Besides the marrow, which is contained in the cavities of the round bones, there is a quantity of oil in the solid part of the bone. The use of this oil, as well as of the marrow, does not appear to be well understood. The idea that the oil of bones serves to render them less brittle, and more flexible, appears to be too mechanical, and does not accord with the nature of the body in question ; for it is probable, that the oil is not diffused through the gene ral body or substance of the bone, but is lodged in small cells like the marrow and the fat. Upon the whole, this is a point respecting which we are not able to give any very satisfactory opinion.
In considering the composition of the bones, an interest ing question presents itself respecting the origin of the phosphate of lime : Is it received into the system along with the food, or have the organs of assimilation and se cretion the power of generating this salt ? Then, if we suppose this point to be satisfactorily explained, we have still to inquire respecting the mode in which the earth is deposited in the bones. If we conceive it to be present in the blood, we have still to learn what causes it to pass into those particular vessels that go to the bones ; and how is it disposed of at those times when it is not required for the growth or the reparation of these parts ? There is also very considerable difficulty in conceiving of the manner in which the earth quits the vessels and is deposited in the mem brane. We may suppose it to be poured out from the ex tremities of the vessels, or that it might be extravasated from their sides, or we may suppose it possible that it may remain lodged in them, so as, in fact, to convert the arte ries themselves into the osseous fibres. Perhaps, of these operations, the first is the most probable, and the one most analogous to the other operations of the animal economy ; yet this affords no explanation of the fibrous form which the bony matter is disposed to assume ; nor is it easy, upon any supposition, to reconcile this mechanical disposition of the bones with any mode in which we can conceive their formation to be effected.
Another subject of inquiry is concerning the manner in which the earthy matter is attached to the membranous part of the bone. It has been supposed by some physio
logists, and it appears to have been the prevailing opinion among the older writers, that the particles of earth exist ed in the blood, and that, being poured out from the se creting arteries, they were deposited between the inter stices of the membranous matter, and lodged there almost after the manner of extraneous bodies. Yet this has beet thought by other writers to afford too mechanical a view of the subject, and not to accord with the ideas of vitality which must belong to every part of an organized body.
With respect to the vital properties of bone, it appears that they exist only in a very imperfect degree. Bones are scantily supplied with blood-vessels, few if any nerves are distributed to them, and we judge of the presence of absorbents rather from observing effects, which cannot be ascribed to any other cause, than from being able actually to demonstrate their existence. Bone is, therefore, ne cessarily devoid of sensibility, and is without contractility ; ,t. partakes, in a small degree only, of the actions of the system ; its changes of all kinds are effected slowly, and often in an imperceptible manner. There are, however, many circumstances, which prove to us that bones are under the influence of the arterial and absorbent systems; for there are many facts which render it highly probable, that the earth of bones undergoes a gradual change, so that all the particles which enter into its composition at one period are, after some time, replaced by others. The diseases to which bones are subject, also show that they are under vascular influence, and are connected with the vital actions of the system. They have affections which are analogous to the inflammation, swelling, and suppura tion of the soft parts, although considerably modified by their situation and structure. Under these circumstances, also, bones are not unfrequently found to possess very great sensibility to pain ; and they exhibit, in an obvious manner, the power of separating the living from the dead parts, and of repairing those portions which have been se parated or destroyed.