[580] See the beautiful exemplification of this subject, and the comparison between the transitory stages of the human teeth in their progress of development, discovered by Mr. Goodsir, with the permanency of these conditions in reptiles. Odontography, p. 182.
Lign. 205. The Lower Jaw of an Iguana.
(From Barbadoes.)
| Fig. | 1.— | The right branch of the lower jaw of an Iguana; viewed on its inner aspect. Nat. size. a. Dentary bone. b. Opercular bone. c. Complementary or coronoid bone. d. Surangular bone. e. Angular bone. f. Articular bone. |
| 2.— | The external aspect of the same. | |
| 3.— | 3.—Inner aspect of three teeth (magnified) attached to the alveolar parapet; with the germ of a successional tooth at the base of the middle tooth; and the sockets of other germs at the bases of the outer two fully formed teeth. | |
| 4.— | External view of the crowns of three teeth; slightly magnified. |
The intimate structure of the teeth consists of a simple pulp-cavity, surrounded by dentine, which is permeated by extremely minute calcigerous tubes, radiating at right angles to the periphery, or external surface of the tooth. One essential modification of this structure consists in the intermingling of cylindrical processes of the pulp-cavity, in the form of medullary or vascular canals, with the finer tubular structure; as in the tooth of the Iguanodon, [Pl. VI. figs. 4b and 4c]. But another modification is that to which allusion was made when describing the teeth of the Lepidosteus (see [p. 616]); in this mode, the dentine preserves its normal character, but the external cement and surface of the tooth are deeply inflected in longitudinal folds around the entire circumference; and this structure is accompanied with corresponding extensions of the pulp-cavity and dentine into the interspaces of these inflected and converging folds.[581] This organization is shown, in its simplest form, in the transverse section of the base of a tooth of the Ichthyosaurus, [Pl. VI. fig. 9]; and attains its most complicated condition in that of the Labyrinthodon, [Pl. VI. figs. 3a, 3b, 3c].
[581] There is a marked difference between the internal structure of the teeth of true Saurians and of Sauroid Fishes. In the former, as well as in the Enaliosauria, the dentine consists of tubes radiating from a slender central pulp-cavity to the periphery of the tooth, without any intermixture of vascular canals. In the sauroid fish (Dendrodus) the central pulp-cavity is produced into numerous irregular canals, from which vascular sinuses radiate to the periphery, sending off branches generally at right angles throughout their entire course; thus, there is an extensive distribution of the vascular system through the body of the tooth, which does not exist in any saurian reptile; the nearest analogy is in the labyrinthine teeth of the gigantic fossil batrachians. (Owen: Odontography; and Art. Teeth, Cyclop. Anat.)
With regard to the mode of development of the teeth, we must briefly state, that the germ of the new tooth is always produced at the side of the base of the old one; that in its progress of growth it presses against the tooth it is destined to supplant, occasions the progressive absorption of the fang, and ultimately displaces its predecessor; in some instances, by splitting the crown of the tooth; in others, by casting it off, according to the oblique or direct position the new tooth attains in its progress, in relation to its predecessor. Thus, in the teeth of the Crocodile, the new tooth is generally found immediately under the conical apex of the crown, and beneath the former a second successional tooth appears, like a series of thimbles of various sizes placed one upon another; for in reptiles the production of new teeth is unlimited. But in the Pleurodont lizards, the new tooth makes its way obliquely, and the crown is often shed entire. [Lign. 205], fig. 3, exemplifies the situation of the successional teeth in the Iguana.
Lower Jaw of Reptiles.—It is well known that the lower jaw in mammiferous animals is composed of a single bone on each side; and that in many genera these pieces become united in front, and form but one bone in the adult state. But in reptiles, the lower jaw consists of six distinct bones on each side, as in [Lign. 205]; and these undergo various modifications of form and arrangement in the different genera. These bones are distinguished by names which have reference to their office and situation, and are as follow:—[Lign. 205], a, the dentary bone, supporting the teeth; b, the splenial or opercular; c, the coronoid or complementary; d, the sur-angular; e, the angular; f, the articular, which forms the upper portion of the jaw, and includes the condyle. The form and disposition of these bones in the Iguana, and other true lizards, are shown in [Lign. 205]; but they differ materially in the Crocodile, Ichthyosaurus, and other genera. We must restrict our comments to this short notice, which, however, will suffice to enable the collector who discovers a fragment of a lower jaw, with any traces of the structure above described, to determine that it is reptilian; and if any portion of the dentary bone remains, indications may be obtained of the family, and perhaps genus, to which it belonged.[582]