HKNTIPORA. 



DKXTITION. 



311 



IvirrvU with vinegar and spices, in which *tte it will keep perfectly 

 well for twelve month*." 

 (Yam-ll, Bntuk /tab*) 



1'KNTIPOKA, [MiUBEI-ORKA.] 



PKNTlKOSTKrX [BIRDS.] 



DKNTITIUN, the formation and evolution of the teeth. Tim 

 varied proctin* by which the teeth are formed, developed, and 

 arranged arc among the most curious and complicated operations of 

 the animal economy. The different stages of dentition, in the human 

 being, mark distinct epochs of human life, in which many important 

 change* occur in the phyiical frame, simultaneously with which new 

 mental power* are developed. 



The teeth differ in their organisation in several important respects 

 from all other organ* of the body. They are of a bony *tructuro, 

 and are placed in the archm of the upper and lower maxillary or 

 jaw-bone*. They consist of two nets, of which the one is intended 

 to last only for a short time, while the other ia destined to last 

 during the whole term of life. The first are called the temporary 

 and the second the permanent teeth. The temporary teeth, 20 in 

 number, are in general considerably smaller than the permanent, 

 have a lea* firm and solid texture, and their characteristic forma 

 and prominences are much less strongly marked. The permanent 

 teeth, 32 in number, are arranged in perfect uniformity, 8 on each 

 side of each jaw, those of the one side exactly corresponding with 

 those of the opposite. They are divided into four distinct classes, 

 which present specific differences in size, form, development, articu- 

 lation, and use ; namely, on each side of each jaw, two incisores, one 

 cuspidatus, two bicuapideft, and three niolarea. 



For our knowledge of the early history of the development of the 

 teeth we are principally indebted to Professor Ooodsir of Edinburgh, 

 who in 1839 published his views upon this subject. In giving an 

 account of his researches and the additional facts that are known 

 on this subject, we shall employ the lectures of Mr. Tomes on 

 4 Dental Physiology,' as one of the most recent and complete works 

 on the subject. 



Preparation U made for the development of the teeth at a very 

 early period of foetal existence. At the sixth week of the existence 

 of the human embryo, on examining the mouth, a groove is observed 

 which is called the primitive dental groove, and in it the first germs 

 of the teeth are observed. At the seventh week a slight projection 

 of the mucous membrane at the bottom of thin groove takes place, 

 and which soon increases in size and forms a papilla. This papilla is 

 the first condition of the tooth-pulp, and ia composed of a mass of 

 cells, each containing a nucleus, or cytoblast. No vessels are yet 

 seen in the pulp, but they pass under it to the mucous membrane. 

 As the papilla increases in size the vessels become elevated, and pass 

 into its substance. The first papilla thus formed constitutes the 

 follicle of the anterior temporary molar tooth, thoae of the upper 

 jaw appearing before those of the lower. Subsequently the papillae 

 of the other teeth are developed, and by the tenth week we have in 

 the dental grooves 20 papilla; corresponding to the 20 temporary 

 teeth. A* the papilla grow the walls of the dental groove increase, 

 and send out lamina toward* each other, which meeting, unite, and 

 form septa. By these mean* the papilla) are inclosed in a series of 

 cell* or cavities with open mouths called follicles. The septa are all 

 developed by the thirteenth week, leaving behind in the dental groove 

 an open portion. 



A change now comes on in the shape of the papillic, which, instead 

 of remaining a* hitherto simple round blunt masses, become changed 

 into special characteristic form*. The papilla-, from their more rapid 

 growth, protrude also from the open mouths of the follicles. With 

 this change in the shape of the papillic a growth of lids or opercula 

 to the open mouths of the follicle* take* place. At the fourteenth 

 week the inner lip uf tin; dental groove has increased in size, and 

 applies itself in a valvular manner to the outer lip. The relative 

 growth of the papilla: and follicles is now reversed, the former begin- 

 ning to *ink by the increased growth of the latter. At this time also 

 a secondary dental groove is observed, which i* declined to furnish 

 the papilla: of the permanent teeth. It gradually appears in the form 

 of imuill crescent-nnaped depressions immediately behind the inner 

 opercula of each of the follicles of the first or milk serio* of papillic. 

 By the fourteenth or fifteenth week, through the adhesion of the oper- 

 cula in the first groove, the follicles have become sacs, and the samp 

 change subsequently occurs in the secondary groove inclosing the 

 permanent Ueth. The (pace left open in the primitive dental groove 

 at the end of the sixteenth or seventeenth week exhibits papilla: 

 which represent the anterior permanent molar teeth. 



From the time of the closure of the milk follicles the pulp* 

 gradually assume their peculiar shape, and those destined for the 

 formation of the molar teeth are divided at the base for the growth 

 of their several roots. With these progressive changes, the sac 

 growing faster than the pulp, an intervening space i* formed, in 

 which i* developed a soft granular substance, which for a time 

 iner***** in quantity, and is adherent to the inner surface of the 

 aac, but not to the pulp, though closely applied to the surface of the 



At this ctage of their growth each sac receive* a twig from the 

 denUl artery ; but this doe* not penetrate the granular substance. 



The next step in the general process of development U the appear- 

 ance of caps of tooth-substance upon the tips of the puljw, and is 

 accompanied by a diminution of the granular substance, which 

 entirely disappears when the tooth-substance U perfectly developed. 

 From the last-mentioned period up to the eighth mouth the cavities 

 devoted to the ten anterior permanent teeth gradually recede from 

 their position between the milk-sacs and the gums, and are now 

 posterior to the milk-sacs. This separation of the sac* has led to 

 the notion that the permanent sacs are formed from the tem- 

 porary one* by a process of gemtniparous fission ; but the obser- 

 vations of Professor Goodsir have demonstrated that this view i* 

 entirely erroneous. 



At about the time of birth the fangs of the incisors begin to be 

 formed. This process is attended with three separate actions: 

 1, elongation of the base of the pulp; 2, deposition of dentine upon 

 it ; 3, adhesion of the contiguous portion of the sac to the surface of 

 this dentine. 



The central incisors pass through the gum, or arc cut as it ia called, 

 about the eighth or ninth month after birth. The crown of tho 

 tooth being perfected, and the formation of the fang advanced by the 

 triplex action already described, an action is set up by whicli tin- 

 edge of the tooth passes through the gum. Here then terminates the 

 saccular stage of the tooth, the sac having been opened by the 

 passage of the tooth through the gum. It must be borne in mind that 

 with the development of dentine for the fang the sac becomes 

 adherent to its surface, but not to the surface of the enamel A probe 

 might be passed down the surface of the enamel to the neck uf the, 

 tooth so soon aa the sac is opened by the edge of the tooth. When 

 once the tooth U cut growth progresses rapidly. The tooth however 

 appears to grow more rapidly than it really does, and from the follow- 

 ing causes: The sac being opened, and its inner surface there! iy 

 rendered continuous with that of the gum, a strong disposition to 

 contract aeems to come into force in that portion lying against the 

 enamel, and as the gum constitutes one fixed point, and the adhesion 

 of the aac to the neck of the tooth the other, the lower is aa it 

 lifted out of the gum by the shrinking of the aac between these two 

 points. Aa a consequence of this movement, the distance between 

 the unfinished end of the fang and the fundus of the alveolus is 

 lengthened. The socket now rapidly adapts itself to the neck of the 

 tooth, to which it becomes accurately moulded. The pulp elongates 

 itself and diminishes at ita base, till at the completion of a tooth it 

 has diminished to the size of a thread and ia constituted principally 

 of the dental vessels and nerves. Aa the temporary teeth have 

 advanced towards the surface, the sacs for the permanent teeth have 

 receded behind them, and have become inclosed in proper bony IT > i ' - . 

 from each of which a foramen proceeds. In the sacs for the 

 front teeth these foramina open immediately posterior to the milk 

 teeth, but thoae for the bicuspid* open into the alveoli of the milk 

 molars. From the apex of each sac a fibrous cord proceeds through 

 the foramen to join the gum near the neck of the corresponding milk 

 tooth, excepting in those under the milk molars, in which the fibrous 

 cord unites with the periosteal lining of the temporary alveolus. 

 These cords, or gubernacula as they are called, are formed of the 

 obliterated portion of the pulp follicle, which it will be remembered 

 was rendered external to the sac by the development and subsequent 

 closure of the opercula. It seems the union of the two sides is some- 

 times incomplete, so that the cord is in fact a tube closed at ita two 

 extremities. The gubernaculura lengthens as the sac recedes from 

 the surface, and disappears only after the tooth passes through the 

 gum. From observing the position and the disappearance of the 

 gubernaculum many have supposed that it leada or directs tho 

 developing tooth to its proper situation in the alveolar arch. Pro- 

 fessor Ooodsir, when speaking of the use of the cords and foramina, 

 says: 



" The cords of communication which pass through these foramina are 

 not tubular, although in some instances a portion of the iinoblitcrated 

 intrn-fiilliciilar compartment of the original little cavity of reserve may 

 be detected in them ; they are merely those portions of the gum \vhieh 

 originally contained the lines of adhesion of the depressions for the 

 permanent teeth in the secondary dental groove, and which have been 

 subsequently lengthened out in consequence of the necessarily retire.l 

 position in which the permanent teeth have been developed during the 

 active service of the temporary set. 



" The cords and foramina are obliterated in the child, either because 

 the former are to become useful as ' gubernacula,' and the latter as 

 ' itinera dentium,' or, much more probably, in virtue of a law which 

 appear* to be a general one in the development of animal Kidie*, 

 namely, that parts or organs which have once acted an important part, 

 however atrophied they may afterwards become, yet never altogether 

 disappear so long as they do not interfere with other part* or 

 functions." 



The sacs of the permanent teeth are supplied with blood-vessels 

 first from the gums but afterwards from the milk-sacs. As the sacs 

 sink deep into the alveolus they receive vessels from the proper dental 

 canals. 



Mr. Ooodsir divides dentition into three stages : " First, the folli- 

 cular, in which stage ia included the papillic when it existed as a 

 simple prominence from the mucous memlmine, and extends to the 



