« PreviousContinue »
enter into the composition of both these substances, and which are chiefly obtained from the materials of common salt, as sulphuric acid and soda; but the proportions are too small to render it necessary to dwell upon them in a course of popular study. Bones, shells, cartilages, and membranes may therefore be regarded as substances of the same kind, differing only in degree of solidity from the different proportions that they possess of albumen and salts of lime.
Teeth, horn, coral, tortoise-shell, fish-scales, and the crustaceous integuments of crabs, millepedes, and beetles*, are all compounds of the same elements combined in different proportions, and rendered harder or softer as they possess a larger or smaller quantity of calcareous salts; ivory and the enamel of teeth possessing the largest quantity, and consisting almost exclusively of phosphate of lime, with a small proportion of animal matter.
The gelatine and albumen are unquestionably generated in the animal system itself from the different substances it receives under the form of food; and it is curious to observe the facility and rapidity with which some animals are capable of producing them. The gastrobranchus cæcus, or hagfish, a small lamprey-like animal of not more than eight inches long, will convert a large vessel of
*The hard integuments of insects have not, as here represented, the same chemical composition as those of other classes of animals: they are chiefly formed of a peculiar chemical substance called entomoline: also, of the animal substances here enumerated as having the same composition, some contain the phosphate of lime, others the carbonate, as their principal ingredient. Ed.
water in a short period of time into size or mucilage, of such a thickness that it may be drawn out in threads. The form and habits of this little animal are singular: Linnæus regarded it as a worm; but Bloch has removed it, and with apparent propriety, into the class of fishes. It is a cunning attendant upon the hooks of the fisherman; and as soon as it perceives a larger fish to be taken, and by its captivity rendered incapable of resistance, it darts into its mouth, preys voraciously, like the fabled vultures of Prometheus, on its inside, and works its way out through the fish's skin.
But though gelatine and albumen are unquestionably animal productions, the one a secretion from the blood, and the other a constituent principle of it, there is a doubt whether lime ought ever to be regarded in the same character. A very large portion is perpetually introduced into the stomach from without. In our lecture on the analogy between the structure of plants and of animals*, I had occasion to observe, that it forms an ingredient in common salt; not, indeed, necessarily so, but from the difficulty of separating the other ingredients from their combination with it: yet it enters not more freely into common salt than into almost every other article, whether animal, vegetable, or mineral, of which our diet is usually composed. And upon this common fact it is more generally conceived, at present, to be a substance communicated to the animal frame, than generated by it.
This opinion, however, is by no means established; and there are many circumstances that may lead us to a contrary conclusion. Though almost *Vol. I. Ser. 1. Lect. vIII.
every kind of food contains some portion of lime, it by no means contains an equal portion; and yet we find that a healthy young animal, whatever be the sort of food on which it is fed, will still provide lime enough from some quarter or other to satisfy the demand of its growing bones, and to maintain them in a due degree of solidity and hardness.
Again, the soil of some countries, as the mountains of Spain, for example, consists almost entirely of gypsum or some other species of limestone; while in other countries these are substances very rarely to be met with. It is a curious fact, that in that vast part of the globe which has been latest discovered, and to which modern geographers have given the name of Australia, comprising New Holland and the islands with which its shores are studded, not a single bed or stratum of limestone has hitherto been detected, and the builders are obliged to make use of burnt shells for their mortar, for which I have lately advised them to substitute burnt coral.* Now, it would be natural to suppose that the animals and vegetables of such a country would partake of the deficiency of its soil, and that the shells and bones which it produces would be less compact in their texture than those of other countries; yet this supposition is not verified by fact: nature is still adequate to her own work; the bones of animals are as indurated and perfect in these regions as in any parts of the old world; while the shells are not only as perfect, but far more
It is understood that some beds of chalk have since been discovered on the farther side of the Blue Mountains, but none is still to be traced on the hither side in any of the settlements of the colony.
numerous; and the frequent reefs of coral, altogether an animal production, that shoot forth from the shores in bold and massy projections, prove clearly that a coral rock, largely as it consists of lime, forms the basis of almost every island.
The prodigious quantity of lime, moreover, that is secreted by some animals at stated periods, beyond what they secrete at other times, seems to indicate a power of generating this earth in their own bodies. The stag, elk, and several other species of the deer-tribe, cast their antlers annually, and renew them in full perfection in about twelve weeks. These antlers are real bones; and those of the elk are sometimes as heavy as half a hundred pounds weight, and in a fossil state in Ireland have been dug up still heavier, and of the enormous measure of eight feet long, and fourteen feet from tip to tip; on beholding which, we may well, indeed, exclaim with Waller,
O fertile head! which every year
Could such a crop of wonders bear.
In like manner, many species of the crab and lobster tribes annually throw off and renew the whole of their crustaceous covering, and apparently without any very great degree of trouble. The animal at this time retires to some lonely and sheltered place, where, in its naked and defenceless state, it may avoid the attack of others of the same tribe which are not in the same situation: a line instinctively drawn now separates the shell into two parts, which are easily shaken off, when the secernent vessels of the skin pour forth a copious efflux or sweat of calcareous matter all over the
body, the more liquid parts of which are as rapidly drunk up by the absorbent vessels, so that a new calcareous membrane is very soon produced, which as speedily hardens into a new calcareous crust, and the entire process is completed in about a fortnight. This genus, also, in many of its species, is capable of reproducing an entire limb, with the whole of its calcareous casing, whenever deprived of it by accident or disease, or it voluntarily throws it off, to extricate itself from being seized hold off; though the new limb is seldom so large or powerful as the original. So, in other animals, we sometimes find a large and preternatural secretion of calcareous matter, in consequence of a diseased habit of particular organs, or of the system generally. The human kidneys are too often subject to a morbid affection of this kind, whence a frequent necessity for one of the most painful operations in surgery. The chalk-stones, as they are erroneously called, that are often produced in protracted fits of gout and rheumatism, are rather lithate of soda than any compound of lime; but instances are not wanting in which one of the lungs has been found converted into an entire mass of limestone.
In the Transactions of the Royal Society there are several cases related of young persons who, in consequence of a morbid habit, threw out a variety of calcareous excrescences, either over the hands and feet, or over the whole body*; and about four years since, a Leicestershire heifer was exhibited for a show in this metropolis, the head and neck of which were completely embedded in horny excres
* See also Mr. Baker's account of the porcupine-man, Phil. Trans, for 1755,