THE TEETH 



1125 



A somewhat later stage in the organogenesis of a tooth is shown in fig. 880. The mesen- 

 chj-mal cells on the surface of the dental papilla, next to the enamel organ, form a single layer 

 of columnar cells, the odontoblasts. These cells form the dentine upon their outer surfaces, 

 gradually retreating toward the center of the tooth as the dentine increases in thickness. The 

 first dentine formed is irregular, enclosing the spatia interglobularia. The odontoblasts re- 

 main through life just beneath the dentine on the surface of the pulp, sending slender processes 

 up into the dentinal tubules as previously noted in the structure of the adult tooth. The re- 

 mainder of the dental papilla becomes the pulp, receiving its vascular and nerve supply at the 

 point opposite the enamel organ, corresponding to the future root. 



The enamel organ (fig. 880) is differentiated into three layers: a thin outer layer attached by 

 the neck to the dental ridge; a thick middle layer (forming the spongy "enamel pulp"); and a 

 single inner layer of cylindrical enamel cells, the adamantoblasts. The latter form the enamel 

 prisms, which are deposited gradually upon the outer surface of the dentine. 



Fig. 883. 



-Dissection Showing the Teeth at about Six Years. 

 Fischelis.) ■ 



(Broomell and 



Surrounding the entire developing tooth there is formed a strong, fibrous connective- 

 tissue membrane, the tooth-sac. The deeper part of this sac later becomes the alveolar periosteum 

 around which the bony alveoli are formed. This bone may entirely surround the tooth-sac, 

 excepting at the summit, where a foramen persists through which a process of connective 

 tissue (gubernacidum dentis) connects the tooth-sac with the overlying gum (see figs. 114, 881). 

 Upon the inner surface of the tooth-sac, next to the root, the bony cementum is deposited upon 

 the dentine. The root gradually elongates, and is usually not completed until long after the 

 eruption. The remaining superficial portion of the tooth-sac undergoes pressure atrophy and 

 absorption. The remnants of the enamel organ, however, persist and form a thin tough 

 cuticle [cuticula dentis], Nasmyth's membrane, which is soon worn off when the crown is exposed 

 at the surface. 



From the remainder of the dental ridge, which lies on the lingual side of the deciduous 

 teeth (fig. 878), the permanent teeth are later derived in a very similar manner. (Rudimentary 

 indications of a prelacteal dental ridge have also been described.) The anlages of the per- 

 manent teeth therefore lie to the lingual side of the deciduous (fig. 883). From the posterior 

 end of the dental ridge a process extends into the jaw behind the deciduous teeth, and from 

 this process the permanent molars (which have no deciduous predecessors) are formed. At 

 birth, although no teeth have yet been cut, there are present in the gums the anlages of not 

 only aU of the deciduous teeth, but also all of the permanent teeth, with two exceptions. Those 

 of the second molars do not appear until six weeks after birth, and of the third molars not until 

 the fifth year. The remnants of the dental ridges become broken up into small masses of 

 epithehal cells, which persist for a variable time. 



