This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2270). By inspecting the accompanying figure (fig. 400) representing a section of the tooth of an Elephant, the disposition referred to will be better understood: the layers of enamel are seen to alternate with plates of ivory, while all the interstices are filled up by the circumfused cementum.
Fig. 400. Structure of the molar teeth of the Elephant.
(2271). During the growth of a compound tooth of this description, the enamel-secreting membranes derived from the capsule of the tooth, of course, interdigitate with the ivory-forming pulps that arise from the bottom of the sockets, and thus the hard materials formed by them take the same arrangement. After these structures have been completed, one or other of the sets of pulps, most probably the enamel pulps, changing their action, fill up all the intervening spaces with the crusta petrosa.
(2272). As during the growth of a quadruped the size of the jaws is continually increasing, a necessity exists for changing the teeth once or oftener during the life of the animal, in order to adapt these organs to the altered conditions required: hence the necessity for shedding the teeth of young animals, and replacing them with others of larger dimensions or more numerous than the first set.
(2273). This is effected in two different ways, each of which demands our separate notice.
(2274). In most quadrupeds, as, for example, in the Carnivora, the Quadrumana, and the greater number of herbivorous genera, the succession of the teeth is provided for precisely in the same way as in our own species, namely, by the formation of a new tooth below each of the deciduous ones (fig. 399, d d); so that when the latter falls out in consequence of the absorption of its fangs, the former is ready to take its place. The germ of the second tooth is at first found imbedded in the jaw-bone, in the immediate vicinity of the roots of the one which it is destined to replace; and as its growth advances, the old and used tooth is gradually removed to make way for the new comer. The steps of this process are exactly similar to those by which the milk-teeth of a child are changed, and the details connected with it are familiar to every anatomist.
(2275). But in the Elephant, and some other genera of Pachydebmata, the succession of the teeth is effected in a different manner, the place of the first-formed being supplied by others that advance from behind as the former become used. Animals exhibiting this mode of dentition have the grinding surfaces of their molar teeth placed obliquely *; so that if they were to issue altogether from the gum, the anterior portion would be much more prominent than the posterior, notwithstanding that the opposed teeth act upon each other in a horizontal plane. The consequence of this arrangement is, that the anterior portion of these teeth is ground down to the roots, and worn away sooner than the posterior portion. Moreover the posterior part of the tooth is considerably wider than the anterior; so that, as the succeeding tooth advances from behind, there is always sufficient room to receive it; and in this way, by the time that the first tooth is quite destroyed and falls out, a new one from behind has already taken its office.
There is, therefore, no absorption of the roots of these teeth, but they are ground down from the crown to the stump.
* Cuvier, Lesons d'Anat. Comp. iii. p. 122.
(2276). The new tooth that thus advances from behind is always of larger dimensions than that to which it succeeds, because the animal itself has grown in the interval, and the jaws have become proportionally developed.
(2277). The Elephant in this way may have a succession of seven or eight teeth on each side in both jaws, or from twenty-eight to thirty-two in all; and nevertheless, seeing that the anterior ones successively fall out, there are never more than two visible at once above the gums on each side, or eight in all; generally, indeed, there is only one visible at a time. Every successive tooth is composed of more laminae than that which immediately preceded it, and a longer time is required to perfect its growth.
(2278). Nearly the same account of this process was found in the Manuscripts of John Hunter *, who lucidly accounts for such an aberration from the ordinary course of proceeding. "These creatures," says that distinguished observer of Nature, "do not shed their teeth as other animals do that have more than one; for those that have more than one tooth can afford to be for some time without some of their teeth: therefore the young tooth comes up in many nearly in the same place with its predecessor, and some exactly underneath; so that the shedding tooth falls sometimes before the succeeding tooth can supply its uses. But this would not have answered in the Elephant; for if the succeeding tooth had formed in the same situation with respect to the first, the animal would have been for some time entirely deprived of a tooth on one side, - or, at least, if it had one on the same side in the opposite jaw, that one could have been of no use; and if this process took place in both sides of the same jaw, and in either jaw, the animal would have been entirely deprived of any use of the two remaining".
(2279). The teeth of Mammalia being thus adapted to so many various offices, and serving, under different circumstances, to hold, to bruise, to cut, to tear, or to grind alimentary substances, we must naturally expect the movements of which the lower jaw is capable to be in correspondence with the nature of the dental apparatus.
(2280). In Man, as the student well knows, in consequence of the laxity of the ligaments that connect the inferior maxilla with the temporal bone, and the thickness of the articular cartilage that is interposed between the convex surface of the condyle and the shallow glenoid cavity, every kind of motion is permitted, in conformity with the omnivorous habits of the human race; and the temporo-maxillary articulation is no longer a mere hinge, but the teeth can be made to act upon each other by rubbing their grinding surfaces in all needful directions. In the Herbivorous quadrupeds these triturating motions are likewise extensive. In the Rodentia the movements of the lower jaw are principally backwards and forwards, thus giving free play to their chisel-like teeth whilst employed in eroding hard substances; and in the Carnivora, where there is no necessity for any grinding motion, the condyle is so locked into a deep and transverse glenoid cavity, that the movements of a hinge only are permitted.