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The researches of the microscope into the organic structure of teeth have been equally successful; not only for the purposes of the physiologist, but, most unexpectedly, for those also of the zoologist and palæontologist; scarcely any means for determining affinities and recognising imperfect fossil remains being so precise and certain as the minute diversities in teeth. The real structure of teeth was first noticed by Leeuwenhoek. In the Philosophical Transactions of 1678, he described the human tooth as 'made up of very small, straight, and transparent pipes,' and in a subsequent paper he calculated that the number of these 'pipes' in a single molar amounted to nearly five millions. He spoke, also, of their existence in the cod and in the haddock. But these observations were so completely overlooked by later writers, that the controverted question, whether teeth possessed an organic structure, was very generally decided in the negative until a recent period. Teeth were considered, in fact, as mere stony exudations from the surface of the tooth-pulps, and so far assimilated to shell. The achromatic microscope, however, had not been long in the hands of anatomists, before the investigation of the structure and development of teeth was taken up anew, under more favourable circumstances; and there is, probably, no single branch of microscopic inquiry which has been, or which is likely to prove, so fertile in valuable results. Into the troubled question of priority we have no desire now to enter; but, content with expressing our obligations to Purkinje, Retzius, Müller, and Agassiz, among continental anatomists, and to Owen, Nasmyth, and Tomes, among our countrymen, we shall proceed with our endeavour to convey a general idea of their labours.

The greater part of the substance of the teeth in the higher vertebrata, and the whole of it in the lower, consists of the substance commonly termed ivory, but which has been more appropriately termed dentine; meaning by the word 'dentine,' what is essential to the composition of the dental structure, and restricting the word 'ivory' to certain varieties of the substance. Dentine, like bone, is made up of an apparently homogeneous substance, composed of animal and mineral matter in intimate union. Like bone, too, it is channelled-out by innumerable pasCHRISTIAN TEACHER.-No. 47.

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sages of extreme minuteness, which, though incapable of admitting blood, seem to convey the nutriment absorbed from it. There is no system of cavernous excavations, however, in true dentine; the tubuli passing continuously onwards, instead of merely traversing the spaces between successive chambers as in bone. In human teeth, and in others whose dental structure is equally simple, the tubules originate from the central pulp-cavity, and radiate outwards with great regularity; their course being slightly sinuous, but having a constant direction towards the exterior of the tooth. The diameter of their calibre near their central commencement does not exceed 1.10,000th of an inch; and this gradually diminishes, as the tubuli pass outwards, and send off lateral branches, until it is reduced at their terminations to the smallest channel that can be distinctly traced through the solid substance. In the teeth, however, of many animals, there is no single central pulp-cavity, but the whole substance is excavated by vascular canals, very analogous to those of bone. We find, in such cases, each canal the centre of a system of radiating tubuli, so that the transverse section of the tooth strongly resembles that of bone, except in the absence of the cavernules. This type of structure prevails chiefly among fishes and it is a curious fact, that in this group, the lowest in which true bones and teeth are found, the characters of the two structures should approximate so much more closely than in the higher classes, where both are elaborated, each in its own fashion, until they become distinct. In other instances, again, where a central pulpcavity does exist, it sends vascular prolongations for a certain distance into the dentine: the outer layers of this substance, however, are very seldom thus penetrated by blood-vessels.

The dentine is capped, in the human tooth, by a layer of a harder substance, familiarly known as enamel. This is the hardest of all the animal tissues; and is indeed so completely mineralised, that the organic basis does not form above two or three per cent. of its components; yet it presents a very distinct organic structure. It is composed of a series of prismatic membranous cells (resembling those of a honey-comb in miniature) arranged side by side with great regularity. Their lower extremities

rest upon the surface of the dentine, whilst their upper ends form the crown of the tooth; and the extraordinary density of the tissue appears to be occasioned by a deposit of calcareous matter, with which the cells are completely filled, and in which phosphate of lime predominates. The strength of these enamel-prisms is increased by the wavy curves in which they are arranged. By this means their alliance is much more dependent and complete, than if each of them were a straight column supporting its own separate share of pressure. The disposition of the enamel varies greatly in different tribes of Mammalia, according to the purposes to which the teeth are to be applied. Thus in the long front teeth of the Rodents, we find it only upon the front surface, where, owing to its superior density, it constantly projects beyond the ivory, and maintains a sharp cutting edge, however much the tooth may be worn down. In the herbivorous Mammals, the surface of the grinding teeth is merely penetrated by vertical plates of enamel, instead of being covered with a continuous layer : and their superior density and resistance are the cause of those ridged inequalities of the grinding surface, which enable them to triturate their tough vegetable food. In Sloths and other edentate Mammals, the enamel is altogether wanting.

A third substance, the 'cementum' or 'crusta petrosa,' has long been known as a constituent of the teeth of herbivorous animals; but it is only lately, by means of the microscope, that its existence in human teeth has been detected. The intimate structure of the 'cementum' is the same with that of bone. It has the same system of cavernules and tubuli communicating with the nearest vascular surface; and its thick layers, whenever they exist, are penetrated by vascular canals. As it invests the fang of the tooth, it forms a bond of vital union between the denser structures of the tooth itself, and the bone in which it is implanted. In herbivorous Mammals, on the contrary, it penetrates, like the enamel, the substance of the grinding teeth; the vertical folds in which it is arranged, being softer than the dentine, are most rapidly worn down; and the inequality produced by the superior density, and consequently slower abrasion, of the enamel, is considerably increased.

Such being the structure of teeth and their three component parts, it is evident from our verbal sketch, that they deserve a far higher character than that of mere inorganic exudations. Without entering into the history of their development, by the consolidation of the soft tissues forming the pulp of the tooth, the knowledge of the fact is of the utmost importance in interpreting the phenomena of disease, and has already led to great improvements in dental surgery.

The varieties of dental structure produced by differences in the arrangement and relative amount of these three components are in themselves very numerous; and they are vastly increased through the diversities exhibited by the minuter characters of each tissue. To some of these we have already alluded; and instead of formally detailing others, we will adduce a few examples, by way of proof, of the assistance which microscopic research has already rendered to the Naturalist.

There have been few questions in paleontology more fruitful of discussion, than that of the precise zoological affinities and habits of life of the great extinct Megatheroid quadrupeds of South America. They present the strange phenomenon of a skeleton more closely resembling that of the sloths than any other; not merely expanded to a colossal size, but developed with a massiveness unparalleled in any other animals of similar dimensions. The feet, on the other hand, were constructed rather upon the plan of those of the ant-eaters and armadilloes; being obviously capable, in virtue of their long, curved, sharp-edged claws, of being used as efficient instruments for digging or excavating the soil. In consideration of these peculiarities of conformation, as long as attention was not paid to the characters furnished by the teeth, an opinion was entertained by many distinguished palæontologists, that these strange beasts must have burrowed in the ground, like the pigmy armadilloes of the present time, and have fed upon the roots which they there met with. From the examination of the minute structure of their teeth, however, ample evidence was obtained by Professor Owen, that the food of the Megatheroids must have been of the same description with that of the existing sloths; that is, it must have consisted of vegetable matter capable of being easily reduced

to a pulpy state, such as the leaves and young shoots of trees for the teeth are entirely destitute of enamel; without which it is impossible that tough fibrous roots could be ground down. The greater part of their substance is made up of coarse vascular dentine; the exterior being composed of vascular cementum, and the harder non-vascular dentine forming only a hollow cylinder between the two. The conformation of the skeleton having been re-investigated by Professor Owen under the guidance of this most important indication, he succeeded in obtaining a most satisfactory solution to the problem of their mode of existence. They prove to be Sloths in all their essential characters-but adapted to live upon the ground, instead of climbing trees; which could not have sustained their enormous weight, even had it been possible to endow them with the climbing power. Not only were their limbs expanded to a colossal bulk, but, from its size and strength, their tail also was obviously designed for an organ of support; forming a firm tripod with the hind legs. Upon this tripod we may conceive the gigantic Mylodon or Megatherium to have reared itself, after having excavated with its trenchant claws the earth around the roots of the trees on whose leaves its hungry eyes were fixed; thus, raising itself upright, and placing its fore-feet against the trunk, it would sway the tree to and fro, until it had brought it down, and so provided itself in its foliage and tender shoots with a supply of food.

The 'Odontography' of Professor Owen will afford us another example of the value of the microscope in determining the nature of a doubtful fossil. This work contains the results of the laborious and accurate observations of its distinguished author upon almost every principal tribe of the three classes of vertebrata in which teeth are present; and must constitute the foundation of all future researches on this subject. In the case in question, a few scattered teeth were almost the only indications of animal life throughout an extensive stratum: and they happened not to be characterised by any peculiarity of external form-so that their real nature would have remained doubtful, unless the microscope had enlightened us on their internal structure. Some years ago, certain detached teeth were found in the keuper-sandstone of Wirtemberg, and were described

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