1124 SPLANCHNOLOGY 



throughout its whole length by a single odontoblast. The central part of the papilla does not 

 undergo calcification, but persists as the pulp of the tooth. In this process of formation of dentin 

 it has been shown that an uncalcified matrix is first developed, and that in this matrix islets of 

 calcification appear which subsequently blend together to form a cap to each cusp : in like manner 

 successive layers are produced, which ultimately become blended with each other. In certain 

 places this blending is not complete, portions of the matrix remaining uncalcified between the 

 successive layers; this gives rise to little spaces, which are the interglobular spaces alluded to 

 above. 



Formation of the Cement. The root of the tooth begins to be formed shortly before The crown 

 emerges through the gum, but is not completed until some time afterward. It is produced by a 

 downgrowth of the epithelium of the dental germ, which extends almost as far as the situation 

 of the apex of the future root, and determines the form of this portion of the tooth. This fold 

 of epithelium is known as the epithelial sheath, and on its papillary surface odontoblasts appear, 

 which in turn form dentin, so that the dentin formation is identical in the crown and root of the 

 tooth. After the dentin of the root has been developed, the vascular tissues of the dental sac 

 begin to break through the epithelial sheath, and spread over the surface of the root as a layer 

 of bone-forming material. In this osteoblasts make their appearance, and the process of ossi- 

 fication goes on in identically the same manner as in the ordinary intramembranous ossification 

 of bone. In this way the cement is formed, and consists of ordinary bone containing canaliculi 

 and lacunae. 



Formation of the Alveoli. About the fourteenth week of embryonic life the dental lamina 

 becomes enclosed in a trough or groove of mesodermal tissue, which at first is common to all the 

 dental germs, but subsequently becomes divided by bony septa into loculi, each loculus con- 

 taining the special dental germ of a deciduous tooth and its corresponding permanent tooth. 

 After birth each cavity becomes subdivided, so as to form separate loculi (the future alveoli) 

 for the deciduous tooth and its corresponding permanent tooth. Although at one time the whole 

 of the growing tooth is contained in the cavity of the alveolus, the latter never completely encloses 

 it, since there is always an aperture over the top of the crown filled by soft tissue, by which the 

 dental sac is connected with the surface of the gum, and which in the permanent teeth is called 

 the gubernaculum dentis. 



Development of the Permanent Teeth. The permanent teeth as regards their development 

 may be divided into two sets: (1) those which replace the deciduous teeth, and which, like them, 

 are ten in number in each jaw: these are the successional permanent teeth; and (2) those which 

 have no deciduous predecessors, but are superadded distal to the temporary dental series. These 

 are three in number on either side in each jaw, and are termed superadded permanent teeth. 

 They are the three molars of the permanent set, the molars of the deciduous set being replaced 

 by the premolars of the permanent set. The development of the successional permanent teeth 

 the ten anterior ones in either jaw has already been indicated. During their development the 

 permanent teeth, enclosed in their sacs, come to be placed on the lingual side of the deciduous 

 teeth and more distant from the margin of the future gum, and, as already stated, are separated 

 from them by bony partitions. As the crown of the permanent tooth grows, absorption of these 

 bony partitions and of the root of the deciduous tooth takes place, through the agency of osteo- 

 clasts, which appear at this time, and finally nothing but the crown of the deciduous tooth remains. 

 This is shed or removed, and the permanent tooth takes its place. 



The superadded permanent teeth are developed in the manner already described, by extensions 

 backward of the posterior part of the dental lamina in each jaw. 



Eruption of the Teeth. When the calcification of the different tissues of the tooth 

 is sufficiently advanced to enable it to bear the pressure to which it will be afterward 

 subjected, eruption takes place, the tooth making its way through the gum. The 

 gum is absorbed by the pressure of the crown of the tooth against it, which is 

 itself pressed up by the increasing size of the root. At the same time the septa 

 between the dental sacs ossify, and constitute the alveoli; these firmly embrace 

 the necks of the teeth, and afford them a solid basis of support. 



The eruption of the deciduous teeth commences about the seventh month after 

 birth, and is completed about the end of the second year, the teeth of the lower 

 jaw preceding those of the upper. 



The following, according to C. S. Tomes, are the most usual times of eruption: 



Lower central incisors 6 to 9 months. 



Upper incisors - . . . 8 to 10 months. 



Lower lateral incisors and first molars . . 15 to 21 months. 



Canines 16 to 20 months. 



Second molars 20 to 24 months. 





