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and in 58 per cent. of European, Semitic, and Egyptian skulls; according to Zuckerkandl, in 735 per cent. of the lower races and 45.6 per cent. of Europeans.) The third upper molar has three tubercles much more frequently than four amongst Europeans (four only in 36 per cent., although it has four more frequently in certain lower races). It should be remarked that, while there are practically always four tubercles in the first molar, still there is a tendency to the disappearance of the postero-lingual one, which tendency grows more pronounced as we pass backwards to the second and third molars. The other tubercles are practically constant.
The three roots of the upper molars (Figs. 881, 882, and 885) are, a large palatine, subcylindrical in shape, and two labial roots, smaller and flattened from before backwards. The palatine root, which is placed opposite the posterior labial root, is often united to one of the others. The lower part of the maxillary sinus generally extends down between the palatine and the two labial roots (Fig. 879, p. 1113), but the latter project on its floor more frequently than the palatine root. In the last molars the three roots are frequently more or less united into a single conical process (Fig. 881).
Lower Molars.-The crowns are more massive than those of the upper molars, and are elongated antero-posteriorly (Fig. 884). A crucial groove separates the four chief tubercles from
one another; this bifurcates behind to enclose the fifth, which lies slightly to the labial side of the middle of the tooth. The number of tubercles present in the lower molars is as follows:The first has usually five (62 per cent. of all races, 61 per cent. of Europeans); the second has four, as a rule (five in only 24 per cent. of all skulls); the lower dens serotinus has four a little more frequently than five (five in 46 per cent. of all skulls), but like the upper last molar tooth it is extremely variable.
The roots of the lower molars (Fig. 881), two in number, are flattened from before backwards, and very wide. The anterior of these has two root-canals; the posterior but one (Fig. 885). The dens serotinus has commonly two roots like its fellows; occasionally the two are united. In determining the side to which a lower molar belongs, it should be remembered that the deep part of the root is generally curved backwards, and also that the blunter margin of the crown (see above) and the fifth tubercle, if present, are on the labial side.
Arrangement of the Teeth in the Jaws.-The teeth are arranged in each jaw in a curved row-the arcus dentalis-of approximately a semi-oval form (Figs. 884 and 885). The curve formed by the upper teeth, arcus dentalis superior, however, is wider than that formed by the lower set, arcus dentalis inferior, so that when the two are brought in contact the upper incisors and canines overlap their fellows in front, and the labial tubercles of the upper premolars and molars overlap the corresponding ones of the lower teeth (Fig. 886, p. 1120). It will also be seen that, as a rule, the teeth in one jaw are not placed exactly opposite their fellows, but rather opposite the interval between two teeth, in the other jaw
STRUCTURE OF THE TEETH.
As mentioned above, the teeth are composed of three special tissues, substantia adamantina or adamant (O.T. enamel), substantia eburnea or ivory (O.T. dentine), and substantia ossea (O.T. crusta petrosa or cement), in addition to the pulp which occupies the tooth cavity. The chief mass of the tooth is formed of substantia eburnea, which surrounds the tooth cavity and extends from crown to root; outside this is a covering of substantia adamantina on the crown, and a layer of substantia ossea on the root.
The substantia adamantina is the dense, white, glistening layer which forms a cap, thickest over the tubercles, for the portion of each tooth projecting above the gum (Fig. 888). At the neck it ceases gradually, being here slightly overlapped by the substantia ossea.
It is composed chiefly of phosphate and carbonate of lime (phosphate of calcium, 89-82 per cent, carbonate of calcium, 4:37 per cent., magnesium phosphate, 1.34 per cent., a trace of calcium fluoride, other salts, 88 per cent.), and has generally been considered to contain about 36 per cent. of organic cavity substance; but Tomes has recently shown this to be inaccurate: "That which has heretofore been set down as organic matter is simply water combined with the lime salts. The substantia adamantina is to be regarded as an inorganic substance composed of lime salts, which have been deposited in particular patterns and formed under the influence of organic tissues, which have themselves disappeared during its forma
The adamantine substance consists of calci fied microscopic prisms, prismata adamantina radiating from the surface of the ivory, on which their inner ends lie, to the surface of the crown, on which they terminate by free ends. These prisms are hexagonal in shape, solid, and of considerable length, for most of them reach from the ivory to the surface of the crown without interruption. The prisms, which are calcified themselves, are held together by the smallest possible amount of calcified matrix (Tomes). In old teeth the cap of adamantine substance is often worn away over the tubercles. the ivory is then exposed, and is easily recog nised by its yellowish colour, which contrasts strongly with the whiteness of the adamant. Whilst adjacent adamantine prisms are in general parallel to one another, they do not usually take a straight, but rather a wavy course, and in alternate layers they are often inclined in opposite directions, thus giving rise to certain radial striations seen by reflected light (Schreger's lines). Certain other pigmented lines, more or less parallel to the surface, are also seen in the adamant (brown striæ of Retzius). They are due to true pigmentation (Williams), and mark the lines of deposit of the adamant during its develop ment. The adamantine prisms are more or less tubular in certain animals-viz., in all marsupials except the wombat, in the hyrax, certain insectivora, and certain rodents.
Alveolar periosteum or root-membrane
FIG. 888.-VERTICAL SECTION OF CANINE TOOTH to illustrate its various parts, and its structure.
Cuticula dentis (O.T. Nasmyth's membrane) is an extremely thin (2000 of an inch) cuticular layer which covers the adamant of recently-cut teeth, and is very indestructible, resisting almost all reagents. Two chief views are held as to its origin. One that it is the last formed layer of adamant, which has not yet been calcified, and therefore the final product of the adamant cells. The other that it is produced by the outer layer of cells of the adamant organ. This latter seems to be the more probable view.
Substantia eburnea or ivory (O.T. dentine) is the hard and highly elastic substance, yellowish white in colour, which forms the greater part of the mass of every tooth (Fig. 888). Like the adamant it is highly calcified, but it differs from it in containing
a very considerable amount of organic matter and water incorporated with its salts, which are chiefly phosphate and carbonate of lime.
Fresh human ivory contains 10 per cent. of water, 28 per cent. of organic and 62 per cent. of inorganic material. The organic matter is composed chiefly of collagen, and to a less extent of elastin. The organic matter consists of (1) calcium phosphate (with a trace of fluoride), (2) calcium carbonate, and (3) magnesium phosphate, the percentages present in dried dentine being 66-72, 3:36, 108, respectively.
Ivory consists of a highly calcified organic matrix, which is itself practically structureless, although everywhere traversed by tubes-the canaliculi dentales, or dentinal canaliculi-which give to this tissue a finely striated appearance, the striæ usually running in wavy lines. The canaliculi begin by open mouths on the wall of the pulp cavity, whence they run an undulating, and at the same time a somewhat spiral course, towards the periphery of the ivory. They give off fine anastomosing branches, and occasionally divide into two. Somewhat reduced in size, they usually end in the outer part of the ivory.
The canaliculi dentales are generally described as being lined by special sheaths (dentinal sheaths of Neumann) which are composed of a most resistant material, and possibly are calcified. It should be mentioned that the presence of these sheaths as separate structures is doubted by some authorities, who hold that the part described as the sheath is only a modified portion of the ivory which forms the tubules.
The canaliculi dentales are occupied by processes, prolonged from the outermost cells of the pulp the odontoblasts. These processes are called after their discoverer, Tomes' fibrils (dentinal fibrils), and they are probably sensory in function.
The concentric lines of Schreger, frequently seen in the ivory, are due to bends in successive canaliculi along regular lines running parallel to the periphery of the ivory. Other lines (the incremental lines of Salter), due to imperfect calcification, are found arching across the substance of the ivory, chiefly in the crown. There must also be mentioned the interglobular spaces, intervals left in the ivory, as a result of imperfect calcification, bounded by the fully calcified surrounding tissue, the contour of which is in the form of a number of small projecting globules. These interglobular spaces are very numerous in the outer or "granular layer" of the ivory, particularly beneath the osseous substance.
The substantia ossea (O.T. cement) is a layer of modified bone which encases the whole of the tooth except its crown. It begins as a very thin stratum, slightly overlapping the adamant at the neck. From there it is continued, increasing in amount, towards the apex, which is formed entirely of this substance. It is relatively less in amount in the child, and increases during life. In places the ivory seems to pass imperceptibly into the substantia ossea, the "granular layer" marking the junction of the two, and some of the canaliculi dentales are continuous with the lacunæ of the substantia ossea. Like true bone, it is laminated, it possesses lacunæ, canaliculi, and, when in large masses, it may even contain a few Haversian canals.
The pulpa dentis occupies the tooth cavity and the root-canals of the teeth. It is composed of a number of branched connective tissue cells, the anastomosing processes of which form a fine network, containing in its meshes a jelly-like material, in addition to numerous vessels and nerves, but no lymph-vessels. The most superficial of these cells are arranged in the young tooth as a continuous layer of columnar, epitheliumlike cells, lying on the surface of the tooth pulp against the ivory; they are known as odontoblasts, for they are the active agents in the formation of the ivory. From the outer ends of these odontoblasts processes are continued into the canaliculi dentales, where they have been already referred to as Tomes' fibrils. The vessels of the tooth pulp are numerous, and form a capillary plexus immediately within the odontoblasts. The nerves form rich plexuses throughout the pulp, but their exact mode of ending is unknown.
The periosteum alveolare is a layer of connective tissue free from elastic fibres, but well supplied both with blood-vessels and nerves, which fixes the root of the tooth in the alveolus, being firmly united by perforating fibres of Sharpey, to the substantia ossea on the one hand, and to the bone of the alveolus on the other. It establishes a communication between the bone of the jaw and the substantia ossea, and it is continuous with the tissue of the gum. Its blood comes chiefly from the arteries, which subsequently enter the apical foramina for the supply of the tooth pulp, but in part also from the vessels of the surrounding bone and of the gum (hence the relief obtained in dental periostitis by lancing the gum).
The tongue is a large mobile mass, which occupies the floor of the mouth and forms the anterior wall of the oral part of the pharynx (Fig. 889). It is composed chiefly of muscular tissue, and is covered by mucous membrane.
Whilst the sense of taste resides chiefly in its modified epithelium, the tongue is also an important organ of speech, and, in addition, it assists in the mastica
FIG. 889.-SAGITTAL SECTION THROUGH MOUTH, TONGUE, LARYNX, PHARYNX, AND NASAL CAVITY. The section was slightly oblique, and the posterior edge of the nasal septum has been preserved. The specimen is viewed slightly from below, hence the apparently low position of the inferior
tion and deglutition of the food-functions which it is well fitted to perform. owing to its muscular structure and great mobility. In length it measures about three and a half inches (9 cms.), when at rest, but both its length and width are constantly varying with every change in the condition of the organ, an increase in length being always accompanied by a diminution in width
and vice versa.
In describing the tongue we distinguish the following parts: the corpus lingua (body), made up chiefly of striped muscle, and forming the mass of the organ; the dorsum linguæ (Fig. 890), which looks towards the palate and pharynx, and is free
in its whole extent; the base, the posterior wide end which is attached to the hyoid bone; the apex linguæ, the pointed and free anterior extremity; the margo lateralis, which is free in its anterior half or more, i.e. in front of the attachment of the anterior palatine arch (Fig. 890). Finally, the unattached portion on the inferior aspect, seen when the apex is turned strongly upwards (Fig. 892), constitutes the facies inferior, or inferior surface; whilst the thick posterior portion, fixed by muscles and mucous membrane to the hyoid bone and mandible, is known as the radix linguæ or root.
The dorsum of the tongue, when the organ is at rest, is strongly arched antero-posteriorly in its whole length (Fig. 889), the greatest convexity corresponding to the attachment of the glosso-palatine arch. When removed from the body Hypoglossal nerve
Internal jugular vein
FIG. 890.-HORIZONTAL SECTION THROUGH MOUTH AND PHARYNX AT THE LEVEL OF THE PALATINE TONSILS. The stylopharyngeus muscle, which is shown immediately to the medial side of the external carotid artery, and the prevertebral muscles, are not indicated by reference lines.
the tongue, unless previously hardened in situ, loses its natural shape, and appears as a flat, elongated oval structure, which gives a very erroneous idea of its true form and connexions.
Both in structure and in function, as well as in embryological history, the dorsum linguæ is divisible into two areas-an anterior or oral part, which lies nearly horizontally on the floor of the mouth, and constitutes about two-thirds of the length of the whole tongue (Fig. 890); and a posterior or pharyngeal part, the remaining third of the organ, which is placed nearly vertically, and forms the anterior wall of the oral pharynx (Fig. 889). The separation between these two parts, which differ in appearance as well as in direction, is indicated by a distinct. V-shaped groove, called the sulcus terminalis (Fig. 890), the apex of which is