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COMPARATIVE ANATOMY
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See also:COMPARATIVE See also:ANATOMY . . The details of the See also:teeth vary so greatly in different animals and See also:groups of animals, and, on See also:account of their being the most durable tissues of the See also:body, are so important for classificatory purposes, that they are dealt with freely in the various zoological articles. All that can be done here is to give a broad See also:general survey of the subject, taking the details of See also:man's dentition, already set forth, as a point of departure. In some fishes the teeth are continuous over the edges of the jaws with the scales on the See also:surface of the body, and there is no doubt that teeth should be regarded as modified scales which have migrated into the mouth. In the See also:Cyclostomata (lampreys and hags) the teeth are horny cones, but beneath them there are papillae of the mesoderm covered with ectoderm which resemble the dental papillae and See also:enamel See also:organs although no calcification occurs except in Bdellostoma. In the Elasmobranchii (cartilaginous fishes) the teeth are arranged in several rows, and as those of the front See also:row fall out the hinder row take their See also:place; sometimes they are triangular and very See also:sharp as in the sharks, sometimes flattened and arranged like a See also:pavement for crushing as in rays. These teeth only represent the crowns of man's teeth, and they are not embedded in sockets except in the See also:case of the teeth in the saw of the saw-See also:fish (Pristis); moreover the dentine of which they are largely composed resembles See also:bone and fills up the whole pulp cavity. From its structure it is known as osteodentine. In the Teleostomi (teleostean and ganoid fishes) there is See also:great variability; sometimes, as in the See also:sturgeon, there are no teeth at all, while at others every bone bounding the mouth, including the branchial See also:arches, bears teeth. As an example of a very full tooth See also:armature the See also:pike's mouth and pharynx may be instanced. Both in the pike and the See also:hake hinged teeth occur; these See also:bend backward during the passage of See also:prey down the See also:throat, but are re-erected by elastic ligaments. As a See also:rule, the dentine of the Teleostomi is of the variety already described as osteodentine, but sometimes, as in the hake, it is vascular and is known as vasodentine. In the See also:Amphibia teeth are not so numerous as in the fishes, though like them they are not confined to the jaws, since vomerine teeth are very See also:constant. The See also:toad is edentulous, while the See also:frog has no teeth in the See also:lower See also:jaw. An See also:extinct See also:order of tailed amphibians, the Stegocephali, are often called labyrinthodonts on account of the complex way in which the enamel is involuted into the interior of the teeth. Amphibians' teeth are usually anchylosed to the jaw, that is to say, directly See also:united by bone. In the Reptilia many and various arrangements of the teeth are found. In the Chelonia (turtles) there are no teeth, although the ectodermal ingrowth (dental See also:band) from which they are See also:developed in other animals is See also:present in the embryo. The place of the teeth in these See also:reptiles is taken by horny jaw-cases. In the Ophidia the non-poisonous See also:snakes have two rows of teeth in the upper jaw, one on the maxillae and another on the See also:palatine and pterygoid bones, while in the lower jaw there is only one row. These teeth are sharp pegs anchylosed to the bones and so strongly recurved that one of these snakes would be unable, even if it wished 502 to do so, to let any prey which had once entered its mouth See also:escape. The poisonous snakes have a See also:special See also:poison See also:fang in the maxilla of each See also:side; these have a deep groove or See also:canal See also:running down them which transmits the poison from the poison gland. In the colubrine snakes, such as the See also:cobra, the poison fang is always erect, but in the viperine, such as our own See also:adder and the See also:rattlesnake, there is a mechanism by which the tooth is only erected when the jaws are opened for striking. At other times the teeth See also:lie See also:flat in the roof of the mouth. In the lizards or Lacertilia the teeth usually consist of a See also:series of pegs in the upper and lower jaw, each resembling the one in front of it; sometimes, as in the See also:chameleon, they are anchylosed by their bases to the bone, but at others, as in the See also:iguana, they are fused by their sides to a See also:ridge of bone which forms a See also:low See also:wall on their lateral surface. In the former case the dentition is spoken of as " acrodont," in the latter as " pleurodont." In the Crocodilia the teeth are fitted into definite sockets as in mammals and are not anchylosed with the jaws. This arrangement is spoken of as " thecodont." Existing birds are toothless, but palaeontology shows that they originally had teeth of a reptilian See also:character. In all these lower vertebrates, then, the teeth are similar or nearly similar in character; at least they are not divided into definite incisor, canine, premolar and molar regions. Their dentition is therefore known as " homodont." Another characteristic is that in almost all of them there is an arrangement for a continuous See also:succession of teeth, so that when one is lost another from behind takes its place, and to this arrangement the See also:term " polyphyodont " is applied. With a few exceptions a -homodont dentition is also polyphyodont. In the See also:Mammalia the different groups of teeth (incisor, canine, &c.) already noticed in man are found, and these animals are characterized, with some exceptions, by having a " heterodont " as opposed to a homodont dentition. In the mammals too the polyphyodont or continuous succession of teeth is reduced to a ' diphyodont " dentition, which means that there is only one relay of teeth to replace the first set. In the marsupials the reduction of the succession is carried still further, for only one premolar in each segment of the jaw is replaced, while in the toothed whales there is no succession at all. When one set has to do See also:duty throughout See also:life the dentition is called " monophyodont." There is a great See also:deal of discussion as to how the complex back teeth of mammals with their numerous cusps were derived from the See also:simple conical teeth which are generally assumed, though not by all, to have been the See also:primitive arrangement. One simple way of accounting for the See also:change is by the concrescence theory, namely that several conical homodont teeth have fused and so formed a single multitubercular tooth; but, although this See also:process may be partly true, it does not account for all the facts at our disposal. Another theory, which is more favoured at the present See also:time, is known as the " tritubercular," and is largely based on the researches of E. D. See also:Cope and H. F. See also:Osborn, two See also:American palaeontologists. According to this theory a simple peg-like, or. as it is called, " haplodont," tooth develops two additional smaller pegs or cones, one in front and one behind the See also:original See also:main See also:cone, possibly owing to the irritation of the teeth against which it bites in the other jaw. This is known as the triconodont See also:stage, and it is found in some of the See also:oldest extinct mammals. As a later See also:adaptation it is found that the two small cones, the anterior of which is called the " paracone " and the posterior the " metacone," become See also:external to the original " protocone " in the upper jaw and See also:internal in the lower. The surface of the tooth has now a triangular shape with a cone at each See also:angle, and this is the " tritubercular tooth " which is of very See also:common occurrence among the ancestral mammals. Other cusps may be developed later, and so the quadricuspid and quinquecuspid molar teeth of man and other mammals are accounted for. This theory, although in a brief outline it sounds feasible enough, has really many points of difficulty, and those who are interested in the subject will find a See also:fuller account in C. S. Tomes' Dental Anatomy (See also:London, 1904), and in W. L. H. See also:Duckworth's See also:Morphology and See also:Anthropology (See also:Cambridge, 1904), in both of which references to the original literature, which is now very voluminous, are given. Marett Tims (J. Anat. and Phys., vol. See also:xxxvii. p. 131) suggests that the See also:evolution of the mammalian teeth is to be explained partly by the tritubercular and partly by the concrescence theory. It is impossible, in the space assigned, to give even a brief See also:review of mammalian odontology, but it may clear the ground for the special zoological articles if an See also:attempt is made to define what is meant by the different classes of teeth. Incisor teeth are those which in the upper jaw have their sockets in the premaxillary bone; they are generally See also:chisel-shaped, and with their opponents of the lower jaw See also:act like See also:scissors. They are specially well marked in the rodents, and in these animals the pulp throughout life continues to See also:form fresh dentine, so that the teeth are ever growing, and it is absolutely necessary for their owners to be continually gnawing in order to See also:wear them away at their cutting edges. The tusks of the See also:elephant and the single tusk of the male See also:narwhal are modified incisors, while in the ruminants the incisor teeth are wanting in the upper jaw. Th' canine tooth is the first tooth behind the premaxillo-maxillarysuture, provided it be not far behind it; it is almost always the first of the premaxillary series, speaking accurately, which is elan. gated and sharply pointed. As its name implies it is well marked in See also:dogs and other See also:Carnivora, but is found in many other orders, It is the special offensive and defensive weapon of many mammals, and is greatly developed in some of the ungulates which are without horns, e.g., the See also:musk See also:deer. The tusks of the See also:walrus and See also:wild See also:boar are canines. In many of the Insectivora, especially the See also:mole, the canine is very hard to identify, as in these animals an incisor or a premolar may take on caniniform characters, or there may be no tooth at all with these characters. The premolar teeth are those in the maxillary bone which are preceded by See also:milk teeth. This See also:definition, of course, includes the canine as a modified premolar, and so it should no doubt be considered, though, if it is desired to keep it distinct, " behind the canine " must be added. Unfortunately for an accurate definition the first premolar behind the canine is not always preceded by another tooth, and so it becomes an unsettled question whether, in these cases, the tooth is a retained milk tooth or a permanent one which has had no predecessor; it is probable, however, that the latter is the right See also:interpretation. The molar teeth are those, behind the premolars, which are not preceded by temporary teeth. As was pointed out, in man's dentition they are probably teeth of the first or milk dentition which appear See also:late. In front of the premolar teeth, and between them and the canine, if it be present, or the incisors, if it be absent, there is often a space called the " diastema." It is best marked in the orders of See also:Rodentia and See also:Ungulata, and in the See also:horse is See also:familiar as the place where the See also:bit lies. In recording the teeth of any particular mammal it saves time and space if a dental See also:formula be used. This simply means setting down the number of each See also:kind of tooth in one side of the upper and lower jaw in their order from before backward. Thus man's formula would be, incisors 2, canines I, premolars 2, molars 3. This is condensed into 2.1.2.3- 2.1.2.3 Some other types of dental formulae are 2.1.2.3 Catarrhine (old See also:world) monkeys 2.1.2.3 2.1.3.3 Platyrhine (new world) monkeys 2.1.3.3 2.1.3.2 Marmosets 2.1.3.2 2.1.2.3 Most lemurs 2.1.2.3 2.1.3.3 or 2.1.3-3 2.1.3.3 Insectivorous bats (full series) . . 3.1.3.3
(The upper incisors and both premolars may
be reduced by one)
Frugiverous bats . . . . ? 1 2 3 2.1.3.3
(The molars may be reduced)
Insectivora (teeth variable and somewhat uncertain)
See also:Hedgehog
Mole .
(Five different dental formulae have been assigned to this See also:animal)
Carnivora
See also:Cat See also:family (Felidae) . See also:Dog family (Canidae) See also:Bear family (Ursidae)
See also:Civet family (Viverridae) Racoon family (Procyonidae)
Hyaena family (Hyaenidae)
See also:Weasel family (Mustelidae)
Eared See also:seal family (Otariidae)
Seal family (Phocidae)
Walrus family (Trichechidae), adult
In a See also:young animal (probably)
3.1.3.3
' 2.1.2.3 3.1.4.3 3.1.4.3
3.1.3.1 3.1.2.1 3.1.4.2 3.1.4.3
3.1.4.2 3.1.4.2 3.1.4.1 3.1.3.1 3.1.4.1 3.1.4.2 3.I.4.I Oe 2.1.4.1 3.1.4.1 3.1.4.1 1.1.3.0 0.1.3.0 3.1.3.2 3.1.3.1
2.1.4.1 2.1.4.3 3.1.4.3
3.1.4.3 1.1.3.3 3.1.2.3 0.1.3.3
• 3.1.3.3 0.(O Or 1).3.3
3.1.3.3 0.0.3.3
• 3.1.3.3
3.1.4.3
3.1.3.3
3.1.3.3.
3.1.3.3 (0—2).0.4.3 (o-1).(o—I).4.3
(1-2).0.4.3
2.0.4.3
d. i. I. i.1. c. o. d. m. (3-4) M. 3 0 o o (3-4) 3
In this animal there are no premolars, but the milk molars (d.m) and true molars gradually replace one another from before back-See also: The homodont dentition of the whales is a See also:retrograde process, and is therefore not comparable to the homodont dentition of the vertebrates below mammals. Sirenia.—The dentition is monophyodont. The manatee has i. ?, c. 0--, back teeth I 1 2 0 II In the See also:Edentata the See also:ant-eaters (Myrmecophagidae) and pangolins (Manidae) are toothless, though the latter have foetal tooth germs. The aard varks (Orycteropodidae) are somewhat heterodont, while the armadillos (Dasypodidae) and sloths (Bradypodidae) have a homodont dentition, which, like that of the whales, is retrogres- sive. In the See also:giant See also:armadillo (Priodon gigas) the formula is 5. This animal therefore has a See also:hundred teeth. In none of the Edentata are the teeth covered with enamel. In the See also:Marsupialia the typical formula is 3'''3'4' They are 3.1.3.4. divided into diprotodont, in which there are not more than 3 -incisors, often i as in kangaroos, and polyprotodont, in which the incisors are more than 3, as in the Tasmanian See also:wolf (Thylacinus) and Tas- manian See also:devil (Sarcophilus). The marsupial teeth are often regarded as all milk teeth, yet the order is not really monophyodont because the germs of the permanent teeth are formed and aborted. See also:Modern See also:research, however, casts See also:grave doubt on the accuracy of this view. In the See also:Monotremata the See also:Echidna or spiny ant-eater is quite edentulous, while the See also:duck-mole (Ornithorhynchus) has functional molar teeth in youth, though in the adult these are lost, and their place is taken by horny plates. Reviewing the various tooth iormuiae a mammals the following is usually regarded as typical: 3.1.4.3 3.1.4.3 This, it will be noticed, is the formula of the See also:pig, and it is also that of almost all the See also:Eocene Ungulata. Although the See also:majority of mammals are diphyodont, or, in other words, the working teeth belong to two dentitions, evidences have lately been submitted of vestiges of two other series, one on the labial side of the milk teeth and one on the lingual side of the permanent series. If these are substantiated there would be four dentitions—(1) pre-milk; (2) milk; (3) permanent; (4) See also:post-permanent. The theory, though it See also:bridges over the See also:gap between the polyphyodont lower vertebrates and the apparently diphyodont mammals, is not by any means established. As the teeth are of such importance in the See also:classification of animals, it will See also:save continually repeated explanations in other articles if some of the See also:chief terms by which they are described are recapitulated and briefly defined here. 1. Acrodont, a tooth which is anchylosed by its See also:base to the See also:summit of a See also:parapet on the jaw. 2. Bilophodont, a molar tooth having two transverse ridges on its grinding surface, as in the See also:tapir. 3. Brachyodont, a low-crowned molar tooth—the opposite of hypsodont. 4. Bunodont, a tooth bearing conical cusps. 5. Diphyodont, having two series of teeth (milk and permanent). 6. Diprotodont, a marsupial with not more than 3 incisors, often only one on each side of the mandible. 7. Haplodont, a tooth having a simple conical See also:crown with a single See also:root. 8. Heterodont, a dentition in which the teeth are not all alike, chiefly characteristic of the Mammalia. 9. Homodont, a dentition in which the teeth are all alike as in many of the lower vertebrates and some mammals. to. Hypsodont, a high-crowned molar tooth, such as that of the horse,—the opposite to brachydont. 11. Lophodont, a transversely ridged molar tooth; cf. bilophodont. 12. Monophyodont, having only one dentition (cf. diphy- and polyphy-odont). 13. Multituberculate, a tooth, the crown of which bears numerous conical cusps; held by some to be the primitive See also:condition of the mammalian teeth. 14. Pleurodont, a tooth anchylosed to the inner side of a parapet on the jaw. 15. Polybunodont, a synonym for multituberculate. 16. Polyphyodont, having an endless succession of teeth, as in most vertebrates below the mammals. 17. Polyprotodont, a marsupial having an incisor formula of more than 3 18. Protodont, a stage met with in fossil mammals which is an advance on the haplodont tooth in that two small cusps are added to the main cone. 19. Secodont, a back tooth adapted to cutting as in many of the Carnivora. 20. Selenodont, a molar tooth with crescentic ridges on its grinding surface as in most ruminants. 21. Thecodont, a tooth embedded in a socket or alveolus, as in mammals. 22. Triconodont, a fossil stage in advance of the protodont, There are three well-marked cones in an antero-posterior See also:line. 23. Tritubercular, a fossil stage succeeding the triconodont. The main cone is external in the lower teeth and internal in the upper. A very common form of back tooth in fossil forms and one which gives its name to the " tritubercular theory." (F. G. Additional information and CommentsThere are no comments yet for this article.
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