the language is clear; there are no redundant matters or language; and on the whole, this treatise on "Diseases of the Liver" is one which none but a man well versed in his profession could have composed. And we do not doubt that the tolerably full digest which we have given, and the quotations we have made, will induce our readers to seek in the work itself that large amount of valuable pathological facts and inductions which we confidently promise them.

ART. IV.

1. Odontography; or a Treatise on the Comparative Anatomy of the Teeth; their Physiological Relations, &c. By RICHARD OWEN, F.R.S. &c. &c.-London, 1840-45. Royal 8vo, pp. 656. With 168 Plates. 2. Report on the Microscopic Structure of Shells. Part I. By WILLIAM B. CARPENTER, M.D. F.R.S. 8vo, pp. 24. With 20 Plates.

3. Observations on the Structure of the Shells of Molluscous and Conchiferous Animals. By JAMES SCOTT BOWERBANK, F.R.S. &c. Vol. I.— London, 1844. Royal 8vo, pp. 34. With 5 Plates.

It will be in the recollection of our readers, that, in our last Number, we devoted a considerable space to the analysis of Prof. Owen's admirable researches on the comparative structure and development of the teeth; of which the publication is now complete. Our survey embraced a general account of the elementary types of Dental tissue; together with an outline of the principal forms which the apparatus presents, in the classes of fishes and reptiles. We now take up the subject where we left off; and, passing over the class of birds, whose total want of teeth leaves us nothing to say on the subject, we proceed with the dental apparatus of mammals; after which we shall give a brief notice of the recent discoveries made in a corresponding field of investigation, by Dr. Carpenter and Mr. Bowerbank.

MAMMALS. It is in the class of mammals, that the Dental apparatus presents its most special development, and is of the greatest importance in classification. This importance may be over-rated however; and has actually been, in Prof. Owen's opinion, by Frederick Cuvier. It is requisite to bear in mind, that there is a considerable diversity in the limits of variation, among different orders; thus the dental system of the Cetacea and Bruta has a much greater range of variation, and a less constant relation to the other characters on which the families and genera are founded, than in the ungulate and higher unguiculate groups.

"It is true, indeed, that the most manifestly natural mammalian genera are those, the species of which are provided with absolute similar molar teeth; and that those genera, which include species with molars of different forms, do not present the same character of unity. But it does not follow that, by combining species of mammals with similar molars, a group will be formed perfectly analogous to those which may be considered as the most natural or perfect. Neither the molar teeth, nor any other solitary character will serve to establish a natural classification.

"The molar teeth will least mislead in this respect, where their modification is most extreme, as when they are adapted to divide the flesh of animals, in which case they must of necessity be associated with the faculties and instruments for seizing and destroying prey. But molar teeth may be similarly modified, and

equally well-adapted for crushing vegetable substances, which substances may be sought for by one species on the dry land, by a second in marshes, and by a third in the sea, or on the banks of rivers. The grinding surface of the molar tooth, for example, may for this purpose be elevated into a pair of transverse ridges; and we find such molars in the kangaroo, the tapir, and the manatee, as also in the extinct diprotodon, nototherium, and dinotherium. The small anterior molars of the mastodon giganteus likewise present this form. It would be difficult to select from the mammalian class the constituents of the more heterogenous group, than would be constituted by the character which M. F. Cuvier has assigned as the true guide in the formation of the most natural and uniform genera in mammalogy." (Introduction, p. lxxi.)

Even in regard to teeth adapted to carnivorous habits, if these characters were to form the sole guides in classification, species of placental mammalia would be associated with those of the ovo-viviparous sub-class. This was actually done by F. Cuvier; and other zoologists have followed in the same track with wonderful pertinacity. There is no limit to the absurdities which result from the adoption of any single character as the uniform guide in classification, however valuable such a character may generally be as an exponent of others. There is certainly no single character, which can be so much relied on in the arrangement of Mammals, as the conformation of the teeth; yet, as we have seen, the exclusive adoption of it leads to the most erroneous results. Next to the teeth, we may rank the extremities, as serving to afford important grounds of union or distinction; but to rely exclusively upon these, as is done by a British zoologist of some eminence, is to bring together the monkeys and opossums, because they have an opposable thumb, although in other respects they are nearly at the opposite extremities of the scale; and to reunite the carnivorous and herbivorous cetacea, which all truly scientific zoologists have agreed to separate, on account of their difference in all essential characters, their agreement being only in those which adapt them to the fish-like form and habits.

The class of Mammals, like the preceding, includes a few species which are entirely devoid of teeth, or of any representatives of those organs; these all belong to the group of anteaters, with the exception of the monotreme Echidna, which resembles the rest in regimen, though differing so remarkably in its generation. A few mammals have the jaws provided with horny substitutes for teeth, as the whalebone-whales and the ornithorhyncus. In the rest of the class, true teeth are present; these are always confined to the jaws; but the lingual and palatal teeth of reptiles and fishes are not without their representatives.

"In the feline tribe, the epithelium of the tongue is thickened at the fore part of its dorsum, and invests the papillæ there with hard sheaths like prickles, which are analogous to the lingual teeth of certain fishes and batrachians. The back part of the dorsum of the tongue in the echidna is provided with a plate of horny denticles, which bruise its food against the hard and prickly epithelium covering the palate. Horny processes, analogous to the palatal teeth of fishes and reptiles, are likewise developed upon the roof of the mouth of the great bottle-nose dolphin, thence termed hyperoodon by Lacépède." (p. 296.)

The average number of teeth in mammals, is that which characterizes man, the apes of the old world, and the true ruminants ;-namely, 32. In some species of cetacea, however, this number is reduced to two; and in

the narwhal or sea unicorn, only one is ordinarily developed, but this grows to an unusual length. The elephant has never more than one entire molar, or parts of two, in use on each side of the upper and lower jaws; making, therefore, at the most, eight molars in all; to which we have to add the two tusks, developed in the intermaxillary bones. The examples of excessive number of teeth are presented, in the order Bruta, by an extinct armadillo, which has 98 teeth; and among the Cetacea, by the cachalot or sperm-whale, which has upwards of 60 teeth, though most of them are confined to the lower jaw, -by the common porpoise, which has between 80 and 90 teeth,-by the gangetic dolphin, which has 120 teeth, -and by the true dolphins, which have from 100 to 190 teeth, yielding the maximum number in the class Mammalia. Where the teeth are in excessive number, as in the species just cited, they are, as among reptiles and fishes in similar circumstances, nearly alike in form; being for the most part simply conical, and evidently destined rather for the prehension than for the mastication of the food. In several of the bruta, again, such as the smaller armadillos, the orycterope, and the three-toed sloth, in which the anterior teeth are deficient, the remainder are nearly all of one form, being sub-cylindrical with broad triturating surfaces; they differ, however, in size. In almost all the other Mammalia, particular teeth have special forms for special uses; thus, the front teeth, from being commonly adapted to effect the first coarse division of the food, have been called cutters or incisors, and the back teeth which complete its comminution, grinders or molars; large conical teeth, situated behind the incisors, and adapted by being nearer the insertion of the biting muscles to act with greater force, are called holders, tearers, lamaries, or more commonly canine teeth, from being well developed in the dog and other carnivora, although they are given, likewise, to many vegetable-feeders, for defence or combat. These names are not, however, by any means indicative of the shape of the crowns of the teeth which occupy the respective positions just described; and it has been found necessary to consider the position, and also the mode of succession of the teeth, in order to make them of definite use in classification.

Comparative anatomists, by common consent, now apply the term 'incisor,' arbitrarily to those teeth which are implanted in the intermaxillary bones, and in the corresponding part of the lower jaw. When the tooth which succeeds the incisors, or the first of the upper maxillary bone, is conical, pointed, and longer than the rest, it is called a canine,' as is also its analogue in the lower jaw, which always passes in front of it when the mouth is closed Of the remaining teeth, those which are shed and replaced vertically, or which have successors descending into their place in the upper, and ascending in the lower jaw, are called *premolars, or false molars, and in human anatomy bicuspides; the remaining teeth, which are not displaced by vertical successors, but which follow each other from behind forwards, in both jaws, are called molars,' or true molars." (p. 298.)

The ordinary teeth of mammalia have so much more definite and complex a form than those of fishes and reptiles, that we can usually recognize in them the crown or exposed part, as not only distinct from the fang or inserted part, but as divided from it by a constricted portion or neck. In those teeth which grow uninterruptedly, however, the exposed part is not separated by a neck from the implanted part, which generally maintains

It is

to its extremity the same size and shape as the implanted crown. peculiar to the class Mammalia to have teeth implanted in sockets by two or more fangs; but this can only happen to the teeth of limited growth, and generally characterizes the molars and premolars. In no mammal does anchylosis of the teeth with the jaw constitute a normal mode of attachment, as in the cold-blooded vertebrata. Each tooth has its particular socket, to which it firmly adheres by the close co-adaptation of their opposed surfaces, and by the firm adhesion of the alveolar periosteum to the organized cement, which invests the fang or fangs of the tooth; but in some of the cetacea, at the posterior part of the dental series, the sockets are wide and shallow, and the teeth adhere more strongly to the gum than to the periosteum; so that Prof. Owen has seen, in the cachalot, all the teeth brought away with the ligamentous gum, when it has been stripped from the sockets of the lower jaw. True teeth implanted in sockets are confined, in this class, to the maxillary, intermaxillary, and lower maxillary bones, and to a single row in each. In general, teeth are situated in all these bones; but they may be absent in the intermaxillaries, as in the ruminants and most bruta; or they may be present in those bones only, as in the narwhal. In man, the intermaxillaries coalesce with the maxillary bones at an early period; it was formerly supposed that he was the only mammal in which there is no interspace in the dental series of either jaw, a character resulting from the shortness of the jaws, and the equal lengths of the crowns of the teeth; but the importance of this character as associated with the peculiar attributes of human organization, has been somewhat diminished by Cuvier's discovery of a like contiguous arrangement of the teeth, in the jaws of the extinct Anoplotherium. For a general account of the substance of the teeth of mammals, we shall have recourse to Prof. Owen's unabridged description.

"The teeth of mammalia usually consist of hard or unvascular dentine, defended at the crown with an investment of enamel, and everywhere surrounded by a coat of cement. The coronal cement is of extreme tenuity in man, quadrumana, and terrestrial carnivora; it is thicker in herbivora, especially in the complex grinders of the elephant; and is thickest in the teeth of the sloths, magatheroids, morse, and cachalot. Vertical folds of enamel and cement penetrate the crown of the tooth in ruminants, and in most rodents, and pachyderms, characterizing by their various forms the genera of the two last orders; but these folds never converge from equidistant points of the circumference of the crown towards the centre. The teeth of the quadrupeds of the order bruta have no true enamel; this is absent likewise in the molars of the dugong, zeuglodon, and the cachalot. The tusks of the narwhal, walrus, elephant, mastodon, and dinotherium, consist of modified dentine, which in the large proboscidian animals is properly called 'ivory,' and is covered by cement.

"The central part of the fully-formed tooth in man and most other animals, contains an irregular kind of osseous substance, which is most abundant in the cachalot, and forms around foreign bodies which may gain admission to the pulpcavity of tusks. A fifth substance which, from the number and regular position of the vascular canals in it, I have termed the vascular dentine,' forms the body or axis of the tooth in the sloth tribe, and is present in smaller proportion in the centre of the teeth of the armadillos. The teeth of the orycteropus consist of congeries of long and slender prismatic columnar denticles, each consisting of a body of dentine, with a coating of cement, by which they are united together to form a composite tooth, as in some of the cartilaginous fishes." (p. 302.)`

XLII.-XXI.

5

"The dentine of the long and slender prismatic denticles, which are aggregated to form the compound tooth of the orycterope, is unvascular, and is characterized chiefly by the frequent division and wide angles at which the branches of the bifurcating calcigerous tubes diverge, before resolving themselves into their minute wavy terminations." (p. 303.)

The resemblance between one of the composite teeth of the mammiferous orycterope, as figured in plate 78 of the Odontography, and that of the saw-fish shown in plate 9, is very remarkable; and it shows what care is requisite in drawing general conclusions respecting the limitation of special forms of dental structure to particular groups, until the examination has been most extensively prosecuted.

As the account of the dental structures, which was contained in the preceding article, was principally founded upon the characters which they exhibit in mammalia, we need not stop here to go over the same ground again; but have only to add to our generalities a few words regarding the peculiarities of dental development in this class.

The primary pulp, which first appears as a papilla on the free surface of the gum, and by the calcification of which the dentine is formed, sinks into a cavity, and becomes surrounded by a closed capsule, at an early period of the formation of the tooth, in every mammal. It is also completely enclosed in the substance of the jaw-bone; from which the crown of the growing tooth extricates itself by exciting the absorbent process, whilst the cell is deepened by the same process and by the growth of the jaw, into an alveolus for the root of the tooth. In those teeth which possess enamel, the mould or pulp of that constituent is developed from the capsule covering the coronal part of the dentinal pulp. In the complex teeth of the herbivorous and some omnivorous quadrupeds, the enamel-pulp sends processes into depressions of the crown, which vary in depth, breadth, direction, and number, in the several groups; the dentinal pulp, thus penetrated, offers corresponding complications of form, and as the capsule follows the enamel-pulp in all its folds and processes, the external cavities or interspaces of the dentine become occupied by enamel or cement, the cement, like the capsule which formed it, being the outermost substance, or having the enamel interposed between it and the dentine. The dental matrix presents the most extensive interdigitation of the dentinal and enamel pulps, in the capybara and elephant.

The ordinary process of the formation of the fang is, in its essential characters, the same with the continued production of the tooth which takes place in certain cases, as the tusks of the elephant, and the front teeth of the rodentia; the only difference being, that the pulp is reproduced in perpetually decreasing quantity in the former case; whilst it is regenerated below, as fast as it undergoes conversion above, in the latter.

"In the formation of a single fang, the activity of the reproductive process becomes enfeebled at the circumference, and is progressively contracted within narrower limits in relation to a single centre, until it ceases at the apex of the fang; which, though for a long time perforated for the admission of the vessels and nerves to the interior of the tooth, is in many cases finally closed by the ossification of the remaining part of the capsule. When a tooth is destined to be implanted by two or more fangs, the reproduction of the pulp is restricted to