486 SPECIAL HISTOLOGY. 



alveoli of tlic corresponding milk teeth, but those of the premolars* open 

 into the bottom of the alveoli of the deciduous molars. 



The sacs of all these teeth are produced at their apices into a solid 

 cord, which extends either to the gum, or, as in the two premolars, to 

 the periosteum in the bottom of the alveoli of the two deciduous molars 

 (Fig. 195, i), and has been erroneously described as a guhernaculum, or 

 guiding cord for the teeth in their eruption. 



The sac of the anterior of the three permanent molars, arises, together 

 with its papilla, in the sixteenth or seventeenth week, quite indepen- 

 dently, from the posterior extremity of the primitive dental groove, and 

 closes in such a manner that a reserve sac remains between it and the 

 mucous membrane (Mikr. Anat. Fig. 206). It is not till the seventh or 

 eighth month after birth that the latter elongates behind the first sac, 

 arches into the margin of the jaw, produces a papilla from its base, and 

 becomes constricted off into the sac of the second molar. The remainder 

 of the cavity falls into a line with the other sacs, and forms that of the 

 wisdom tooth. 



The formation of milk teeth begins at about the fifth month of foetal 

 life, and at the seventh, ossification has commenced in all of them. The 

 first step is the formation of a little scale of dentine upon the apex of 

 the pulp ; in the molar teeth there are at first several of these scales, 

 corresponding with the several elevations of the pulp, but they soon coa- 

 lesce. Immediately after the appearance of this dentinal scale, a thin 

 layer of enamel is deposited from the so-called enamel organ upon the 

 roof of the sac, and whicli coalescing with the dentine, forms the first 

 rudiment of the crown of the teeth. The scale of dentine extends over 

 the pulp and becomes thicker, so that it soon rests like a cap upon the 

 pulp, and finally forms a sort of capsule for it, which, as ossification 

 proceeds and the pulp diminishes, closely and completely embraces it; 

 the deposition of enamel goes on simultaneously, so that it soon proceeds 

 from the entire surface of the enamel organ, and becomes more and more 



* [Instead of the loose phraseology " small " and " large "' molars, &c., we have adopted 

 the philosophiinil nomenclature of the teeth, introduced by Professor Owen (see his Article 

 on the Teeth, in Todd's Cyclopedia), and thus explained by him : " Those teeth which are 

 implanted in the premaxillary bones, and in the corresponding part of the lower jaw, are 

 called ' incisors,' whatever be their shape or size ; the tooth in the maxillary bone, which is 

 situated at or near to the suture with the premaxillary, is the ' canine,' as is also that tooth 

 in the lower jaw, whicli in opposing it, passes in front of its crown when the mouth is closed. 

 The other teeth of the first set, are the 'deciduous molars;' the teeth which displaces them 

 vertically, are the ' premolars :' the more posterior teedi which are not displaced by vertical 

 successors, are the molars properly so called." 



It results from this, that the so-called bicuspid and "first molar " of the permanent set in 

 man (Professor Kolliker's ",small molars") are the premolars; being, in fact, the third and 

 fourth of the typical dentition ; the first and second premolars and the third incisor of the 

 typicardentition, not being developed in man. The nomenclature of the teeth, from being 

 merely technical and arbitrary, has thus, by Professor Owen's recourse to development 

 become scientific. — Tiis.] 



