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mals of different zoological groups. Thus the cavernules of Reptilian bones are distinguishable from those of Birds and Mammals by their great length in proportion to their breadth; and those of Fish may be usually recognised by their angular form, and by the small number of their radiating tubules. In certain species of the last-named class, however, there is an approach to reptiles, in general conformation; the minute structure of the bones also exhibits the same transitional character. The cavernules of the bones of Birds and of Mammals do not differ considerably in size: but there are diversities in the origin and course of the radiating tubules, which usually render it very easy to distinguish them. For our knowledge of these diversities we have principally to thank Mr. Quekett and Mr. Bowerbank. We will now adduce an example or two of their applicability as distinctive characters, both in zoology and palæontology.
We have already noticed the position of the Lepidosiren as one of the most interesting questions at present under discussion among naturalists; and have adverted to the large size of its blood-corpuscles, as significant evidence towards determining its real place in the scale. We may now add, that in the minute structure of its bones, its relationship to the perenni-branchiate reptiles, rather than to fish, is no less decided,—the form and size of the cavernules, and the distribution of the tubuli, corresponding closely with what is seen among the former, there being nothing parallel in the latter class.
In the determination of the real nature of fossil bones, whose imperfect preservation or whose insufficient characters render their recognition difficult and uncertain, the value of the microscope has been no less satisfactorily established. Dr. Falconer, the distinguished investigator of Himalayan palæontology, having met with certain small bones, about which he was doubtful, placed them in the hands of Mr. Quekett for minute examination, and was informed, on the authority of the microscopic test, that they might certainly be pronounced reptilian, and probably belonged to an animal of the turtle tribe. They subsequently proved to be the toe-bones of the Colossochelys Atlas, the gigantic fossil tortoise, nearly twenty feet in length, discovered by Dr. Falconer in the Sivalik region.
The test has been applied, with equal success, by Mr. Bowerbank to the determination of some doubtful wingbones found in the chalk near Maidstone. The question of their ownership lay between the long-winged sea-birds, such as the Albatross, and the ancient Pterodactyles, or winged lizards. The evidence from external form inclined somewhat in favour of the former; and, as no Pterodactyle of a greater spread of wing than five or six feet had been previously known, it was thought an additional proof of the ornithic character of these bones, that they must have belonged to an animal whose wings measured at least eleven feet, when fully extended. But the minute structure of the bones of undoubted Pterodactyles is decidedly characteristic of their reptilian nature; and they have additional well-marked peculiarities, such as have not yet been found in any other animal. The fact, therefore, that a complete identity of structure has been ascertained to exist between the bones of the Maidstone fossil, and those of the genuine Pterodactyle, appears to us to settle the question of the real nature of the former. The structure in question is as unlike that of any bird, as it is accordant with that of this peculiar reptile. And the essential characters furnished by the minutiae of organization, are now universally admitted by competent judges to possess a higher value than those adaptive characters drawn from external configuration, which have reference only to the purposes of the organ. It is true, we must extend our ideas of the dimensions of the flying reptiles, which took the place of birds in the atmosphere of the ancient world. But this will not be a real difficulty in the apprehension of any one at all conversant with the other gigantic forms of reptilian life now extinct. Some future palæontologist, whose first ideas of the marine birds of our epoch had been formed upon the fossil remains of a Gull, might, with equal reason, object to the idea that a bird, of the dimensions of an Albatross, ever soared over our ocean. We cannot doubt but that the general application of this test will prove most advantageous in the determination of the true nature of fossil bones, whose external characters are ambiguous; and we may hope thus to see the termination of those conflicting statements, which have in so many instances obscured the truth, and led to acrimonious discussion.
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.
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.