313 



DENTITION. 



DERMATINE. 



314 



closure of the opercula. Secondly, the saccular, which commences 

 with the closure of the opercula, and ends with the passage of the 

 tooth through the gum. Thirdly, the eruptive, which commences 

 when the tooth appears through the gum, and extends to the period 

 when the permanent teeth are fully developed. These three stages, 

 when considered in reference to any particular tooth, are well defined, 

 but when viewed in reference to the whole set, or two sets, they are 

 intermingled ; thus, when one tooth-germ is in the saccular, another 

 is in the follicular stage ; again, when the temporary teeth are in the 

 eruptive, the permanent teeth are in the saccular condition." 



For an account of the development of the tissues which form the 

 tooth, we must refer our readers to the article TEETH, in which the 

 tissues themselves are described. 



The age at which the teeth first make their appearance varies con- 

 siderably, frequently without any apparent reference to the constitu- 

 tional powers of the child. Instances are not wanting in which 

 children have been born with two or more teeth. In many other 

 cases the teeth have not come through the gum until fourteen or 

 sixteen months, or even as late as two or three years. In general, 

 however, dentition may be said to commence at the age of from five 

 to eight months. The following table from Mr. Tomes's Lectures gives 

 the result of the observations of several writers on this subject : 



These periods are however only given as a general rule, liable to 

 continual exceptions, not only in the time at which the different teeth 

 appear, but also in the relative order of their precedence. 



According to l)r. Ashburner, who has paid great attention to this 

 subject, and who has had ample opportunities of observation, the teeth 

 of the first dentition commonly cut in couples ; the two anterior 

 incisors of the lower jaw appear first ; then, in perhaps from fifteen 

 to twenty days the two anterior incisors of the upper jaw come 

 through ; to these succeed the lateral couple of incisors of the lower 

 jaw ; then come those of the upper jaw. After these the two molar 

 teeth nearest to the lateral incisors of the lower jaw appear ; then 

 the first molars of the upper jaw ; after which come the lower two 

 canine ; then the upper canine ; then the two second molar of the 

 lower jaw ; and afterwards the corresponding molar of the upper jaw. 

 The period occupied in the process is about two years from the appear- 

 ance of the first tooth. 



The first formed of the permanent teeth are the anterior molars, on 

 which the first point of ossification may be seen at birth. At about 

 the age of twelve months the ossification on these teeth has proceeded 

 to a considerable extent ; also on the permanent incisors, and it has 

 commenced on the lower cuspidati, the upper ones being generally 

 two or three months later. About the time when all the temporary 

 teeth have made their appearance, ossification is found on the points 

 of the bicuspids, and the bony shells of the teeth before mentioned 

 have acquired considerable size. 



Most of the permanent teeth are larger than those which precede 

 them. They are placed during their progress a little behind them ; 

 hence they are confined within the segment of a smaller circle ; con- 

 ."i|ui'ntly, as they approach more and more nearly to their ultimate 

 size, they must become very much crowded in the jaw. " The exami- 

 nation of the maxillary bone of a child of about five years old," 

 observes Mr. Bell, " will show this fact in a very striking manner. At 

 this period the jaws being considerably deepened by the development 

 of the alveolar processes, the sockets in which the permanent teeth 

 are lodged will be found placed beneath those of the temporary, some 

 higher than others, and the bony shells are closely packed in such a 

 imimier as to occupy the least possible space. Thus, in the upper 

 jaw, the central incisors are situated immediately beneath the nose, 

 the lateral incisors thrown back behind the points of the cuspidati, 

 and the bases of the latter scarcely a quarter of an inch below the 

 orbit ; in the lower jaw the cuspidati are placed at the very base of 

 the bone, with only a thin layer beneath them ; but the crowding is 

 much less considerable than in the upper jaw, from the smaller com- 

 parative size of the incisors. 



" At from six to seven years of age the whole of the permanent 

 t.t-i-tli are more or less ossified, excepting the denies sapientuc ; so 

 that," previously to the shedding of any of the temporary teeth, there 

 are at this time no less than forty-eight teeth in the two jaws; 

 namely, twenty deciduous, the whole of which are perfected, and 

 twenty-eight permanent, in different degrees of development, within 

 the lj< 



At a particular epoch of human life, the temporary teeth are 

 exchanged for a more numerous set, of a stronger and more durable 

 structure, and of increased power of mastication. The original teeth 

 In-come loose in their sockets, their roots are eaten away, their crown 

 crumbles MX! recedes" from the gums, and at last they fall out. This 



change takes place in the temporary teeth exactly in the order in 

 which they were originally formed, and in which they cut through 

 the gums. Thus, the central incisors of the lower jaw fall away first, 

 then those of the upper jaw, then go the interior lateral incisors, and 

 so on. 



The mode in which this change is effected, which constitutes the 

 shedding of the teeth, is by a process of absorption. The anterior 

 parietes of the cavities in which the permanent teeth are contained 

 are removed by the absorbent vessels, in consequence of which the 

 teeth are allowed to advance ; next the sockets, then the roots, auoT 

 lastly the crowns of the temporary teeth are absorbed. This absorp- 

 tion cannot be solely the effect of pressure produced by the advancing 

 permanent teeth, for the process goes on when such pressure cannot 

 possibly have existed ; it is, in part at least, like the preparation of 

 the cell for the reception of the permanent pulp, a true process of 

 anticipation. Neither do the advancing permanent teeth displace the 

 receding deciduous teeth ; but the jaw grows and enlarges consenta- 

 neously with the increasing bulk and number of the teeth which it is 

 destined to receive. 



The change of the temporary for the permanent teeth commences, 

 in the majority of instances, at about seven years of age ; " though," 

 says Mr. Bell, " I have occasionally known it to occurNis early as five 

 and as late as eight years and a half." The first permanent molars 

 usually pierce the gum before the loss of the temporary central inci- 

 sors, and their appearance may be considered as indicative of the 

 approaching change. The following are about the medium periods at 

 which the different permanent teeth are generally cut, but so irregu- 

 lar are they in this respect that comparatively little dependence can 

 be placed on such a statement. Those of the lower are here indi- 

 cated, and they most commonly precede the upper by about two or 

 three months : 



Anterior Molars 6J years 



Central Incisors 7 



Lateral Incisors ...... 8 



Anterior Bicuspids 9 



Posterior Bicuspids 10 



Cuspidati. . 11-12,, 



Second Molars 12-13,, 



Third Molars, or Denies Sapientiic . . 17-19,, 



It has been supposed that the permanent teeth are subject to much 

 irregularity in the time of their appearance. This however is an 

 error, as within a limited period the appearance of the teeth in healthy 

 children is very constant. The following extract from Mr. Saunders's 

 work on 'The Teeth a Test of Age,' 'will be found to confirm 

 this: 



" Thus, then, it appears that of 708 children of nine years old, 389 

 would have been pronounced on an application of this test to be near 

 the completion of their ninth year ; that is, they presented the full 

 development of that age. But on the principle already stated that 

 of reckoning the fourth tooth as present when the three are fully 

 developed a still larger majority will be obtained, and instead of 389 

 the proportion will be as follows : of 708 children no less a number 

 than 530 will be fully nine years of age. What, then, are the devia- 

 tions in the remaining 178? They are the following : 126 would be 

 pronounced eight years and six months, and the remaining 52 eight 

 years of age ; so that the extreme deviations are only twelve months, 

 and these only in the inconsiderable proportion (when compared with 

 the results obtained by other criteria) of 52 in 708. , 



" Again, of 338 children under thirteen years of age, no less than 

 294 might have been pronounced with confidence to be of that 

 age. The remaining 44 would have been considered as follows : 

 36 in their thirteenth and 8 near the completion of their twelfth 

 year." 



Such are the main phenomena which relate to the process of denti- 

 tion when this operation is performed in a perfectly natural and 

 healthy manner. But this process is exceedingly apt to become 

 deranged, producing evils in the system often most serious, and even 

 fatal ; and the preceding account of the natural process will enable us 

 readily to understand the nature, extent, and causes of the diseases 

 which so often result from the morbid progress of the function. 

 [DENTITION DISEASES, in ARTS AND So. Div.] 



(Bell, On the Anatomy, Physiology, and Diseases of the Teeth; Ash- 

 burner, On Dentition and some co-incident Disorders ; Meckel, Man. 

 d'Anat.; Serres, Nouvelle Theorie de la Dentition; De la Barre, 

 Seconde Dentition; Owen, Odontoyraphy ; Goodsir, On the Teeth; 

 Edinburgh Med. and Surg. Journal, vol. li. ; Tomes, On the Structure 

 of the Teeth ; Proc. Roy. Soc., 1838 ; Tomes, Lectures on Dental Phy- 

 sioloijy and Suryery ; Huxley, On the Development of the Teeth, in 

 Quarterly Microscopical Journal, vol. i.) 



DEODARA. [ABIES.] 



DERBYSHIRE SPAR. [FLUOR-SPAR.] 



DERC^EA. [STENELYTBA.] 



DERMATINE, a Mineral found in the serpentine quarry near 

 Waldheim. It occurs in reniform masses, rarely globular, and in 

 thin coatings or crusts. Its colour is dark olive-green or liver-brown. 

 Streak yellow inclining to gray. The fracture is conchoidal. It 

 feels greasy, but does not adhere to the tongue. Its hardness is 



