But, independent of the deposition of earthy matter iR the bone, the membranous part itself undergoes a com plete change in its structure. The original cartilage is of a uniform consistence in all its parts, whereas, in the fully formed bone, the centre is nearly hollow, and the more solid parts which surround it have their density much in creased as they approach to the surface, while the whole is composed of plates disposed in a specific manner, with numerous cavities or interstices between them. How then, it may be asked, is the cartilage converted into the membranous matter which enters into the composition of bone ? The only way in which we can well suppose this thane to be effected, is by the gradual absorption of the cartilage, and the deposition of other particles of animal matter, which may constitute the future bone ; but upon this supposition we arc at a loss to explain by what means the vessels have their actions or powers so directed, that the mechanical structure of the part should be so much altered, or that the position of the new particles should be so different from that of the former matter.
The manner in which bones are repaired, when they have suffered from external violence, is as remarkable as that by which they are originally formed ; and this pro cess has, in like manner, given rise to much speculation and controversy. The common opinion among the older writers was, that a soft mucus or jelly is, in the first in stance, deposited between the ends of a broken bone, that it is condensed by heat or pressure into a hard gluten, which they called callus, and they conceived that it always retained its membranous state, and was never converted into proper osseous matter. Some physiologists supposed that this callus was produced from effused and coagulated blood, and others that it was derived from the membrane enclosing the old bone. It is now, however, generally un derstood, that the reparation of bone is effected by a process very similar to that by which it was originally pro duced. The arteries of the divided bone throw out a soft matter, of a mucous or gelatinous nature, which gradually hardens into a cartilaginous substance, or is replaced by it ; in this the earthy matter is deposited, and the whole is new-moulded, in the manner which has been described above. What is the immediate cause by which this pro cess is effected, why the arteries throw out this substance, how'it is moulded into the proper form, whence the sup ply of earth is derived, just at the exact period when it is required by the wants of the system, are questions that have not yet been satisfactorily answered. The hypothe
ses that have been formed upon the subject have been, in some cases, the mere expression of the fact in different words ; in others, the substitution of the final for the effi cient cause, or they have proceeded upon the assumption of some imaginary agent, created by the fancy of the wri ter, to meet the present emergency. We cannot doubt that there is a proper efficient cause for this, as well as for every other change which occurs in the system ; and that, were our knowledge of the animal economy complete, we should be able to refer it to the general laws by which the body is directed. At present, however, our acquaintance with many of the minute operations of nature is extremely limited, and we are only retarding the advancement of knowledge by premature attempts at explaining them.
The chemical nature of bone was very imperfectly un derstood until about forty years ago, when Galin discover ed that it contains an earthy salt, composed of phosphoric acid and lime. Later and more accurate researches, espe cially those of Berzelius, have shown that the salt is in the state of a sub-phosphate, and that it is mixed with a small quantity of phosphate of magnesia and fluate of lime. The earth of bones is insoluble in water, bears a high tem perature without being decomposed, and is, in all respects, of a very imperishable nature. We accordingly find, that the compact bones of animals are frequently met with in a tolerably perfect state, after the lapse even of many cen turies, and after having been exposed to all the revolutions to which the surface of the earth has been subjected. In deed, from some of the discoveries that have been lately made by Cuvier and other modern naturalists, we are in duced to believe that bones still remain belonging to ani mals that must have existed before any traditional or his torical records of which we are in possession, and when the earth was peopled by animals of a different kind from any of its present inhabitants.
We are indebted to Mr. Hatchett for the discovery of the nature of the -membranous part of bone, which he found to possess all the characters of coagulated albumen; the substance which has been already stated to be the basis of membranous matter of all descriptions. 'We find, also, that bones contain a quantity of jelly, which can be ex tracted by boiling ; and that this jelly is much more abun dant in the bones of young than of old animals.