792 THE TEETH. 



retraction of the gum. By its connection with the surrounding mem- 

 branous structures it contributes to fix the tooth in the socket. It is the 

 seat of the bony growths or exostoses sometimes found upon the teeth. 



D. Osteodentine (Owen), secondary dentine (Tomes), or the horny substance 

 of Blumenbach, is a hard substance which begins to be deposited on the 

 inner surface of the dentine after the age of twenty years or later, so that 

 the central cavity of a tooth becomes gradually diminished in size, whilst 

 the pulp slowly shrinks or disappears. This additional substance, formerly 

 regarded as an extension of the cement into the interior of the tooth, has 

 been shown to have a distinct structure, in part resembling dentine, and in 

 part bone. It is traversed by canals, which contain blood-vessels, and are 

 surrounded by concentric lamellae, like the Haversian canals of bone. From 

 these canals, numerous tubules radiate in all directions, larger than the 

 canaliculi of bone, resembling, in this respect, and also in their mode of 

 ramification, the tubes of the dentine. This newly added structure may or 

 may not coalesce with the previously formed dentine ; it appears to be pro- 

 duced by a slow conversion of the dental pulp. 



Among special works on the teeth may be noticed, Retzius, in Muller's Archiv, 

 1837; Nasmytk, Researches on the Teeth, 1839; Owen, Odontography, 1840-45; 

 Tomes, Lectures on Dental Physiology and Surgery, 1848, also in the Phil. 

 Transactions, 1849 and 1850, and in Quart. Journ. of Micr. Science, 1856; Salter, 

 in Quarterly Journal of Microscopic Science, 1853, in Guy's Hospital Reports, third 

 series, vol. i. ; and in Trans. Path. Soc., 1854 and 1855 ; Czermak in Zeitschrift fur 

 wissensch. Zoologie, 1850; Huxley in Quarterly Journal of Microscopic Science, 1853. 



DEVELOPMENT OP THE TEETH. 



Although the general phenomena of the growth and succession of the 

 teeth had received considerable attention from various anatomists, the ob- 

 servations of Arnold and Goodsir, made independently of each other, more 

 especially the latter, were the first to give precision to our knowledge 

 concerning their origin and the earlier stages of their formation. More 

 recent researches have, it is true, shown that their account of the primor- 

 dial condition of the dental germs may require some modification ; but 

 nevertheless these authors were the first to establish the primordial con- 

 nection of the teeth with the mucous membrane covering the edges of the 

 maxillary arches, and Goodsir was the first to give a consistent view of the 

 earlier steps of the formative process in the temporary and permanent series 

 of teeth. (Arnold in Salzbtirger Med. Zeitung, 1831 ; Goodsir in Edin. 

 Med. and Surg. Journal, 1839.) The changes which take place in the 

 bones of the jaws relate only to the formation of the sockets of the teeth. 

 In their earliest condition these bones present no appearance of alveoli, 

 but, concurrently with certain changes in the mucous membrane, to be 

 immediately described, a wide groove is developed aloi.'g the edge of the jaw, 

 which gradually becomes deeper, and is at length divided across by thin 

 bony partitions, so as to form a series of four-sided cells. These bony septa 

 are not distinctly formed until near the fifth month of foetal life. By the 

 subsequent growth of the bone, these cavities or loculi are gradually closed 

 round, except where they remain open at the edge of the jaw. By the end 

 of the sixth month they are distinctly formed, but continue afterwards, in 

 proportion to the growth of the teeth, to increase in size and depth, by the 

 addition of new matter which widens and deepens the jaw. 



The first stages in the development of the teeth, as observed by Arnold 

 and Goodsir, consist of certain changes in the mucous membrane cover- 



