DEVELOPMENT OF THE TEETH. 1247 



The following, according to Tomes, may be taken as representing the average. The 

 lower central incisors appear first, usually between the sixth and ninth months ; then 

 follows a rest of a few months. Next come the four upper incisors, followed by a rest of 

 a few months. Then the lower lateral incisors and the four first molars erupt, succeeded 

 by a rest of a few months. Next appear the canines, and finally the four second molars, 

 which are all cut by the end of the second year. 



The following statement is simple, and perhaps is sufficient for all ordinary purposes. 

 The deciduous teeth usually appear in the following order: Central incisors, lateral 

 incisors, first molars, canines and second molars; the eruption commences between the 

 sixth and the ninth month, and is usually completed by the twenty-fourth the lower 

 teeth, as a rule, preceding the upper. 



Formation of Adamant and Ivory. Different opinions are held as to the method in which 

 the substantia adamantina is produced by the cells concerned in the process. One view main- 

 tains that it is secreted and shed out by the cells (Kolliker). According to the other view, 

 part of the substance of the cells is actually converted or transformed into adamant (Tomes). 

 In connexion with this latter view, which seems to receive more support at present, Tomes 

 has discovered that there projects from the base of each, cell, towards or into the most recently 

 formed enamel, a fibrillar process, which has received the name of Tomes' process, and he holds 

 that the snbstantia adamantina is formed by calcification taking place in or around the process. 



Similarly, two views are held as to the production of ivory by the odontoblasts ; one, that 

 the odontoblasts secrete the matrix ivory, and the other, that their substance is actually con- 

 verted into the matrix of the ivory. The odontoblasts, when active, are branched columnar- 

 shaped cells, and from their outer ends one or more processes extend towards and into the ivory ; 

 between these processes a matrix appears produced probably by the odontoblasts and soon this 

 matrix becomes calcified. In this way the ivory is formed, and the process is repeated until 

 its full thickness is attained. The branches of the odontoblasts, encased in ivory, just mentioned, 

 are the Tomes' fibrils already described ; the canals in which they lie are the dental canaliculi ; 

 and the fibrils themselves are concerned in the production of the sheaths of Neumann which 

 line the tubes. 



The tooth-sacs, when fully developed, are large and distinct fibrous bags which lie 

 in the alveoli of the maxilla and mandible, and are continuous above with the tissue of 

 the gum. On the lingual side of the sacs of the deciduous teeth are found the germs of the 

 permanent teeth, surrounded by their own sacs. These latter are at first very small, and 

 are partly embedded in the posterior wall of the deciduous tooth-sacs, but subsequently 

 they come to lie in distinct but incomplete bony cavities of their own. The bone sur- 

 rounding the tooth-sacs, temporary and permanent, is always wanting over the summit of 

 the sac, and the band of connective tissue by which the sac is connected with the over- 

 lying gum tissue, through the deficiency, is known as the gnbernaculum dentis. 



All the points mentioned are easily demonstrated on the mandible of a child at birth, 

 particularly when the tissues have been allowed to soften a little. If, in such a specimen, 

 the gum and periosteum are reflected upwards from the labial and lingual surfaces of the 

 mandible, and freed as far as the superior border of the jaw, the gum, with the tooth-sacs 

 depending from it like small bags, can be pulled away out of the bony groove of the jaw ; 

 and if the operation has been successfully performed, the tooth-sacs of the three front 

 permanent teeth may be seen, varying in size from a small pin's-head to a hemp-seed, 

 hanging down behind the superior part of the corresponding deciduous sacs. As already 

 explained, the tooth-sacs are produced simply by a condensation of the connective tissue 

 around the developing tooth, the condensation going on to the formation of a distinct 

 membranous bag. 



Formation of Alveoli and Eruption. At first the developing teeth lie in an open 

 ny groove or channel between the labial and lingual plates of the young jaw. This 

 groove is subsequently divided up into separate compartments for the sacs of each of the 

 deciduous teeth. As development proceeds these compartments or alveoli surround the 

 sacs more completely, but never actually close over the summit. When the eruption of 

 the deciduous teeth is about to take place, the anterior wall and roof of the alveolus are 

 absorbed ; the tooth passes through the sac and appears above the gum, and then the 

 alveolus, which up to this was much too large to give actual support, is re-formed more 

 closely around the tooth. Meanwhile the root, which was only partly formed at the time 

 of the eruption, continues to be added to, possibly for a few years more, and, as it grows, 

 the alveolus is completed around it. When the permanent tooth, or as much of it as 

 is then formed, is about to be erupted, it makes its way from its own bony cell through 

 the posterior wall of the alveolus of its temporary predecessor ; the root of the deciduous 

 tooth undergoes absorption at the same time, but quite independently of pressure from 

 the permanent tooth. "The alveolus, now occupied by both teeth, is again much enlarged 



