Its Latin term, os, is supposed to be derived from the Hebrew word ozam, strength. The bones of animals constitute their firm, solid support; and their varied articulations give the animal that flexibility, those minute and complicated motions, on which our numerous functions depend. There appears to be no bone in the human body that does not possess every movement required, except such as would be inconsistent with its more general and useful purposes. The solidity of bones depends on an earthy deposition, in a manner to be afterwards described; a deposition which some have styled a secretion, though with little propriety, as a bony substance is sometimes deposited by all the ex-halant arteries: we have even seen ossifications in the corpora cavernosa penis. The bones are said by minute anatomists to be 304 in number.
Bones consist of fibres crossing each other in different directions. In the middle of the long bones these fibres are closely united, so as to form an almost solid substance. At their extremities the fibres are more distant, forming a distinct reticulated texture. As the Jong bones are solid, so they are hollow, to give resistance in a greater proportion than weight: the extremities are enlarged in bulk, to afford a more advantageous attachment to the muscles; but their cavities are divided into cells, and in these there is no part free from this reticulated substance. In thisway, we believe, the weight of a given length at the extremity or the middle is not very different.
The substance of bones is now known to be a calcareous phosphat, which is deposited by exhalant arteries; and this substance is constantly renovated, while an equal portion is carried off by absorption. The part absorbed is carried off by urine; and we find in the disease styled mollities ossium the urine containing a large portion of this calcareous neutral. Lime is found in a great variety of our aliment; but the acid is the production of the animal economy, or rather perhaps a modification of some common acid, (probably the muriatic,) by means yet unknown to us. This calcareous salt is deposited in fibres in the long bones, and these fibres placed longitudinally are in the middle of these closely compacted, but diverge and separate towards the joints, leaving a space connected by cross threads of bone to form the net work. We find by proper preparation, not only this fibrous but a laminated structure also; that is, the fibres are disposed even in the hardest bones in strata, connected by crossing threads of a bony matter. The flat bones have no cavities; but, when they rise in protuberances, the same reticulated structure is observed in their substance: so cautious, apparently, is nature to give every advantage to the muscular power, without adding inconveniently to the weight.
The form which bones ultimately assume, is in a great degree owing to the action of the muscles and of the arterial system. In the youngest foetus indeed the general shape is observable; but the extremities are larger, and the cavities of the long bones still retain the reticulated bony structure. When the action of the strong muscles of the extremities condense the bony substance, these last cancelli disappear as no longer useful; but the tendons which possess no contractile power, cannot have the same effect on the ends of the bones, though these are in some measure condensed; for. by increasing age, their bulk becomes less, and their density greater. In old age the contrary change takes place. The bones indeed retain their form, but the proportion of the calcareous phosphat is lessened, and their specific gravity greatly diminished.
The process of the formation of bone was for a long lime overlooked or mistaken; and even so late as the era of Duhamel, it was generally thought that bouy layers were deposited from the periosteum, as the woody layers of trees are from their bark. Traces indeed of more philosophical and correct views appear in earlier authors, but they were disregarded; and it was singular that it should not have occurred to Duhamel and his followers, that, if their system were true, man must yearly increase in bulk like a tree. Another error long prevailed, that bone was only a hardened cartilage. Were this the case, we should find the one gradually changing to the other: they are united indeed by bony protuberances in appearance, shooting into the cartilage, but it is at once obvious where bone ends, and cartilage begins.
In the foetus, at its earlier stages, we find the future bone a gelatinous substance, covered with a membrane, the future periosteum. It gradually becomes firmer and whiter; but even at birth the bony system is flexible, so as to admit of considerable motion in all its parts, to adapt itself to the passage in birth. For a time a considerable proportion of the extremities of the bones is cartilaginous, which has occasioned the idea just mentioned; but when the change is carefully observed, new blood vessels are perceived to penetrate the cartilage, and the osseous matter is deposited from them in the body of the latter, which enlarges till it extends to the bone. This additional portion is for a long time easily separable from the rest of the bone, as united only by a layer of cartilage, and called an epiphesis; and when the union is more complete, this additional portion is called an apophysis. The object of this mechanism is easily explained. Bones are formed from the osseous matter deposited in points, which are either added to longitudinally, or in rays, as from a centre. Where great strength is required, the former structure is observable; but a continuation of the longitudinal fibres would weaken the bone, and a different direction is necessary; besides, as from bony centres, a substance is more quickly produced, when the child begins to walk, his strength will thus increase with a rapidity proportioned to his increase of bulk. We know that bones are nourished by arteries, since in young animals they are reddened by an injection thrown into the general arterial system; and in more advanced age, are coloured by feeding an animal with madder. The experiment so often tried and quoted to support many different sys terns, was not understood till lately examined by Dr. Rutherford. He found it to arise from an affinity between the colouring matter and earth, forming what the painters call a lake. If to a solution of madder in distilled water with muriat of lime phosphat of soda is added, a double decomposition takes place: the mu-riated soda continues in solution, and the colouring part of the madder is precipitated in union with the phosphat of lime. It is thus attracted in the bloodvessel-, by the calcareous phosphat, and deposited with it.