and slide easily upon the bones. 2. To keep in due order, and to support the vessels in their passage to the bones. 3. By being firmly braced on the bones, to assist in setting limits to their increase, and to check their overgrowth. 4. To strengthen the conjunction of the bones with their epiphyses, ligaments, and cartilages, which are easily separated in young creatures, when this membrane is taken away. 5. To afford convenient origin and insertion to several muscles which are fixed to this membrane. And, lastly, to warn us when any injury is offered to the parts it covers.
The Bones are the most hard and solid parts of the body, and generally of a white colour; only in a living creature they are bluish, which is owing to the blood in the small vessels under their surface.
Bones are composed of a great many plates, each of which is made up of fibres or strings united by smaller fibrils; which being irregularly disposed and interwoven with the other larger fibres, make a reticular work. This texture is plainly seen in the bones of fœtuses, which have not their parts closely compacted, and in the bones of adults, which have been burnt, long exposed to the weather, or whose composition has been made loose by diseases.
The plates are said to be firmly joined to each other by a great number of claviculi, or small bony processes, which, rising from the inner plates, pierce through some, and are fixed into the more external ones.
Though the exterior part of bones is composed of firm compact plates, yet they are all more or less cavernous internally. In some, the solid sides are brought so near, that little cavity can be seen; and in others, the cavities are so large, that such bones are generally esteemed to be hollow or fistular. But the internal spongy texture is most evident in young animals.
This spongy, cavernous, internal part of bones, is generally called their cancelli or lattice-work.
The twisting and windings which these cancelli make, and the interstices which they leave, differ considerably in figure, number, and size; and therefore form little cells, which are as different, but communicate with each other.
The cancelli sustain the membranous bags of the marrow which are stretched upon them, and thereby hinder these membranous parts from being torn or removed out of their proper places, in the violent motions and different postures which the bones are employed in.
The depressions between the fibres of the external plates of bones appear like so many furrows on their surface, into each of which the periosteum enters.
Both on the ridges and furrows, numerous little pits or orifices of canals are to be seen, by which the vessels pass to and from the bones.
After a successful injection, the arteries can be traced in their course from the pits to the plates and fibres.
We may conclude, from arteries being accompanied with veins, so far as we can trace them in every other part of the body, that there are also veins in the bones.
The bones of a living animal are fo insensible that they can be cut, rasped, or burnt, without putting the creature to pain, and the nerves distributed in their substance cannot be shewn by dissection; from which it might be inferred, that they have no nerves distributed to them: But the general tenor of nature, which bestows nerves to all the other parts, should prevent our drawing such a conclusion.
The vascular texture of bones must make them subject to obstructions, ecchymoses, ulcers, gangrenes, and most other diseases which the softer parts are affected with; and therefore there may be a greater variety of caries than is commonly described.
On the internal surface of the solid parts of bones, there are orifices of canals, which pass outwards through the plates to open into other canals that are in a longitudinal direction, from which other transverse passages go out to terminate in other longitudinal canals; and this structure is continued through the whole substance of bones, both these kinds of canals becoming smaller gradually as they approach the outer surface.—These canals are to be seen to the best advantage in a bone burnt till it is white: When it is broken transversely, the orifices of the longitudinal canals are in view; and when we separate the plates, the transverse ones are to be observed.
Most bones have one or more large oblique canals formed through their sides for the passage of the medullary vessels.
The bones sustain and defend the other parts of the body.
Bones are lined within, as well as covered externally, with a membrane; which is therefore commonly called periosteum internum.
The internal periosteum it an extremely fine membrane; nay, frequently it has a loose reticular texture; and therefore it is compared by some to the arachnoide coat of the spinal marrow: so that we cannot expect to divide it into layers as we can divide the external periosteum. We can, however, observe its processes entering into the transverse pores of the bones, where probably they are continued to form the immediate canals for the marrow distributed through the substance of the bones; and along with them vessels are sent, as from the external periosteum, into the bone. These processes being of a very delicate texture, the adhesion of this membrane to the bone is so small, that it separates commonly more easily from the bone than from the marrow which it contains.
From the internal surface of the internal periosteum, a great number of thin membranes are produced; which, passing across the cavity, unite with others of the same kind, and form so many distinct bags, which communicate with each other; and these again are subdivided into communicating vesicular cells, in which the marrow is contained.
The Marrow is the oily part of the blood, separated by small arteries, and deposited in these cells. Its colour and consistence may therefore vary according to the state of the vessels, and their distribution on the membranes of the cells.
Besides the arteries already mentioned as being sent from the bones to the marrow, there is at least one artery for each bone; several bones have more, whose principal use is to convey and secern this oily matter.
