The bone is every where surrounded by a proper mem brane, which is supplied with blood vessels; the mem branous part of the bone itself seems also tc possess a set of vessels for its support, and the bones are pierced with numerous holes, through which arteries enter to nourish the internal membrane, and probably to secrete the mar row. Few nerves have been traced into the bones, and, in their natural state, they are without sensation, yet like other parts, into the composition of which dense mem brane enters, they become extremely painful in certain states of disease.
The process by which bone is formed, called ossifica tion, is one of the most curious that occurs in the animal economy, and respecting which there has been an abun dance of speculations and hypotheses. In the fcetal state, the body is without any pruper bones, those parts which afterwards acquire the osseous texture, possessing the ge neral figure of the future bones, but being composed merely of soft membranes. These membranes seem to become gradually more and more dense, until at length they acquire the nature of cartilage ; then small points of bone appear on their surface ; from these, as from cen tres, bony fibres radiate in all directions, and these ossified spots gradually enlarge, until at length they unite, and the whole becomes a solid body. These facts were as certained by Haller, who minutely examined the gradual evolution of the chick in ovo, during the different stages of its incubation, and it appears that the same changes occur in the embryo of viviparous animals.
Before the nature of bone was thoroughly understood, it was conceived that the membrane became converted into bone by some mechanical means, as by the compres sion of the contiguous muscles, by the evaporation of its watery parts, or the condensation of the membrane by the heat of the body. Duhamel formed an hypothesis, which professed to be founded upon experiment, and was, for a long time, very generally received. He supposed that there was an analogy between the bones of animals and the stems of trees, and imagined that bone was gene rated by the condensation of successive layers of the in vesting membrane, in the same manner as the annual rings of wood, in the trunk of a tree, are formed from the bark of the preceding season. He founded his experiments upon the fact which had been discovered, that if the root of madder was mixed with the food of animals, it commu nicated a red tinge to their bones; and he affirms that, by feeding animals for a certain period with madder, and then omitting it for some time, again resuming its use, and again discontinuing it, the bones of animals that had been thus treated were composed of successive rings of red and white matter. Mr. John Bell shrewdly remarks,
that when speculators perform experiments, they gene rally find exactly what they desire to find, and so it seems to have been with Duhamel. We are now assured that the membrane surrounding the bone is of a totally different nature from the osseous part of the bone, and could never be formed from it, without an entire change of its nature; that the different parts of the bone are tinged with the madder, in proportion to the quantity of earthy matter which they contain; and that the membrane which sur rounds the bone is not itself affected by it. Dr. Ruther ford discovered that this curious effect of madder depends upon the attraction between this body and the earth of bone, by which they unite and form a compound of a red dish purple colour. When the particles of madder are received into the stomach, like many other extraneous substances, they are absorbed and carried into the blood ; and when, in the course of the circulation, they arrive at the bones, they are separated by their affinity for the earthy matter. As the external layers of the bone con tain the most earth, they will soonest acquire the red co lour, will exhibit the deepest shade, and retain it longer than the other parts.
With respect to the manner in which the earth is car ried into the cartilage, so as to convert it into bone, it seems to be a case of secretion analogous to many other operations in the body, where the arteries possess the power of either separating particles already existing in the blood, and appropriating them to some specific pur pose, or forming new combinations, which may be after wards detached from the mass, and employed in different ways.
As to the cause which determines this effect to be pro duced at particular periods of our existence, we can say little more than that we find it to be a matter of fact. It is a part of the general constitution of the animal system, that at regular times, certain changes should take place, without our being able to assign any physical cause for them. The final cause of the present order of things is sufficiently obvious, and we cannot conceive of an arrange ment better adapted to the situation of the animal at the commencement of its existence, or to the alteration which it afterwards experiences. In the first instance, softness and flexibility are absolutely requisite, and hardness would be injurious; while, as the necessity For resisting external violence gradually arises, the capacity for resistance is proportionably produced.