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Originally appearing in Volume V26, Page 503 of the 1911 Encyclopedia Britannica.
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COMPARATIVE ANATOMY. . The details of the teeth vary so greatly in different animals and groups of animals, and, on account of their being the most durable tissues of the 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 general survey of the subject, taking the details of 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 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 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 enamel 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 row fall out the hinder row take their place; sometimes they are triangular and very sharp as in the sharks, sometimes flattened and arranged like a 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 case of the teeth in the saw of the saw-fish (Pristis); moreover the dentine of which they are largely composed resembles 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 great variability; sometimes, as in the sturgeon, there are no teeth at all, while at others every bone bounding the mouth, including the branchial arches, bears teeth. As an example of a very full tooth armature the pike's mouth and pharynx may be instanced. Both in the pike and the hake hinged teeth occur; these bend backward during the passage of prey down the throat, but are re-erected by elastic ligaments. As a 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 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 constant. The toad is edentulous, while the frog has no teeth in the lower jaw. An extinct 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 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 band) from which they are developed in other animals is present in the embryo. The place of the teeth in these reptiles is taken by horny jaw-cases. In the Ophidia the non-poisonous snakes have two rows of teeth in the upper jaw, one on the maxillae and another on the 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 escape. The poisonous snakes have a special poison fang in the maxilla of each side; these have a deep groove or canal running down them which transmits the poison from the poison gland. In the colubrine snakes, such as the cobra, the poison fang is always erect, but in the viperine, such as our own adder and the rattlesnake, there is a mechanism by which the tooth is only erected when the jaws are opened for striking. At other times the teeth lie flat in the roof of the mouth. In the lizards or Lacertilia the teeth usually consist of a series of pegs in the upper and lower jaw, each resembling the one in front of it; sometimes, as in the chameleon, they are anchylosed by their bases to the bone, but at others, as in the iguana, they are fused by their sides to a ridge of bone which forms a low 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 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 succession of teeth, so that when one is lost another from behind takes its place, and to this arrangement the term " polyphyodont " is applied. With a few exceptions a -homodont dentition is also polyphyodont. In the 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 duty throughout life the dentition is called " monophyodont." There is a great deal of discussion as to how the complex back teeth of mammals with their numerous cusps were derived from the simple conical teeth which are generally assumed, though not by all, to have been the primitive arrangement. One simple way of accounting for the change is by the concrescence theory, namely that several conical homodont teeth have fused and so formed a single multitubercular tooth; but, although this 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 time, is known as the " tritubercular," and is largely based on the researches of E. D. Cope and H. F. Osborn, two 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 original main cone, possibly owing to the irritation of the teeth against which it bites in the other jaw. This is known as the triconodont stage, and it is found in some of the oldest extinct mammals. As a later adaptation it is found that the two small cones, the anterior of which is called the " paracone " and the posterior the " metacone," become external to the original " protocone " in the upper jaw and internal in the lower. The surface of the tooth has now a triangular shape with a cone at each angle, and this is the " tritubercular tooth " which is of very 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 fuller account in C. S. Tomes' Dental Anatomy (London, 1904), and in W. L. H. Duckworth's Morphology and Anthropology (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. xxxvii. p. 131) suggests that the 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 review of mammalian odontology, but it may clear the ground for the special zoological articles if an 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 chisel-shaped, and with their opponents of the lower jaw act like scissors. They are specially well marked in the rodents, and in these animals the pulp throughout life continues to 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 wear them away at their cutting edges. The tusks of the elephant and the single tusk of the male 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 dogs and other 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 musk deer. The tusks of the walrus and wild boar are canines. In many of the Insectivora, especially the 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 milk teeth. This 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 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 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 Rodentia and Ungulata, and in the horse is familiar as the place where the bit lies. In recording the teeth of any particular mammal it saves time and space if a dental formula be used. This simply means setting down the number of each 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 Some other types of dental formulae are Catarrhine (old world) monkeys Platyrhine (new world) monkeys Marmosets Most lemurs or 2.1.3-3 Insectivorous bats (full series) . . (The upper incisors and both premolars may be reduced by one) Frugiverous bats . . . . ? 1 2 3 (The molars may be reduced) Insectivora (teeth variable and somewhat uncertain) Hedgehog Mole . (Five different dental formulae have been assigned to this animal) Carnivora Cat family (Felidae) . Dog family (Canidae) Bear family (Ursidae) Civet family (Viverridae) Racoon family (Procyonidae) Hyaena family (Hyaenidae) Weasel family (Mustelidae) Eared seal family (Otariidae) Seal family (Phocidae) Walrus family (Trichechidae), adult In a young animal (probably) ' 3.I.4.I Oe • 0.(O Or 1).3.3 • (0—2).0.4.3 (o-1).(o—I).4.3 (1-2).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-ward throughout life, so that there are never more than two back teeth in each segment of the jaw at any one time. Rodentia Typical rodents (Simplicidentata) Hares and rabbits (Duplicidentata) Cetacea.—In the living toothed whales (Odontoceti) the denti- tion is homodont and may be as great as 6 , There is every reason to believe, however, that they are derived from heterodont ancestors. In the whalebone whales (Mystacoceti) the teeth are replaced by the whalebone in the adult, but in the embryo slightly calcified teeth are present which are afterwards absorbed. The homodont dentition of the whales is a 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 Edentata the 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 giant armadillo (Priodon gigas) the formula is 5. This animal therefore has a hundred teeth. In none of the Edentata are the teeth covered with enamel. In the Marsupialia the typical formula is 3'''3'4' They are 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 wolf (Thylacinus) and Tas- manian 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. Modern research, however, casts grave doubt on the accuracy of this view. In the Monotremata the Echidna or spiny ant-eater is quite edentulous, while the 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: This, it will be noticed, is the formula of the pig, and it is also that of almost all the Eocene Ungulata. Although the 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) post-permanent. The theory, though it bridges over the 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 classification of animals, it will save continually repeated explanations in other articles if some of the chief terms by which they are described are recapitulated and briefly defined here. 1. Acrodont, a tooth which is anchylosed by its base to the summit of a parapet on the jaw. 2. Bilophodont, a molar tooth having two transverse ridges on its grinding surface, as in the 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 crown with a single 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 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 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. P.)

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