84 



FIRST CLASS OF THE VERTEBRATED ANIMALS. 



with which alone we are at present concerned, may further be restricted by the cir- 

 cumstance already alluded to, that they are only found upon the margins of the maxil- 

 lary bones. 



It was for a long time supposed that the teeth were bones, that they were produced 

 in the same manner, and had a similar structure. This view of tiie subject has been 

 wholly abandoned, since the publication of the admirable treatise of the Baron Cuvter 

 upon the grinders of the Elejihants, in the Annales du Museum d'Histoire Naturelle, 

 tome viiij in the year 1S06. Although the differences between teeth and bones appear 

 to be very numerous and essential, there seems, however, to be a considerable ana- 

 logy between them, especially when considered in a point of view purely anatomical. 

 "When physiologically considered, they possess many peculiarities in common with the 

 horns, nails, and hair. 



At first, the constituent matters of the teeth and bones are precisely the same ; 

 and if we revert to the first formation of these bodies, it appears that they are 

 equally secreted and deposited by proper vessels. Under this point of view, the 

 teeth may be considered as bones, the vessels of which are united in a single mass, 

 and deposit the osseous matter ai'ound them; while the bones may, on the other hand, 

 be viewed as teeth, within which the minute subdivisioiis of the vessels cause this 

 matter to circulate in every direction. 



At their origin, and during the greater part of their existence, the teeth are com- 

 posed of a secreting organ and a secreted substance. The former, or secreting organ, 

 is always concealed in the lower part of the tooth, or in the interior; and w'hen en- 

 tirely formed, consists of three, or at least two other organs. It is essentially com- 

 posed of vessels and nerves, which communicate directly with the remainder of the 

 organization. The latter, or secreted substance, is merely deposited outside the first. 

 It is composed of a greater or less number of different substances, and being deprived 

 of all vessels and nerves, bears no necessary or immediate connexion with the other 

 organs. 



The secreted substance is of a calcareous appearance, and composed of two parts ; 

 the one external, called the crown (fust or couronne) of the tooth; the other being 

 more or less concealed, is termed the root (racine). The intermediate point is dis- 

 tinguished by the appellation neck (collet). 



The crown of the tooth may be composed of different kinds of matter, deposited 

 one over the other. In the most complicated kinds of teeth, three of these may be 

 obtained by a mechanical analysis. The central part is termed the ivory; the second 

 the enamel, and the most external part the cortex. These three substances are found 

 combined in four different ways. Some teeth are composed of ivory, enamel, and 

 cortes ; others only of ivory and enamel. Some, again, are formed of ivory and 

 cortex, the enamel being wanting ; others of ivory alone, this last being never observed 

 to be deficient except in those Hammaha which are wholly destitute of teeth. 



The root may be real or apparent. In the first case, it is formed of ivory alone, as 

 in Man, the Carnassiers, and the Ruminantia; or of ivory and cortex, as in the Cacha- 

 lots. In the second case, the root is merely a continuation of the crown, and has all 

 the characters of the latter. Such are the roots of all tusks proj^erly so called, the 

 incisive teeth of all the Rodentia ; the molars of Hares, of Guinea-pigs, and of the 

 Cabiais (^Hydrochcerus). 



The secreting organ of the tooth or dentary capsule, according to M. Frederic 

 Cuvier, appears to be dependant on, or produced by, the nei'ves and maxillary vessels. 

 It is not, however, without relation to the contiguous parts, being even united to 

 the gums ; but much less than some authors have imagined. It is certain that the 

 secreting organ of the second teeth, for a long time after its formation, is altofrether 

 independent of these parts, and it is only subsequently that it becomes united to the gums. 



The dentary capsule corresponds, both in its structure and functions, with the 

 substances or materials of which the teeth are composed, in such a way, that it is 

 more simple in teeth formed of one substance alone, than in those composed of two 

 or three. It is the same with its forms, as well as its growth, in relation to the 

 forms and growth of the teeth, the one always being the consequence of the others. 

 The most complicated kind of dentary capsule, being that observed whenever the 

 teeth are composed of three substances, is itself formed of three very distinct secret- 

 ing organs. The central one, called the buJh, produces the ivory'; the second, 

 under the form of a membrane, secretes the enamel, and may thence be termed the 

 enamellating viembrane ; and the third, which surrounds all the other parts, produces 

 the cortex or external ivory. The last may be termed the external viembrane. 



The bulb which secretes the ivory by its external surface, appears to be entirely 

 composed of nerves and vessels. Several arterial trunks, which extend from the 

 one extremity to the other, are ramified infinitely before arriving at its extremities, 

 where their divisions sometimes form tufts and fringes of an almost imperceptible 

 degree of fineness. This part of the teeth may be studied with the greatest facility 

 when they first begin to form; it is then found to be naturally injected, and is not 

 exposed to injury during the abstraction of those bony portions in which the teeth 

 are enclosed, while a very slight degree of maceration is sufficient to extract the 

 bulb from the coating of ivory by which it is surrounded. It seems to be homo- 

 geneous throughout, and always has the same shape as the tooth will ultimately have. 

 In fact, it is the mould upon which the tooth is modelled. 



The enamellating membrane produces the enamel by its internal surface. It sur- 

 rounds the bulb entirely, and follows all its circuitous outlines, thus possessing the 

 same form, except at the base of the bulb, corresponding to the neck of the tooth 

 where it abuts and terminates. M. F. Cuvier was unable to detect any vessels in 

 this membrane. It is transparent and brittle when thick and about to deposit 

 enamel; but it soon softens, becomes of a milky whiteness and great elasticity. 

 Finally, it ends by disappearing altogether, when it has no longer any function to 

 discharge, that is, when the external membrane, by depositing the cortex, resumes 

 Its place. The transparency of this membrane, its extreme thinness thereafter, and 

 its final obliteration, in those teeth where the ivory is formed, have been tiie cause 

 that many Naturalists have failed to observe it. Bat it may be seen very easily 

 upon the parts contiguous to the molars of the Ruminantia, and especially on the 

 hinder ones, at the moment when these animals are born ; and, if once remarked 

 here, it becomes easy to detect it upon all teeth possessed of enamel. 



The external membrane, like the bulb, is of a nature essentially vascular, and 

 may be considered as an external bulb. It is homogeneous in respect to its intimate 

 structure; but its two faces have not always the same forms, nor do they perform 

 the same functions. It deposits the cortex by its internal surface, and follows all 

 the contortions of the tooth. In the compound teeth it juts outwards, whenever 

 they present any hollows. The parts which line these cavities are not merely mem- 

 branes, at least when the cortical matter is about to be deposited, for they then have 

 the same thickness as these cavities, and this gives them all the appearance of bulbs. 

 Before the above period, it is sufficiently thin upon the surface of the compound 

 teeth, and this observation is applicable to most teeth. But it may be presumed 

 that the external membrane is always of a great degree of thickness in the capsules 

 of those teeth where the ivory has to be covered with a great thickness of cortex, 

 as may probably happen in the molar teeth of the Cachalots. Its external surface 

 is always simple, being merely the protecting and uniform envelope of the entire den- 

 tary organ, and its form when complete is always more or less spherical. It is 

 pierced at its summit by the evolution of the tooth ; but its margins are attached to 

 the gums, and become in some measure a continuation of thera. 



These three parts, composing the dentary capsule, are intimately united, and be- 

 come confounded together towards the inferior part of this organ, at the point where 

 the vessels and principal nerves are introduced, at least from the time vfhen the 

 roots begin to develop themselves, and to become distinguished from the crown. It 

 appears that all the three parts originate from this point, and likewise all the essen- 

 tial vessels which traverse and nourish them pass from thence. Their other 

 portions are from the very commencement entirely independent of each other. The 

 external membrane may be raised without occasioning the slightest injury to the 

 enamellating membrane, which detaches itself without effort from the layers of enamel 

 just deposited ; and the bulb may be separated from its cones of ivory like a blade 

 from its scabbard ; or if the cones be broken, it may be disengaged and displayed 

 without being destroyed, or in any way injured. 



This capsule, however, is not entirely formed before the teeth are secreted, in 

 those at least which have roots. It develops itself successively, and in proportion 

 as the different parts are formed, beginning from the summit of the crown, and finish- 

 ing by the extremity of the root. 



The bulb and enamellating membrane seem to deposit simultaneously the matters 

 which they respectively secrete ; and the first molecule of ivory receives the first mole- 

 cule of enamel. It is only at a later period, that the external membrane deposits the 

 cortex, being at the time when the crown is already formed, and when the bulb, as 

 well as the enamellating membrane, cease to deposit matter in this part of the tooth, 

 for these secreting organs have still to give birth to the roots. 



The above detailed analysis of the most complicated kind of dentary capsule, en- 

 ables us to pass rapidly over those destined solely to secrete the ivory and enamel, 

 or the ivory and cortex, or the ivory alone, and being consequently of a more simple 

 structure. 



Those capsules intended to form teeth composed of ivory and enamel alone, are 

 not, on that account, deprived of the external membrane, but this body always ap- 

 pears to be thinner, instead of being thick, as in the preceding kind of teeth, when 

 about to deposit the cortex. It is raised with difficulty and by shreds, and seems 

 only to be intended to protect the function of dentition ; it accordingly envelopes the 

 organ in every part. The enamellating membrane presents itself in these capsules 

 with all the general characters which have been assigned to it. The bulb does not 

 differ from that belonging to teeth composed of three substances. 



V*''ith respect to those teeth which are composed of ivory and cortex, such as the 

 molai'S of Cachalots, we also find the external membrane in thera to be of a cer- 

 tain thickness, in addition to the bulb, which is never wanting. 



Having thus shown that the dentary capsule of the most complicated kind of teeth, 

 product's three distinct and different substances which can be accurately separated 

 from each other, it now remains for us to consider the secreted bodies themselves, 

 composing, as they do, the proper substance of the teeth. 



The ivory forms the essential and fundamental part of the tooth. As it covers 

 the organ by which it is secreted, it is deposited from without, inwards, and does 

 not appear to be absolutely identical in all kinds of teeth. In some, as the tusks of 

 the Elephants for example, it is deposited by concentric beds, in such a manner 

 that they are composed of cones, the one encasing another, and being numerous in 

 proportion to the length of the tusk. This conical appearance is especially shown 

 in fossil tusks, as the cones themselves do not appear to have been separated artifi- 

 cially. Other teeth have a more homogeneous kind of ivory, but the differences of 

 texture which this substance presents are very numerous. These tusks of the Ele- 

 phants show on their transverse sections a number of segments of circles regularly 

 disposed, which intersect each other, and form a waving mark, by which the true 

 ivory may always be recognised. The teeth of Man, the Quadrumana, and Carnas- 

 siers, possess an ivory of a silky appearance, apparently composed of fibres. Those 

 of the Cetacea, the tusks of the Hippopotamus and others, have their ivory simple, 

 and of the most uniform texture; those of the Rat-Moles {Bathytrgus) seem 

 formed of longitudinal and parallel fibres, like those of a rush. These charac- 

 ters arise doubtless from the peculiar structure of the bulbs which secrete these dif- 

 ferent kinds of ivory ; yet their essential differences have not been determined by 

 experiment, but will probably be ascertained hereafter, when these bulbs are sub- 

 mitted to a more minute investigation. 



This central part, being the most important and considerable portion of the crown 

 of the teeth, is chiefly furmed of a very compact gelatinous substance. The calca- 

 reous matter which gives it the external appearance, is merely deposited in the meshes 

 of this substance, and composes only the smallest portion. It may be abstracted by 

 means of a small quantity of dilute acid, and the gelatine remains pure, and of the 

 same form as the ivory. This calcareous matter, the only part of the tooth really 

 destitute of life, is a phosphate of lime. 



The Enamel is deposited in a manner contrary to the ivory, being from within, 

 outwards, and always immediately over the latter; this it appears to do by a kind of 

 crystallizing piocess. On being examined upon a section of a tooth, it is found to 



