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298

TEETH OF ORANG AND CHIMPANZEE.

backwards; all the divisions are sharp-pointed, and the crown of each molar thus resembles the trident or fishing-spear; the two fangs of the first molar in both jaws are connate. In the Stenorhynchus serridens the three anterior molars on each side of both jaws are four-lobed, there being one anterior and two posterior accessory lobes; the remaining posterior molars (true molars) are five-lobed, the principal cusp having one small lobe in front, and three developed from its posterior margin; the summits of the lobes are obtuse, and the posterior ones are recurved like the principal lobe. Sometimes the third molar below has three instead of two posterior accessory lobes. Occasionally, also, the second, as well as the first molar above, has its fangs connate: but the essentially duplex nature of the seemingly single fang, which is unfailingly manifested within by the double pulp-cavity, is always outwardly indicated by the median longitudinal opposite indentations of the implanted base.

Teeth of Quadrumana.—The chief aim of comparative anatomy being the better comprehension of the structure of man, we shall finally describe those modifications of the dental system which throw more immediate light on the nature of the teeth in the human subject, and which are met with, as might be expected, in the order (Quadrumana) of mammalia that makes the nearest approach to that represented by the genus homo.

Through a considerable part of the quadrumanous series, c. g., in all the apes and monkeys of the Old World, in all the genera indeed which are above the lemurs (catmonkeys and slow monkeys) of Madagascar, the same number and kinds of teeth are present as in man; the first deviation being the disproportionate size of the canines and the concomitant break or "diastema" in the dental series for the reception of their crowns when the mouth is shut. This is manifested in both the chimpanzees and orangs, together with a sexual difference in the proportions of the canine teeth.

In that large ape of tropical Africa, called the "gorilla" (Troglodytes gorilla), which in some important particulars more resembles man than does the smaller kind of chimpanzee (Troglodytes niger), the dentition seems to approach nearer to the carnivorous type, at least in the full-grown male (see Fig. 50, p. 261). It is nevertheless strictly quadrumanous in its essential characters, as in the broad, flat, tuberculate grinding surfaces of the molar teeth; but in the minor particulars in which it differs from the dentition of the orang, it approaches nearer the human type. In the upper jaw the middle incisors are smaller, the lateral ones larger than those of the orang; they are thus more nearly equal to each other; nevertheless the proportional superiority of the middle pair is much greater than in man, and the proportional size of the four incisors both to the entire skull and to the other teeth is greater. Each incisor has a prominent posterior basal ridge, and the outer angle of the lateral incisors, i 2, is rounded off as in the The incisors incline forwards from the vertical line as much as in the great orang. The characteristics of the human incisors are, in addition to their true incisive wedge-like form, their near equality of size, their vertical or nearly vertical position, and small relative size to the other teeth and to the entire skull. The diastema, between the incisors and the canine on each side, is as well marked in the male chimpanzee as in the male orang. The crown of the canine (ib.), c, passing outside the interspace between the lower canine and premolar, extends, in the male Troglodytes gorilla, a little below the alveolar border of the under jaw when the mouth is shut: the canines in both jaws are twice the size of those teeth in the female gorilla.

orang.

Both premolars are bicuspid; the outer cusp of the first and the inner cusp of the

TEETH OF ORANG AND CHIMPANZEE.

299

second being the largest, and the first premolar consequently appearing the largest on an external view. The anterior external angle of the first premolar is not produced as in the orang, which in this respect makes a marked approach to the lower quadrumana. In man, where the outer curve of the premolar part of the dental series is greater than the inner one, the outer cusps of both premolars are the largest; the alternating superiority of size in the champanzee accords with the straight line which the canine and premolars form with the true molars.

The three true molars are quadricuspid, relatively larger in comparison with the bicuspids than in the orang. In the first and second molars of both species of chimpanzee a low ridge connects the antero-internal with the postero-external cusp, crossing the crown obliquely, as in man. There is a feeble indication of the same ridge in the unworn molars of the orang; but the four principal cusps are much less distinct, and the whole grinding surface is flatter and more wrinkled than in the chimpanzee. The repetition of the strong sigmoid curves, which the unworn prominences of the first and second true molars present in man, is a very significant indication of the near affinity of the gorilla and the chimpanzee, as compared with the approach made by the orangs or any of the inferior quadrumana, in which the four cusps of the true molars rise distinct and independently of each other. The premolars as well as molars are severally implanted by one internal and two external fangs, diverging, but curving towards each other at their ends as if grasping the substance of the jaw. In no variety of the human species are the premolars normally implanted by three fangs; at most the root is bifid, and the outer and inner divisions of the root are commonly connate. It is only in the black varieties, and more particularly that race inhabiting Australia, that I have found the wisdom tooth, or last true molar, with three fangs as a general rule; and the two outer ones are more or less confluent.

The molar series in both species of chimpanzee forms a straight line, with a slight tendency in the upper jaw to bend in the opposite direction to the well-marked curve which the same series describes in the human subject. This difference of arrangement, with the more complex implantation of the premolars, the proportionally larger size of the incisors as compared with the molars; the still greater relative magnitude of the canines; and, above all, the sexual distinction in that respect illustrated by the skull of the full-grown male gorilla (Fig. 50, p. 261), stamp the chimpanzees most decisively with not merely specific but generic distinctive characters as compared with man. For the teeth are fashioned in their shape and proportions in the dark recesses of their closed formative alveoli, and do not come into the sphere of operation of external modifying causes, until the full size of the crowns has been acquired. The formidable natural weapons, with which the Creator has armed the powerful males of both species of chimpanzee, form the compensation for the want of that psychical capacity to forge destructive instruments which has been reserved as the exclusive prerogative of man. Both chimpanzees and orangs differ from the human subject in the order of the development of the permanent series of teeth; the second molar, m 2, comes into place before either of the premolars has cut the gum, and the last molar, m 3, is acquired before the canine. We may well suppose that the larger grinders are earlier required by the frugivorous chimpanzees and orangs than by the higher organized omnivorous species with more numerous and varied resources, and probably one main condition of the earlier development of the canines and premolars in man may be their smaller relative size.

In the South American quadrumana the number of teeth is increased to thirty-six,

300

TEETH OF MONKEYS AND LEMURS.

by an addition of one tooth to the molar series on each side of both jaws. It might be concluded, à priori, that as three is the typical number of true molars in the placental mammalia with two sets of teeth, the additional tooth in the cebine would be a premolar, and form one step to the resumption of the normal number (four) of that kind of teeth. The proof of the accuracy of this inference is given by the state of the dentition in any young spider-monkey (Ateles), or Capucin-monkey (Cebus), which may correspond with that of the human child in Fig. 26, i. e., where the whole of the deciduous dentition is retained, together with the first true molar (m 1) on each side of both jaws. If the germs of the other teeth of the permanent series be exposed in the upper jaw (as in Fig. 26), the crown of a premolar will be found above the third molar in place, as well as above the second and first. As regards number, therefore, the molar series, in the South American monkeys (Mycetes, Ateles, Cebus), is intermediate between that of the genus Mustela and of Felis (Fig. 17); the little premolar, pi, in Mustela, shows plainly enough which of the four is wanting to complete the typical number in the South American monkey, and which is the additional premolar distinguishing its dental formula from that of the Old World monkeys and man.

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Zoologists have rightly stated, as a matter of fact, that the little marmoset monkeys (Hapale, Ouistiti) "have only the same number of teeth as the monkeys of the Old 4 1-1 5-5 World-viz., But the difference is much greater than this 4' 5-5 numerical conformity would intimate. In a young Jacchus penicillatus I find that there are three deciduous molars displaced by three premolars, as in the other South American quadrumana, and that it is the last true molar, m 3, the development of which is suppressed, not the premolar, p 2, and thus these diminutive squirrel-like monkeys actually differ from the Old World forms more than the Cebida do; i. c., they differ not only in having four teeth (p 2 which the monkeys of the Old World do not

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the Cebida, actually have. It is thus that the investigation of the exact homologies of parts leads to a recognition of the true characters indicative of zoological affinity.

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together with remarkable modifications

Most of the Lemurine have p of their incisive and canine teeth, of which an extreme example is shown in the pectinated tooth of the galeopithecus. The inferior incisors slope forwards in all, and the canines also, which are contiguous to them, and very similar in shape.

In the hoofed quadrupeds with toes in uneven number (perissodactyla), whose premolars, for the most part, repeat both the form and the complex structure of the true molars, such premolars are distinguished by the same character of development as those of the artiodactyla, or ungulates, with toes in even number; although here the premolars are distinguished also by modifications of size and shape. The complex ridged and tuberculate crowns of the second, third, and fourth grinders of the rhinoceros, hyrax, and horse, no more prove them to be true molars than the trenchant shape of the lower carnassials of the lion proves them to be false molars. It is by development alone that the primary division of the series of grinding teeth can be established, and by that character only can the homologies of each individual tooth be determined, and its proper symbol applied to it.

In Fig. 20, the three posterior teeth of the almost uniform grinding series of the

HOMOLOGIES OF THE TEETH.

301 horse's dentition are thus proved to be the only ones entitled to the name of "true molars ;" and, if any one should doubt the certainty of the rule of counting, by which the symbols, p 4, p 3, and p 2, are applied to the three large anterior grinding teeth (ib.), which are commonly the only premolars present in each lateral series of the horse's jaws, yet the occasional retention of the diminutive tooth (p 1), would establish its accuracy, whether such tooth be regarded as the first of the deciduous series unusually long retained, or the unusually small and speedily lost successor (p 1) of an abortive (d 1).

The law of development, so beautiful for its instructiveness and constancy in the placental diphyodonts, is well illustrated in the little hyrax, in which the d 1 is normally developed and succeeded by a permanent, p 1, differing from the rest only by a graduated inferiority of size, which, in regard to the last premolar, ceases to be a distinction between it and the first true molar.

The elephant, which by its digital characters belongs to the odd-toed, or perissodactyle, group of pachyderms, also resembles them in the close agreement in form and structure of the grinding teeth representing the premolars, with those that answer to the true molars of the hyrax, tapir, and rhinoceros. The gigantic proboscidian pachyderms of Asia and Africa present, however, so many peculiarities of structure as to have led to their being located in a particular family in the Systematic Mammalogies. And this seems to be justified by no character more than by the singular seeming exception which they present to the diphyodont rule which governs the dentition of other hoofed quadrupeds. In fact, the elephant, like the dugong, sheds and replaces vertically only its incisors, which are also two in number, very long, and of constant growth, forming tusks, with an analogous sexual difference in this respect in the female of the Asiatic species. The molars, also, are successively lost, are not vertically replaced, and are reduced finally to one on each side of both jaws, which is larger than any of its predecessors. These analogies are interesting and suggestive in connection with the other approximations in the "Sirenia" to the pachydermal type.

66

In the mammalian orders with two sets of teeth, these organs acquire fixed individual characters, receive special denominations, and can be determined from species to species. This individualization of the teeth is eminently significative of the high grade of organization of the animals manifesting it. Originally, indeed, the name 66 incisors," "laniaries" or canines," and "molars" were given to the teeth, in man and certain mammals, as in reptiles, in reference merely to the shape and offices so indicated; but they are now used as arbitrary signs, in a more fixed and determinate sense. In some carnivora, e. g., the front teeth have broad tuberculate summits, adapted for nipping and bruising, while the principal back teeth are shaped for cutting, and work upon each other like the blades of scissors. The front teeth in the elephant project from the upper jaw, in the form, size, and direction of long pointed horns. In short, shape and size are the least constant of dental characters in the mammalia; and the homologous teeth are determined, like other parts, by their relative position, by their connections, and by their development.

Those teeth which are implanted in the premaxillary bones, and in the corresponding part of the lower jaw, are called "incisors," whatever be their shape or size. The tooth in the maxillary bone, which is situated at, or near to, the suture with the premaxillary, is the "canine," as is also that tooth in the lower jaw which, in opposing it, passes in

302

NATURE OF TEETH SHOWN BY THOSE OF THE HOG.

front of its crown when the mouth is closed. The other teeth of the first set are the "deciduous molars; " the teeth which displace and succeed them vertically are the "pre-molars; " the more posterior teeth, which are not displaced by vertical successors, are the "molars" properly so called.

The bog is one of the few existing quadrupeds which retain the typical number and kinds of teeth.

Figure 25, part of the lower jaw of a young hog, illustrates the phenomena of development which distinguishes the premolars from the molars. The first premolar, p 1, and the first molar, m 1, are in place and use, together with the three deciduous

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molars, d 2, d 3, and d 4; the second molar, m 2, has just begun to cut the gum; p 2, p3, and p 4, together with m 3, are more or less incomplete, and concealed in their closed alveoli.

The premolars must displace deciduous molars in order to rise into place: the molars have no such relations. It will be observed that the last deciduous molar, d 4, has the same relative superiority of size to d 3 and d 2 which m 3 bears to m 2 and m 1; and the crowns of p 3 and p 4 are of a more simple form than those of the milk-teeth which they are destined to succeed.

The germ of the permanent canine has not yet appeared below the deciduous one, e; those of the permanent incisors, i 1, i 2, i 3, are seen ready to push out the deciduous incisors d 1, d 2, d3. When the whole of the second set of teeth is in place, its nature 3-3 1-1 4-4 3-3 is indicated by the formula:-i 3-31there are, on each side of both upper and lower jaws, three incisors, one canine, four premolars, and three molars, making in all forty-four teeth; each distinguished by the symbol marked in the cut.

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44: which signifies that

When the premolars and the molars are below their typical number, the absent teeth are missing from the fore part of the premolar series, and from the back part of the molar series. The most constant teeth are the fourth premolar and the first true molar; and, these being known by their order and mode of development, the homologies of the remaining molars and premolars are determined by counting the molars from before backwards, e.g., "one," "two," "three," and the premolars from behind forwards, "four," "three," "6. "two,"

one."

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