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.
(2260). In the male Narwal (Monodon) there are no teeth implanted ' along the margins of the jaws; but from the intermaxillary bone of the left side of the face there projects a single tusk of great strength, which sometimes attains the length of eight or ten feet. This formidable weapon is fully developed only upon one side of the body; nevertheless the corresponding tooth exists in a rudimentary condition, enclosed in the opposite intermaxillary bone.
Fig. 397. Teeth of the Porpoise.
* Mr. Hunter means by "intermediate," interposed between the contiguous plates, not between the "hair" and the laminated whalebone.
1 Preps. Nos. 327 & 328.
2 'The Animal Œconomy,' by John Hunter, with Notes by Richard Owen, Esq., F.R.S., p. 353. London, 1837.
(2261). In the Elephant (a creature which so obviously forms a connecting link between the gigantic Cetacea and terrestrial quadrupeds) tusks, more ponderous even than that of the Narwal, project from both intermaxillary bones: but these, as well as the tusks of other Pachydermata, grow upon a simple pulp, such as that which forms the teeth of the Bottle-nose Whale; are composed of ivory, without any enamel; and their growth is only limited by the abrasion to which they are subject.
(2262). In by far the greater number of quadrupeds the teeth present a more complex structure, and consist of two distinct substances of very different texture - the one analogous to the ivory of the simple teeth described in the last paragraph, the other, called enamel, of crystalline texture and such extreme density as to withstand being worn away by acting upon the hardest materials used as food. Teeth of this description may be advantageously divided into two principal groups: first, those whose growth is continuous during the entire lifetime of the animal; and second, those which are completed at an early period, and then cease to grow.
(2263). The first division includes the incisor teeth of the Rodentia, or dentes scalprarii, as they have been termed. Such teeth are, in fact, chisels of most admirable construction, destined to gnaw the hardest kinds of food, and yet never to all appearance wearing away or becoming blunted by use.
(2264). The annexed figure (398) represents a section of the incisor tooth and of the left ramus of the lower jaw of a Porcupine (Hystrix cristata); and from this example the structure of such teeth will be readily understood. The bulk of the tooth consists of solid ivory (a), which in its texture and mode of growth resembles that of a simple tusk, being continually growing from behind by the addition of new matter produced from the vascular pulp (c); so that, were such a tooth not worn away constantly at the point, it would curl up over the face like the tusk of the Babiroussa: and if by accident the opposing tooth in the upper jaw should be broken off, this circumstance in fact really takes place.
Fig. 398. Growth of incisor tooth in the Porcupine.
(2265). But, besides the ivory-forming pulp (c), there is a vascular membrane which exists only upon the anterior surface of the socket, its limits on each side being distinctly marked by a defined line. This membrane secretes enamel, and coats the convex surface of the tooth with a thin layer (b) of that dense substance. From this beautiful arrangement it results that, while the anterior end of the tooth is perpetually worn away by attrition against hard substances, the ivory is abraded more rapidly than the enamel that coats it in front; thus, therefore, the tooth constantly preserves its chisel-like shape, and presents the sharp cutting-edge formed by the layer of enamel.
(2266). The second kind of teeth, composed of bone and enamel, are limited in their growth; and the entire crown or projecting portion is invested with enamel covering its surface. The teeth of all the Carni-voka, of the Quadrumana, and also of Man, are of this description. From marked differences in their form in different regions of the mouth, such teeth are conveniently divisible into different groups, called respectively incisores, laniares or canine teeth, pseudo-molares or false grinders, and molares or grinding teeth.
(2267). Whatever may be the shape of teeth of this class, their mode of growth is similar to that observed in those of our own species. We have chosen, in order to illustrate this, the growing permanent teeth of a young Lion, wherein the different organs employed in their formation are easily distinguishable. The ivory that forms the bulk of the tooth (fig. 399, b) is formed by the surface of an internal pulp (a); and as it slowly accumulates, encroaching upon the central cavity, and penetrating more deeply into the socket, the fang is gradually formed, and the central pulp shrinks until, in the fully-formed tooth, it becomes reduced to a thin membrane richly supplied with vessels and nerves, which lines the small central cavity that remains.
Fig. 399. Growing teeth of a young Lion.
(2268). Before the progressively advancing tooth issues from the nidus wherein it is produced, the enamel is deposited upon the surface of the ivory by the lining membrane of the capsule (c), and becomes arranged in crystalline fibres placed perpendicularly to the surface of the ivory, until the whole crown of the tooth is adequately coated with this important additional substance. Meanwhile the growth of the tooth still proceeds by the lengthening of its root, until at last the crown issues from the jaw, and the enamel-secreting membrane (c)becomes obliterated.
(2269). The most complex condition of the dental organs is that found in the molar teeth of herbivorous quadrupeds, which, being destined to act the part of millstones in grinding down and comminuting vegetable substances, must necessarily, like the millstones of human contrivance, have a grinding surface, presenting prominent edges and deep sulci, not liable to become worn even by the continual abrasion to which they are subjected. In order to attain this end, the ivory and enamel interdigitate, as it were, in the substance of the tooth, and are, moreover, imbedded in a third material, not met with in the simpler forms, called the cementum or crusta petrosa. In consequence of this arrangement, seeing that the plates of ivory, of enamel, and of cement are all of different degrees of hardness, the softer substances are most easily worn away, and thus these compound teeth always offer an efficient grinding surface.