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TEETH. Since the article DENTITION was written, the 

 teeth have been subject. >i in the most careful micro-copic 

 examination, and the result has Km the 

 great amount of knowledge in regard to both their tnic- 

 turr anil their mode of growth. Indeed there if probably 

 no i>art of physiology in which more remarkable and im- 

 portant proglM* has hern mmlr d' 



than in this, to whirh the naiiv irraphv has been 



given. Tl !!y by 



Professor Purkinje of Breslau and Professor Hct/ius nf 

 Stockholm. The former published his observations in 

 1835, in the inaugural dissertation of Dr. Frnenkel (!>> 



C'ttnri Ijrntium > . and in that of Dr. Rasch- 



(MtUtmwtii circa Dentium L'CO/M/KWWI'I ; and the 

 latter communicated his descriptions to the Koyal Aca- 

 demy of Science* at Stockholm, in whose Transactions 

 Uu-y were published in 1H3G. In 1839 Dr. Schwann pub- 

 lished, in his Mikroskopische Untenuchungm,' to account 

 of the modes in which the scvcrul constituent tissues of 

 the teeth are developed : un<l in tiie same year Mr. Goodsir 

 t'urph .)/ .I'liiniul, vol. li.), car- 



rying out the >ie\v before MiL'gcsU'd by Professor Arnold, 

 ({escribed that method of their carlv growth which is now 

 generally received as the truth. Mr. Owen also, in his 

 ' Odontography,' and in various papers, at the same time 

 that he has confirmed and greatly added to the facts de- 

 scribed by those already mentioned, has proved, by his 

 investigations into their comparative anatomy, that the 

 minute structure of the teeth may be taken as one of the 

 most certain characters for the discrimination of the ge- 

 nera, and even of the species, of both existing and extinct 

 animals ; and he has already applied his knowledge of 

 them to the determination of some of the most difficult 

 questions of palteonto 



In the following account .my of the discoveries 



will be detailed except such as relate to the structure and 

 physiology of the human teeth ; for, various as the struc- 

 ture* are in the different classes of animals, yet there is so 

 much analogy among them, that the description of the 

 tooth of one will, in great measure, explain the general 

 plan of structure in the rest. Besides, the lately published 

 articles on comparative anatomy contain nearly all the 

 important facts regarding the structure of the teeth in the 

 animals of which they treat. 



The best method of preparing teeth for microscopic 

 examination is to immerse them in dilute hydrochloric acid, 

 till their earthy matter is so far dissolved that thin trans- 

 parent slices may be cut from them with a knife ; or, with- 

 >i'teiiing them in acid, to make thin sections, in the 

 vertical and other directions, with a fine saw.and to reduce 

 these to the necessary thinness and transparency by filing 



Fig. 1. 



Pig. I- 



them, and polishing them on a hard and 'smooth whet- 

 stone. rV: \amination, lenses magnifying- about 

 "Ml diameters are sufficient. 



In s ieh ; on of a tooth three distinct ub- 



stanccsare seen ; namely, the ilcntine or r. - 

 which forms the greater mass, and, m it v :.:ould 



of the tooth, and which contains the pulp 

 enamel (rr , by which the crown or , t he 



tooth is covered ; and the hone, ceni. 

 dil , which forms a thin layer around tin 

 that pail at which the vessels enter the ] 

 tinned in a liner and scarcely perceptibli i-r the 



enamel. 



The hone, or cement, has in each animal a minute (struc- 

 ture similar to that of which the bones of its skeleton are 

 composed. In man it consists of a basis of him 

 substance, a compound of. .m! earthy matter, in 



which there are minute caxiti. licate 



branched canals leading from one to the other. On tin- 

 walls of tii 



sited more thickly than in the i 

 when examined by transmitted 

 dark grey. The ca\ ities, arbone-oorptutlex, in man are 



I, and flattened; most of them are betwi 

 nfaof an inch in length, about one-third as much in breadth, 

 and one-sixth :is much in thickness. They !., 

 what jagged edges, from all parts of which th< 

 the fine branching canals, to which the name . 

 rout has been given, and which traverse the IK.' 

 basis of the bone, and communicate irregularly wii: 

 another. The diameter of the- 

 parts, is not more than i4-j of an inch; tha 1 

 smaller branches is between ^ and &}&,. Their general 

 direction is towards the axis of the tooth, around which 

 the corpuscles are arranged in concentric ci 



Fig.-L 



Wjcrojeopic view of boDe-corpiuclo and clcl|[aroui rnnili. 



The enamel is composed of solid prisms, or fibres (/'- 

 a fl\ about ^a of an inch thick, set side by side and U]>- 

 right upon the ivory of the crown of the tooth /> . One 

 end of each prism i< lived in a little di ) u the 



rough outer surface of the is. . (her, which is 



somewhat larL'ci. is turned towards the masticating sur- 

 face of the tooth in the direction in which the chief ex- 

 ternal pressure is to be resisted. The course of the p 

 is more or less wavy, their curves bcinir. for the most pint, 

 parallel i'us. \ . but sometimes opposed. Most of them 

 evtend from the ivory to the surface of the tooth: and 

 where they do not, small complemental prisms fill up> 

 like wedges, the vacant spaces. 



fig.*. 





Fig. 4. 





nn of 



rnn-n.-l (T.r- 



Vlnr of the rranipinrnt of thit rmunl-bvM 

 on the crown of mn iaotoor tooth. 



In the perfect slate the enamel contains so small a 

 quantity of animal matter, that it cannot be dc 

 to the siirht. and the prisms are inseparably cor 

 but in young teeth it is oft, and may be broken up into 



