Fig. 438.—Tooth Structure.
1. Longitudinal section of superior canine tooth, exhibiting general arrangement, and contour markings, slightly magnified; 2 and 3. Portions from same, highly magnified, showing the relative position of bone-cells, cementum at 2, dentine fibres, and commencement of enamel at 3; 4. Dentine fibres decalcified; 5. Nasmyth’s membrane separated and the calcareous matter dissolved out with dilute acid; 6. Cells of the pulp lying between it and the ivory; 7. A transverse section of enamel, showing the sheaths of fibres, contents removed, and magnified 300 diameters.
Czermak discovered that the curious appearances of globular conglomerate formations in the substance of dentine depend on its mode of calcification and the presence of earthy material; and he attributed the contour lines to the same cause. Contour markings vary in intensity and number; they are most abundant in the root, and most marked in the crown. Vertical sections exhibit them the best; as [Fig. 440], No. 1. In preparing a specimen, first make the section accurately, then decalcify it by submersion in dilute hydrochloric acid; dry it and mount in Canada balsam; place the specimen in the hot chamber for some time to soak in the fluid resin before it cools. The white opacity at the extremity of the contour markings gives the appearance of rings to the tooth-fang.
“The tooth-substance appears,” says Czermak, “on its inner surface, not as a symmetrical whole, but consisting of balls of various diameter, which are fused together into a mass with one another in different degrees, and in which the dentine tubes in contact with the germ cavity terminate. By reflected light, dark-ground illumination, one perceives this stalactite-like condition of the inner surface of the tooth-substance very distinctly, by means of the varied illumination of the globular elevations, and by the shadows which they cast.” To see this structure to advantage the preparation should be made from a tooth root, the growth of which is not complete. With such preparations, the ground-substance of the last formed layer of the tooth-substance is seen to be, at least partly, in the form of globular masses, fused together with those of the penultimate layers.
The cementum is the cortical layer of osseous tissue, forming an outer coating to the fangs, which it sometimes cements together. Its internal surface is intimately united with the dentine, and in many teeth it would appear as if the earliest determined arrangement of the fibres of the dentine started from the canaliculi, as they radiate from the lacunæ in the cement. The inter-lacunar layer is often striated, and exhibits a laminated structure: sometimes it appears as if Haversian canals were running in a perpendicular direction to the pulp cavity. The canaliculi frequently run out into numerous branches, connecting one with another, and anastomising with the ends of the dentine fibres. The thick layers of cement which occur in old teeth show immense quantities of aggregated lacunæ of an irregular and elongated form.
Fig. 439.—Transverse section of Tooth of Pristis, showing orifices of medullary canals, with systems of radiating fibres (tubuli) analogous to the Haversian canals in true bone.
Compact Tissues, Cartilage and Bone.—Cartilage is a bluish or yellowish-white, semi-transparent, elastic substance, without vessels or nerves, and surrounded by a membrane, termed pericondrium, of a dense fibrous nature. That kind, however, known as articular cartilage, receives a layer of epithelium from the synovial membrane, but this is confined to marginal portions, in consequence of the central wear which occurs as soon as the parts are subjected to friction, during the movement of the limbs. Cartilage covers the ends of all bones in apposition to form joints, and thus lessens the effects of concussion. Besides the ordinary kind of cartilage, temporary and permanent, there are two modifications of the tissue, confined to certain portions of the body: cellular cartilage, composed of cells lying close together, in a mesh formed of fine fibres; and fibro-cartilage, cells distributed in a matrix of fibrous tissue.
Examined with a low power, cartilage appears to be homogeneous in structure, studded over with numerous round, oval, oblong, semilunar, and irregular-shaped corpuscles, as seen in [Plate XIX]., No. 8, a vertical section of animal cartilage, arranged in columns, and condensed at the lower surface previous to its conversion into bone. The greater opacity of this portion is owing to the increase of osseous fibres, and the multiplication of oil globules, and the intercellular spaces becoming filled with vessels. No. 9 shows a small transverse section of the same, with a further change of the cartilage cells at a into bone cells, and at b with the characteristic canaliculi and lacunæ. No. 7 further shows a section of the large tendon fixed to the back of the heel of the foot, near the juncture of the tendo-Archillis with the cartilage. For the examination of these several changes a high power is necessary, and for the purpose pieces taken from the ox may be easily obtained from the butcher, and fine sections cut with a razor parallel to the surface.
