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to our knowledge of ciliary action has been chiefly the result of the systematic observations of Professors Purkinje, Valentin, and Sharpey, and was one of the earliest and most valuable fruits of the achromatization of the microscope.
That the red colour of the blood of the higher animals does not exist in the fluid, but only in certain particles which float in its current, was known to Leeuwenhoek, and perhaps to still older microscopists: and the forms of these particles were observed to vary in different classes of animals. It was not known until recently, however, that these floating particles are true cells, analogous to those of the simplest Alga-each having an independent life of its own, whilst all are subservient to the life of the being through whose vessels they are carried-though in what precise way they are so has not yet been satisfactorily ascertained. It has been further proved by the researches of Mr. Gulliver, that there is not only a difference of form, but also of size, between the blood-discs of various animals; so that it is in many instances possible to distinguish with certainty between the blood of two animals belonging even to the same natural group. The contrast in size is very strongly marked in particular cases. While the diameter of the circular Human blood-disc averages something less than one three-thousandth part of an inch, that of the circular blood-disc of the Musk-deer is no more than one twelve-thousandth; and the large oval blood-disc of the Proteus measures nearly as much as one three-hundredth of an inch in length, and half that amount across its short diameter. It is a very remarkable fact that all the animals yet known, which agree with the Proteus in the retention of the gills after the lungs are developed,―remaining, in fact, all their lives in the condition of great tadpoles,— agree with it also in possessing blood-discs of unusual size. The fact that this character is presented by the blood-discs of the Lepidosiren, has been regarded as of no mean importance in discussing the real affinities of that curious reptile-like fish, or fish-like reptile.
We shall afterwards advert to the microscopic analysis recently made of those various soft tissues of animals and plants, which are the immediate instruments of their vital operations, and on whose nature any general expression of
the phenomena of life must be founded. But it is more convenient to consider previously the structure of the hard parts of animals, such as bones, teeth, shell, &c.; which present features of great interest, not merely to the physiologist who concerns himself about the vital phenomena of beings now alive, but also to the geologist who seeks in the fragmentary remains of past generations for materials to aid him in constructing a history of the earth. Of the organization of bone, nothing more can be made out by the naked eye, or through the help of ordinary magnifiers, than that the solid substance of its densest portions is traversed by a series of canals, which carry inwards the blood-vessels distributed over the investing membrane (the periosteum); and that the spongy texture, found in the ends of the long bones and between the superficial layers of the thicker flat bones, derives its character from the presence of a vast number of minute chambers, or cancelli, which are separated from each other by irregular and incomplete partitions, and into which also the nutritious vessels are prolonged. Nothing whatever was known of the structure of the bony matter itself; and no one could have even ventured to imagine the elaborate minuteness of its organization. But when a thin section even of the very densest bone is examined by a microscope of sufficient power, the solid substance is seen to be penetrated by innumerable tubuli of extreme minuteness, radiating from a series of isolated excavations, of tolerably regular shape and size, and so interlacing each other as to open a communication among all the neighbouring cavities. To compare small things with great, let our readers conceive themselves in a cavern excavated in the centre of a vast mountain, with no principal entrance, but hundreds of narrow passages piercing its solid walls. Whichsoever of these they follow, it will conduct them, after many windings, into a cavern similar to that which they had left; and after passing through a long series of such passages and caverns, they will at last emerge into open day. Such, on a scale almost inconceivably minute, is the structure of solid bone. Let us reduce the spacious cavern to an excavation of 1.1500th of an inch in length, and 1.5000th of an inch in breadth; and let the passages be contracted from dimensions sufficient to admit of our
entrance to a diameter of 1.20,000th of an inch (which is further diminished in the smallest branches to 1.60,000th of an inch), and we then shall have some idea of the wonderful contrivance with which this solid and apparently impervious substance is channelled out, like a coal-field by the miner. There cannot be a doubt but that this curious organization is in some way subservient to the nutrition of bone. The tubuli are far too minute to allow of the entrance of the blood as a whole; its floating particles, indeed, being nearly as large as the caverns themselves. But the great regularity of their arrangement, which is always adapted to keep up a communication between the interior of the bone and the nearest point to which blood is conveyed, seems to justify the inference that they take up from the circulating current, and distribute through the texture, such portions of the fluid as the bones require for their maintenance and consolidation. Very thin bones are nourished by blood, which is simply distributed over their surface; but larger and thicker bones are traversed by passages, into which the vessels penetrate.
We are too conscious of the difficulty of conveying by words alone a distinct idea of so complex a structure, to venture to be sure that we have carried with us every one of our readers through the preceding description. We trust, however, that we have fully impressed them with the wonderful elaborateness displayed in the minute structure of bone. This elaborateness has no parallel in any of the hard tissues of which the skeletons of invertebrated animals are composed. And it seems designed, on the one hand, to minister to the continual changes which bone must undergo during the period of its growth, and, on the other, to confer upon bone that power of self-reparation which it so remarkably displays after disease or injury. It is interesting to observe that whilst, as in the case of the blood-corpuscles, there is no relation whatever between the size of the animal and the dimensions of the elementary parts of its bony skeleton,-the length of the cavernules and the diameter of the tubuli being nearly the same in the Elephant and in the Mouse, in the gigantic extinct Iguanodon and in the smallest existing Lizard,-there is yet a marked difference in these particulars between ani
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.