TEE 



143 



TEG 



its elementary parts. In the early state also it exhibits 

 portions of a membranous animal substance, consisting 

 of the cells in which each of its prisms was formed ; for, 

 as will be presently shown, the earthy matter is deposited 

 in what might be called a set of moulds formed by the 

 primary cells of the enamel membrane, and, as it accu- 

 mulates, the membrane of the cell is so nearly removed, 

 t hat in the perfect tooth no portion of it can be discerned. 

 Its former existence however seems to be indicated by 

 fine close-set transverse striae upon each prism of the 

 enamel. 



The dentine, or ivory, is composed of a hard fibrous basis 

 of cartilage and earthy substance, traversed by very fine, 

 branching;, cylindrical tubules, which run in an undulating 

 course from the pulp-cavity, on whose internal surface 

 they open (see Fig. 1, bj towards the adjacent part of the 

 exterior of the tooth. Each tubule in its course outward 

 makes two or three chief curves (' primary curvatures,' 

 Owen), and is besides bent at eveiy part in minute and 

 very close undulations, or secondary curvatures ; but the 

 course of those tubules, which are adjacent to each other, 

 is very nearly parallel. It is from the parallelism of these 

 secondary curvatures of the tubules, that the appearance 

 arises, as if the ivory were composed of concentric lamella; 

 .:i-d round the pulp-cavity. 



The chief branchings of the tubules of the dentine are 

 dichotoraous (Fig. 3) ; but they also frequently give off 

 minute branches, which again sending off smaller ones, 

 till up the spaces between the trunks (Fig. G). At the 

 trunk each tubule has an average diameter of about 15 lj of 

 an inch, and the distance between each two tubules is 

 nearly equal to the width of three of them. Both the 

 walls and the cavities of the tubules, as well as the sub- 

 stance between them, are filled by the earthy constituent 

 c.f the ivory, which is deposited in fine granules. The 

 basis of the intertubular substance seems to be com- 

 1 of bundles of flat, pale, granular fibres, whose course 

 is parallel to that of the tubules. 



Fig. 0. 



Views of the tubule* of dentines. 



A separate organ is provided for the formation of each 

 of these three constituent parts of the tooth, though, 

 when they are perfected, they contain no vascular tissue 

 but the pulp within the pulp-cavity, and it is doubtful 

 whether, in the human subject, fresh material is ever 

 formed from this after the tooth has once attained its com- 

 plete development. The first appearance of the pulp of 

 tooth is in the form of a minute process or papilla 

 5 from the bottom of a groove in the mucous mem- 

 brane of the mouth behind the edge of the jaw. In course 

 of time, as the borders of the groove grow around it, the 

 papilla seems to sink into the mucous membrane; and it 

 now appears as if rising from the base of a follicle, or of a 

 fia-sk-like depression, in the edge of the jaw. And lastly, 

 processes of membrane, or operculn, trrow from the sides 

 of the mouth of the follicle, and as they approach each 

 oilier and adhere by their mutually opposed cdircs, they 

 gradually close it, and convert it into a <: ;ic, to 



the bsse of which the first-formed papilla is affixed. In 

 the first-appeariiie tooth, the papillary state may be seen 

 in the human embryo an inch in length: the capsular 

 stage is completed at about the fifteenth, week of embry- 

 onic life. 



These three stages of the formative organs of the tooth, 



namely, the papillary, the follicular, and the capsular, 



; completed, the substances of the tooth itself begin 



to be produced. The dentine is developed from the pa- 

 pilla, which gradually assumes the form and relations of 

 the proper tooth-pulp ; the enamel, from a special organ 

 developed at that part of the capsule which is opposite to 

 the papilla ; and the bone probably from the interior of 

 the capsule itself. 



The papilla and the sac both gradually increase in size, 

 but the growth of the latter is at first more rapid than that 

 of the former, and the space between them is thus en- 

 larged. Within this space there is deposited from the 

 wall of the sac a soft, granular, non-vascular substance, 

 the enamel-organ, or, as Mr. Hunter (Natural History of 

 the Teeth) termed it, the external pulp. And at the same 

 time as this is being produced from the interior of the sac, 

 there is formed on the surface of the papilla a peculiar 

 structureless membrane, which has been called the pre~ 

 fiiriiiative membrane, and which, when the papilla begins 

 to ossify, presents numerous little elevations and depres- 

 sions, on which the enamel fibres are afterwards fixed ; for 

 as the papilla enlarges, the preformative membrane comes 

 in contact with the enamel-organ, and they are exactly 

 moulded the one upon the other. 



Both the papilla, or as it may now be called, the pulp, 

 and the enamel, are composed of primary cells [Nu'rui- 

 TION], and it is by the transformation of these that the 

 tubules of the dentine and the fibres or prisms of the 

 enamel are severally produced. The exact mode however 

 in which the change is effected is not yet known. All 

 that can be seen is that the superficial cells of the pulp, 

 which are at first round or oval, and nucleated, assume the 

 same diameter and direction as the trunks of the, dentine 

 tubules, and then have earthy matter deposited in and 

 around them. And these changes go on gradually from 

 without inwards: as fast as the elongated and branching 

 cells of one layer are ossified, those of the layer beneath 

 them become elongated in preparation for the same change ; 

 and so on, till a great part of the pulp is hardened. It. is 

 due to this gradual ossification of the pulp from without 

 inwards, that in growing animals, to whom madder is 

 alternately given and omitted in their food, the dentine is 

 found to consist of alternate rings of red and white ivory ; 

 for while madder is being taken, all the earthy matter that 

 is deposited in the most superficial layer of the nnossified 

 pulp-cells is dyed by its colouring principle, and when it 

 is discontinued the same material is deposited uncoloured 

 in the layer of cells which is subjacent to that already 

 d and reddened. When nearly the whole of that 

 part of the pulp which was formed in the original papilla 

 is thus hardened by the deposition of earthy matter, its 

 base begins to grow into one or more conical processes, 

 and, by a hardening of these, through a process like that 

 just described, the fangs are formed, and the tooth rises to 

 the surface of the gum. 



In the formation of the enamel, the primary nucleated 

 cells on the inner surface of the enamel-organ become 

 elongated and cylindrical, or prismatic ; they assume a 

 direction which is perpendicular to the surface of the har- 

 dening pulp ; and then, their nuclei disappearing, they 

 also are hardened by the deposition of earthy matter within 

 them, which is continued till they are inseparably com- 

 pacted, and their original membranous wall is not dis- 

 cernible. These changes also, like the preceding, make 

 progress in layers ; but the progress is here from within 

 outwards, and it goes on till nothing is left but a thin ex- 

 ternal enamel-membrane on the surface of the crown o' 1 

 the tooth. As the enamel organ and the papilla, both 

 growing and hardening, approach more nearly to each. 

 other, the preformative membrane also disappears. 



By the transformation of this enamel-membrane, or of 

 the superficial part of the capsule itself, that part of the 

 bone is produced which envelopes the enamel ; and by 

 similar changes in that part of the capsule which has 

 grown in company with the fang-processes of the pulp, 

 that part of the bone is formed which invests the fangs. 

 The changes in this part of the process are probably exactly 

 similar to those through which new bone is produced 

 between a periosteum and the old bone which it sur- 

 rounds. 



TKKTH OF WHEELS. [WHEELS.] 



TKFLIS. [Tm.is.] 



TEF2A. [MABOCCO.] 



TE'GEA. [ARCADIA.] 



TEGERNSEE, THE, is a lake in the circle of the Isar, 



