Molar tooth

“ The enamel also has an important part to play in the structure of the molar teeth. Each tooth is surrounded with the enamel plate, which is so intricately folded that the tooth looks as if it were made of a series of enamel triangles, each enclosing the tooth matter.
This structure is common to all the members of the group to which the water-rat belongs. It is the more remarkable because we find a somewhat similar structure in the molar teeth of the elephants, which, like the rodents, have the incisor teeth largely developed and widely separated from the molars. ”

“ Ali Abbas, another great Persian physician (who died in 994) , wrote a lengthy treatise on theoretic and practical medicine, one chapter of which is dedicated to the diseases of the teeth. When a molar tooth is affected by caries, and the pain cannot be subdued in any other way, Ali Abbas applies, inside the carious cavity, the end of a small metallic tube, into which he repeatedly introduces red-hot needles, leaving them123 in the tube until quite cooled. ”

“ It is, however, impossible to examine the molar teeth satisfactorily during life. Far less frequently an incisor tooth becomes loosened; more often it loses its glistening appearance and looks dull and yellowish. ”

“ Each lower molar, in the final stage of wear, consists of a single elliptical column having an enamel plate only on the posterior surface. The first and second upper molars are single elliptical columns having one enamel plate on the anterior surface and another on the posterior surface. The plates are separated by a tract of dentine on each side of the tooth. The third upper molar is partly bilophodont, and the two lophs are separated by a deep outer re-entrant fold. In many of the species an inner re-entrant fold also is retained, but in the adult tooth it is less distinct than the outer. ”

“ Generally speaking, when there is a marked difference between the first premolar and molars at the end of the series, there is a gradual approximation in structure of a progressive kind. The tubercles become more numerous in successive teeth; but the corollary which is apparently deducible from this, i.e. that the last molar is the most elaborate of the series, is by no means always true. The last cheek tooth indeed is often degenerate. On the other hand, it is very markedly the largest of the series in such diverse types as the Elephant, the hog Phacochoerus, and the Rodent Hydrochoerus. ”

“ The shortening of the jaw is still proceeding, and, although in lower races of man the last molar or wisdom tooth is almost as large as the molars in front of it, in the higher races the wisdom tooth is much smaller and frequently does not develop at all, or begins to decay very soon after its appearance. ”

“ It is no rare thing, also, in these days, not only for unprofessional persons, but also for medical practitioners, to fall into errors of this kind, especially because, in similar cases, the wisdom tooth, having but a limited space in which to erupt, is in the habit of filling the void left by the second molar, where it meets with less resistance. ”

“ Among them are anomalies in situation, as in case of the horizontal position of a wisdom-tooth, which, pressing against the third molar, produced inflammation in it; the projection of teeth through the alveolar wall of the maxilla on the anterior or posterior side; exchanges of situation between certain teeth; and irregularities in the number of teeth. Among anomalies in structure are enamel pearls, that is, drops of enamel adhering to the roots, and having no connection with the crown. Anomalies of size are rare; enlargements affect the root more frequently than the crown. ”

“ It proved to belong to E. primigenius and to be the upper right hindermost molar. Inasmuch as it is worn to the base in front and as the large anterior root is missing, some plates, probably at least two, are missing. There are 22 present. The tooth is worn back to the tenth from the rear. The length, as the tooth is preserved, is 275 mm. The height at the tenth plate from the rear is 160 mm., not including the base of the roots. The greatest thickness is 85 mm. On the lateral face are 9 plates in a 100–mm. line. The base of the tooth is straight; the hinder border of the crown, arched. ”

“ The specialization is of this kind. The grinding teeth (or molars) of beasts are divided into premolars and true molars. The premolars are molars which have deciduous vertical predecessors (or milk teeth) , and any which are in front of such, i.e. between such and the canine tooth. The true molars are those placed behind the molars having deciduous vertical predecessors. Now, as a dentition becomes more distinctly carnivorous, so the hindmost molars and the foremost premolars disappear. ”

“ The teeth are thus nourished like other parts of the body. The exposure of the delicate pulp to the air, due to the decay of the dentine, gives rise to the pain of toothache.
Surrounding the cavity on all sides is the hard substance known as the dentine, or tooth ivory. Outside the dentine of the root is a substance closely resembling bone, called cement. In fact, it is true bone, but lacks the Haversian canals. The root is held in its socket by a dense fibrous membrane which surrounds the cement as the periosteum does bone. ”

“ The small twigs which penetrate into the roots of the incisor and canine teeth are less slender than those which enter the roots of the molars, and are easily to be seen not only in large animals, but also in man. If the tooth of an ox or that of a ram be split through the middle, the mucous substance contained in the interior is seen to be traversed by small bloodvessels; and one perceives, besides, certain fibers, which are probably nerves. All these things, says Eustachius, I have observed many times in different animals, in some cases more, in others less distinctly. ”

“ He admits the existence of distinct germs for the enamel and for the dentine. According to him the incisors are formed from three points of ossification, the canines from one, and the molars from three or four. The tooth after its eruption is an extraneous body “with respect to a circulation through its substance, but they have most certainly a living principle by which means they make part of the body, and are capable of uniting with any part of a living body.” The milk teeth, says Hunter, are not shed by a mechanical action of the second teeth, but by an organizing law of Nature. ”

“ The change about to be mentioned affects all the incisors, canines, and first molars of both upper and lower jaws, the bicuspid being scarcely implicated at all; the second molars are also quite healthy. In the first molars the alterations consist of deficiency of enamel on the upper surface of the crown and the presence of spines of uncovered dentine. In the case of the incisors a considerable portion of the crown of each tooth is totally devoid of enamel, and its dentine is also deficient to some extent, so that the teeth are thin, sharp-edged, and of a dirty yellowish color. ”

“ In tracing the lines of these animals backward in time we have made the following discoveries: First, the infoldings of the enamel of the molars become shallower, and are finally represented by the valleys between four hills or tubercles, which stand to each other so as to be inclosed by a square figure. The number of the molar teeth increases. If incisor teeth were absent, they appear. The toes increase in number, becoming five on all the feet. The step becomes plantigrade or flat-footed, the heel reaching the ground. ”

“ 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. ”

“ For that matter, teeth, as is well known, can be extracted, cleaned and put back again, or teeth from one persons mouth can be put into the place of an extracted tooth in another's mouth and become firmly imbedded and do good service for years. The part of the jaw-bone that embraces the roots of the teeth is called the alveolar process, and it continues to grow and harden for some time after the teeth have been erupted, or after they have changed their places in the jaw. ”

“ Fletcher:—The experiments of the men go to show that bone will become organized when ivory will not. The only part of a tooth that may be organized is the cementum. The line of reorganization or absorbtion is a softened tissue, so when the process reaches the crown and is about complete, the crown drops off. If a very fresh tooth is implanted, the membrane may be accepted. ”

“ 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. ”

“ The enamel is restricted to the front of the incisor, so that in gnawing the softer dentine is worn away at the tip faster than the enamel, thus keeping the tooth sharp. There are no canine teeth. Rodents are mostly herbivorous in diet, are usually small, and have clawed digits. ”

“ The enamel of the teeth is formed once for all during childhood. The substance of the teeth is mineral, chiefly lime. It will therefore be appreciated that the child needs abundance of mineral in order that he may have sound tooth material. This he can get only from mineral in his food (see page 169) , or, before his birth, from his mother’s diet. Good circulation in the jaws is also essential for normal development both for teeth and jaws, therefore the importance of some hard food every day after ten months. ”

“ The form of the teeth demands consideration next in order of importance. Sometimes we encounter cases of teeth that are all nearly alike in form; they have lost that morphological differentiation which already existed in the anthropoid apes; there is an insensible transition from the incisors, all exactly equal in form and dimensions, to the premolars, which also present the same appearance, passing over a tooth which it would be difficult to define either as incisor or premolar (the canine tooth) . Usually in such uniform dentition there are slight diastemata. ”

“ The inequality of the grinding surfaces of the molars enables the horse to reduce the toughest fibre to a pulpy mass; but as many substances little less hard than the tooth itself must frequently be ground up with the food, the molars would, at first sight, appear to be subject to injury, especially as they have ten times the work of the incisors to perform, and the senses of touch, smell, and sight, cannot operate for their protection. ”

“ The visible part or crown of the tooth is covered with a very hard white ivory-looking substance called enamel, which serves to prevent it from being worn down by friction, and into which neither bloodvessels nor nerves have been observed to penetrate. Owing to this structure, the tooth can be safely exposed without sustaining damage—a privilege on which most persons will be disposed to place a higher value after having experienced the pains consequent upon injury of the nerve from a portion of the enamel being broken off. ”

“ 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. ”

“ 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. ”

“ In young teeth, he says, the dental cavity is very large, in proportion to the size of the tooth. According to some anatomists, the central cavity of a tooth is coated by a very soft and thin membrane, formed by a tissue of very small vessels and nerves; and besides, this cavity is filled with marrow, like hollow bones. The observations of the author, however, do not agree with these statements. The dental cavity does not contain any fatty substance analogous to the marrow of bones. As to the above-mentioned membrane, Eustachius doubts its existence. ”

“ This is met, as is characteristic in rodents and grass eaters, by the development of first high-crowned, then permanently growing molars. In acquiring the permanently growing tooth, some of the irregularities of the crown are lost, others which are deep-seated enough to affect the tooth even to the root are maintained, so that especially the external and internal infoldings become a persistent part of the tooth, having been impressed into the dental papilla. ”