THE STOMOBJ'WM. 



639 



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Ai 



From the above epithelial structure, which may be called the cuarael 

 organ, and from the papilla it covers, which may be spoken of as the 

 dental papilla, the wliole tooth is developed. After these parts have 

 become established there is formed round the rudiment of each tooth a 

 special connective tissue capsule ; known as the dental capsule. 



Before the dental capsule lias become definitely formed the enamel 

 organ and the dental papilla undergo important changes. The rounded 

 epithelial cells forming the core of the 

 enamel organ undergo a peculiar transfor- 

 mation into a tissue closely resembling or- 

 dinary embryonic connective tissue, while 

 at the same time the epithelium adjoining 

 the dental papilla and covering the inner 

 surface of tiie enamel organ, acquires a some- 

 what different structure to the epithelium 

 on the outer side of the organ. Its cells 

 become veiy markedly columnar, and form 

 a very regular cylindrical epithelium. This 

 layer alone is concerned in forming the 

 enamel. The cells of the outer epithelial 

 layer of the enamel organ become somewhat 

 flattened, and the sui-face of the layer is 

 raised into a series of short papillje which 

 project into the highly vascular tissue of 

 the dental sheath. Between the epithelium 

 of the enamel organ and the adjoining con- 

 nective tissue there is everywhere present 

 a delicate membrane known as the mem- 

 bran a prseformativa. 



The dental papilla is formed of a highly 

 vascular core and a non-vascular superficial 

 layer adjoining the inner epithelium of the 

 enamel organ. The cells of the superficial layer are arranged so as almost 

 to resemble an epithelium. 



The first formation of the hard structures of the tooth commences at 

 the apex of the dental papilla. A calcification of the outermost layer of 

 the papilla sets in, and results in the formation of a thin layer of dentine. 

 Nearly sin)ultaneous1y a thin layer of enamel is deposited over this, 

 from the inner epithelial layer of the enamel organ (fig. 428). Both 

 enamel and dentine continue to be deposited till the crown of the tooth 

 has reached its final form, and in the course of this process the enamel 

 organ is reduced to a thin la\er, and the whole of the outer layer of the 

 dental papilla is transformed into dentine — while the inner portion remains 

 as the pulp. 



The root of the tooth is formed later than the crown, but the enamel 

 organ is not prolonged over this part, so that it is only formed of dentine. 



By the formation of the root the crown of the tooth becomes pushed 

 outwards, and breaking through its sack projects freely on the surface. 



The part of the sack which surrounds the rf)ot of the tooth gives rise 

 to the cement, and becomes itself converted into the periosteum of the 

 dental alveolus. 



The genei-al development of the enamel organs and dental papillse is 



Fig. 428. Diagkam shewing 

 the development of the teeth. 

 (From Gegenbaur.) 



p. dental i^apilla; e. enamel 

 organ. 



