TEETH. 



[ 758 ] 



TEGEOCRANUS. 



side being in apposition witli tlie capsule. 

 It forms a spongy tissue, composed of ana- 

 stomosing stellate cells or reticular areolar 

 tissue; in its inside is the enamel-mem- 

 brane, consisting of c^lindrical epithelium 

 (fig. 738 c). 



The enamel is formed by the direct cal- 



Fig. 739. 



Surface of the pulp of a newly-born infant, a, ivory- 

 cells; 6, their inipendagfs ; (■, vascular part of the pulp. 



Magnified .300 diameters. 



cification of the epithelium, the prisms re- 

 sulting from the calcification of the long 

 cylindrical cells. 



" Ossification commences by the deposition 

 ofcalcareous matter in the cells of tlieivory- 

 membrnne at the summit of the pulp ; this 

 is soon folloAved by similar deposition in the 

 cells of the enamel-membrane. By the fur- 

 ther formation of new cells and fresh depo- 

 sition, the structure of the teeth becomes 

 more and more consolidated, the spongy 

 tissue of the enamel graduallj' being ab- 

 sorbed. 



When the entii'e enamel and a consider- 

 able portion of the ivory have been formed 

 in the capsules, these become too small to 

 contain the teeth, which tlien rupture them, 

 and continue to grow at the root, until the 

 crown projects above the margin of the jaw. 

 The remainder of the capsule then forms 

 the periosteum of the alveoli, and, by depo- 

 sition from the side next the tooth, pro- 

 duces the cement. 



The permanent teeth are formed upon 

 the same plan :■ — the three last molars in 

 the remains of the primitive dental groove; 

 tlie others in distinct sacs, called reserve 



sacs, and formed in the wall of the follicles 

 of the milk-teeth. 



The teeth of animals present numerous 

 interesting varieties, to which we can but 

 briefly refer. Thus, in the Mammalia the 

 enamel is often absent, the cement fre- 

 quently extends over the crown, the three 

 component structures are folded, the teeth 

 are compound, the ivory contains Haver- 

 sian canals, and the ivory-tubes enter the 

 enamel. In Eeptiles the teeth are often an- 

 chylosed to the jaws. In Fishes the teeth 

 are often solid ; the ivory is furnished with 

 Haversian canals, sometimes isolated, and 

 each surrounded by a layer of ivory and ce- 

 ment, so that the teeth appear to consist of 

 aggregations of little teeth ; the vessels 

 often branch and anastomose freely ; the 

 ivory-tubes are often very large or absent, 

 the ivory then consisting of a finely granu- 

 lar base with numerous vascular canals, true 

 enamel appe.aring to be absent. 



The oral and gastric teeth of the Mol- 

 lusca (see Tongue), Vermes, and Arthro- 

 poda, are composed of chitine, which is some- 

 times impregnated with lime or silica. The 

 teeth of Echinida are composed essentially 

 of thin leaflets aggregated into a radial 

 lamina ; and they are composed of elongated 

 prisms of carbonate of lime, somewhat 

 curved at their extremities. These lustrous 

 calcareous plates lie between the prisms, and 

 present a fine plexus of anastomosing cana- 

 liculi. 



It may he said generally that the teeth 

 of the higher Invertebrata are to be re- 

 garded as epithelial structures, and that in 

 the lowest Yertebrata they are chiefly com- 

 posed of peculiarly modified and ossified 

 connective tissue. 



The method of maMug sections of teeth 

 is described under Peepakation. They 

 should be very thin, and preserved in the 

 dry state. 



BiBL. Owen, Oclontot/raphi/, and Todd^s 

 Cyd. iv. 864; Goodsir, Ed. Med. and Svn/. Jii. 

 1839, i. ; Tomes, Dental Surgcn/, and Phd. 

 Tr. 1849, 1850; Hanover, Verh. Leopold. 

 Carol. Ak. xxv. 2, Mic. Jn. 18.'57, v. 166; 

 Huxley, Qu. Mic. Jn. 1854, 18.>5, 1857; 

 Lankester, Qu. Mic. Jn. 1867 ; Boll, Zahn- 

 jndpa, Arch. Mih. An. iv. 1868 ; Cutler, Den- 

 tal Comos, 1867; EoUeston, Qu. Mic. Jn. 

 1872; Waldeyer, StricJicr's Hum. c^- Comp, 

 Hist. i. ; Wliite, Mn. Mic. Jn. vii. 2(;3 ; 

 Tomes, M. M. Jn. xiii. 85 ; Frey, Hist, and 

 the lit: 



TEGEOC'RANUS, Mich.— A new genus 



