulare dentis of Koelliker). 



The external epithelium of the enamel organ may, perhaps, persist also 

 in the form of the " enamel cuticle." 



The subsequent falling out of the milk teeth is preceded by re-absorp- 

 tion of their roots. 



The successive eruption of the thirty-two permanent teeth commences 

 in the seventh year, lasting until the end of the second decade, when the 

 wisdom teeth make their appearance. 



The cause of the falling out of the teeth at an advanced age has not 

 yet been sufficiently cleared up. It is probable, however, that a narrow- 

 ing of the canaliculi, and degeneration of Tomes' fibres, prepares the way 

 for the decay of the organs. 



The origin of dental caries requires also further investigation. In it 

 we remark in succession, softening and destruction of the enamel mem- 

 brane and enamel, of the dentine in its ground -substance, and of the 

 dental sheaths and fibres. In this process vibriones and filiform fungi 

 make their appearance. 



The so-called tartar of the teeth consists of albuminates and allied 

 matters from the fluid of the mouth, together with a large proportion of 

 phosphates. The former amount, according to Berzelius, to 21, the latter 

 to 79 per cent. 



Hypertrophies of various external portions of the teeth are of fre- 

 quent occurrence : they generally affect the cement or dentine, or both 

 together. 



There likewise occurs frequently enough a new formation of dentine on 

 the internal surface of the tooth and an ossification of the pulp. To 

 compensate for the wear and tear of the crown also, produced by chewing, 

 and also of loss of substance on the external surface through disease, 

 there are new layers of dentine laid down by the pulp on the interior of 

 the central cavity. 



Teeth which have been drawn may again become attached, and healed 

 in their alveoli on being replaced. 



The formation of teeth in strange localities occurs also as a rarity, 

 especially in the ovary, but occasionally in other situations. 



