Tooth (gear)

“ The tooth, which previously seemed to be on the eve of piercing the gum, after the extraction of the milk incisor never came up,—nor will any person who has thought for an instant, wonder if the violence, necessarily used, does occasionally injure or rupture the delicate vessels and gelatinous tissues of the pulp. More often the breeder’s impatient interference breaks the tooth off at the neck and leaves the fang in the jaw. ”

“ This hollow centre may be seen occasionally filled up by a sharp conical tooth like the phragmocone of a belemnite.
The lower portion of the tooth exhibits a much more complicated texture. Externally it is marked by deep longitudinal furrows, that run down the enamelled sides and sink into the jaw. When cut across at this ribbed part, the tooth is found to present the most complex and graceful internal structure. ”

“ If, owing to the chucking of a horse’s head, or to a fall, or to other accident, a tooth gets knocked out of the head of a youth of ten or twenty years, if the tooth be at once replaced in its socket and bound to the teeth on each side of it, it will certainly take root again, though it will remain somewhat weaker than its neighbours ”

“ Within the mouth there are many teeth. I will not now speak of their exact number, nor of some other particulars concerning them, because I mean to return to them presently; but I look only at their general structure and origin. Each tooth consists of three distinct parts, the central portion, which is ivory; the exceedingly hard, polished, glassy coat of the crown, which is enamel; and a thin layer of bone around the fang, which is the cement.
Before either of these appeared, a minute papillary process of vascular pulp was formed in a cavity of the jaw. ”

“ A milk tooth, says Brunner, ought never to be extracted unless there be manifest signs of the presence of the corresponding permanent tooth, or when it is painful and decayed. Badly grown teeth can often be put in order solely by the pressure of the fingers frequently repeated, but when this is not sufficient, one must have recourse to waxed threads or to special contrivances. ”

“ The alveolar border swells slightly where it forms the dental base; the outer side of the tooth is sculptured like the rest of the bone, but in a less or finer degree (see the magnified view, fig. 5, a) . At rather less than a line in advance of this tooth is a minute one like the fourth ; in advance of this is the base of a larger denticle (ib. b) , the fracture of which shows a cavity filled by pyritic matrix; and at a line in advance of this is the fractured hollow base of a smaller denticle: these hollows might at first sight be mistaken for sockets. ”

“ In the cheek-teeth the sectorial or scissor-like cutting function is developed at the expense of the tubercular or grinding, there being only one rudimentary tooth of the latter form in the upper jaw, and none in the lower. They are, however, sufficiently strong to break bones of large size. ”

“ Then they fall to with their spades, cutting the flesh away from the lower part of the tooth and loosening it, and completing the extraction by main strength, very much after the fashion of a dentist, but by means of the falls instead of forceps. Often the loop slips off of the tooth suddenly, letting down the men who are swaying away on the falls, and starting shouts of laughter. Or the whole strip of teeth may come together, held together by the gums. When the teeth have all been drawn the jaw is sawed into slabs for convenient use, for the jawbone is very hard and close-grained. ”

“ Both the mandibular and maxillary tooth-rows are relatively longer than in the generalized skull, providing a longer block for the planing action of the lower molariform teeth. All teeth, especially P4 and M3, are longer. In M3 the heel (posterior loph) in particular is elongated. Both the anterior and posterior enamel plates usually are retained in M1 and M2. ”

“ The inside of the tooth is not quite so dead as the outside—one sometimes gets the impression during a visit to the dentist that it isn’t dead at all. The tooth, inside the enamel, is mostly bone; and bone is mostly lime, like clam-shells, mortar and chalk, plaster, and the great boulders and ledges of rock in a limestone country. The rest of the bone is living substance, scattered cells far apart from one another with long roots, that look as if they had grown out into the bone like tree roots into the soil. Really, however, it is the other way. Before there were bones, there were bone cells. ”

“ As the tooth wears, the surface, besides the external enamel layer as in an ordinary simple tooth, shows in addition a second inner ring of the same hard substance surrounding the pit, which adds greatly to the efficiency of the tooth as an organ for biting tough, fibrous substances. ”

“ As for our teeth, the hard white enamel on the outside is just about as much alive as a clam shell. The baby tooth, as I have already explained, is formed in a little pocket in the gum. The inner part of the tooth grows up from the cells at the bottom, very much as a hair grows out from the bottom of the still smaller pocket where it starts. In fact, the tusks of pigs and the long front gnawing teeth of squirrels and rats are still more like hairs, for they keep growing from the root, and wear off at the outer end. ”

“ This peculiarity in the manner of teeth-shedding is characteristic of some reptiles, each of whose teeth is merely a hollow cone tilled in the interior with a soft pulp which supplies the material for the external bony layer. When the tooth becomes worn and useless, a new one is formed beneath the shell of the first by the pulp in the interior, which gradually ousts the old from its socket. ”

“ In like manner, the horses and cows and sheep would soon wear their teeth too smooth to be of any use, if they did not have a way of making them once more rough. In these grinding, millstone teeth, the hard enamel, instead of being on the outside of the tooth as with us, is doubled into folds and plates, and mixed in with the bone of the tooth like the streak of fat and streak of lean in bacon, or like the two colors in marble cake. So the bone wears down faster than the enamel, and leaves the enamel standing up in sharp ridges. ”

“ In the jaws of a very young individual these plates may be seen as separate parts. As the tooth continues to grow, the plates become cemented together, and when the ends of the plate are worn down it may be observed that each consists of a layer of enamel surrounding a flat central core of dentine. The type of construction is rather more obvious in the mammoth tooth than in that of a horse, partly because of the larger size, and partly because of the relative simplicity of construction. ”

“ The hard parts of a tooth commonly enclose a central pulp-cavity into which projects the pulp, a papilla of the dermis including blood-vessels and nerves. As long as growth continues the outer layers of this pulp become successively calcified and added to the substance of the dentine. In young growing teeth the pulp-cavity remains widely open, but in mammals the general rule is that as a tooth gets older and the crown becomes fully formed, the remainder of the pulp becomes converted into one or more tapering roots which are imbedded in the alveolar cavities of the jaws. ”

“ There is, moreover, a greater roughening of the outer surface of the enamel, which on the worn disk produces a faint crimping not generally observed in northern specimens; otherwise the tooth is un distinguishable from the same molar of the mammoth.
3. Two fragments of germ molars:—
a. A fragment of an upper tooth, holding 8–9 posterior ridges, including a broad, flat, digitated talon. The plates are of precisely the same character as in the preceding tooth, but longer and broader. ”

“ Were the teeth to be without this power, the animal would run a great risk of dying from hunger, the injured tooth not being able either to do its own work, or to aid its companion of the opposite jaw. Either tooth alone would be as useless as a single blade of a pair of scissors.
There is another notable characteristic of these incisor teeth. If you will examine the incisors of any rodent, whether it be a rat, a mouse, a rabbit, or a beaver, you will see that the tips are "bevelled" off just like the edge of a chisel. This shape is absolutely necessary to keep the tooth in working order. ”

“ A clear view of the tooth and its surroundings can be obtained during the whole period of removal.
(2) Force can be applied with greater advantage.
(3) The alveolus can be easily embraced by the fingers, or by the finger and thumb of the left hand.
(4) In removing the tooth from the socket a slight backward movement can be employed.
One disadvantage of shape fig. 29 is that it is difficult to employ much inward movement, and therefore, for teeth lying inwards, namely, with the crown directed towards the tongue, hawk’s-bill-shaped forceps cannot easily be used. ”

“ The mode in which the horse cuts his teeth remains to be pointed out. It is generally said, that the tooth, originally growing in its circumscribed cavity, pushes its way through the parts which oppose its entrance into the mouth. So mechanical an idea is characterized by a small amount of physiological knowledge, and it is time the notions which attributed development to mere force were discarded. The tooth is first a pulpy mass, very soft and highly vascular; the superior part, or that part which primarily shows itself above the gum, is the first to become consolidated. ”

“ When the tooth is even, or nearly so, with the others on each side of it, the best instrument will be the hawk’s-bill forceps, of very small size, and with narrow blades. The instrument being fixed as low on the neck of the tooth as possible, a gentle but firm movement is to be made forwards, so as just to separate the organ from the back part of the alveolar cavity, and then, continuously with this motion, the tooth is at once to be raised out of the socket. ”

“ They appear worn not so much on the outer edge or line of the tooth, as inside this line; but, after this, the edge begins gradually to lose its sharpness, and to present a more flattened surface; while the next outer teeth wear down like the four central ones; and at three months this wearing off is very apparent, till at four months all the incisor teeth appear worn, but the inner ones the most. ”

“ The first four teeth, being cutting teeth (incisors) , are called by the Greeks tomici. These are flanked on both sides by one canine. Beyond this there ordinarily exist, on both sides, five grinders, except in the case of those persons in whom the last molars, which commonly are cut very late, have not yet appeared. The incisors and the canines are fixed with one single root; but the molars at least with two, some even with three or four. In general, the shorter the tooth, so much the longer is its root. A straight tooth commonly has a straight root, a curved tooth has it generally curved. ”

“ The incisor teeth of the horse have all the great peculiarity, not found in the teeth of any other mammal, and only in the Equidæ of comparatively recent geological periods, of an involution of the external surface of the tooth, by which what should properly be the apex, is carried deeply into the interior of the crown, forming a pit, the bottom of which becomes partially filled up with cement. ”

“ At this age the marrow is very considerable and the tooth being small in proportion, it is easy to raise the tooth without injuring it, because it has as yet no root and the opening at the base of the crown is very large. The tooth being raised and the marrow thus laid212 bare, if it is irritated in any way the animal gives signs of the most acute pain. I have often made this experiment, always easily done, on account of the want of thickness of the osseous layers which then form the sockets. ”

“ If a tooth is forced out it should if possible be immediately replaced.
(c) Passage of a foreign body through the isthmus of the fauces.—A foreign body, such as a tooth, a broken piece of forceps or a prop, passing through the isthmus of the fauces may become impacted in either the air or food passages.
In the air passages it may lodge (1) over the entrance of the larynx, (2) in the larynx, (3) in the trachea or bronchus.
In the food passages it may lodge (1) in the pharynx, (2) in the œsophagus, (3) at the pyloric opening of the stomach. ”

“ The visitor should, if possible, obtain a view of the teeth, which are sufficiently powerful to bite through the hardest woods, such as ebony or teak—woods which woefully try the temper of our steel-made tools. In the front of the jaw there are four incisor teeth, chisel-shaped, like those of the rabbit and other rodents, but of enormous depth and thickness. Like the rodent teeth, they are furnished with a pulpy substance at their root, from which fresh tooth-substance is developed, so as to supply new tooth from behind as fast as it is worn away in front. ”

“ The young have milk teeth, and in the upper jaw the last crushing or molar tooth has large cusps behind and projecting inwards, and the incisor teeth are equal in size. In the lower jaw the incisors are grooved in front in a curious manner; and the great molar teeth with five cusps have a curiously wrinkled-looking surface. Underneath the skull looks long, and the hole for the spinal cord is much longer than broad, and the joints or condyles are distant from the front of it. The palate is broad and the nose cavity also, and there is a bony styloid process connected with the ear-bone. ”