The blood, which remains after the secretion of the marrow, is returned by proper veins, which are collected from the membranes into one or two large trunks, to pass out at the same holes at which the arteries enter.
The vessels of the marrow, wrapt up in one common coat from the periosteum, pass through the bones by proper canals; the most considerable of which are about the middle of each bone, and are very oblique.
From the structure of the contents of the bones, we may judge how these parts, as well as others, may be subject to oidema, phlegmon, erysipelas, schirrhus, &c. and may thence be led to a cure of each, before the common consequence, putrefaction, takes place, and frequently oocasions the loss of the limb, if not of the patient.
The marrow is of very considerable use to the bones; for by entering their transverse canals, and passing from them into the longitudinal ones, it is communicated to all the plates, to soften and connect their fibres, whereby they are preserved from becoming too brittle.
When the marrow, after having served the uses mentioned, is reassumed into the mass of blood, it corrects the too great acrimony communicated to the saline particles of our fluids by their circulation and heat; in the same manner as lixivial salts are blunted by oil in making soap.
Since it is the nature of all oil to become thin and rancid when exposed long to heat, and bones have much oil in their firm hard substance, we may know why an ungrateful smell, and dark-coloured thin ichor, proceed more from corrupted bones than from other parts of the body; and we can understand the reason of the changes of colour which bones undergo, according to their different degrees of mortification.
Though bones so far agree in their structure and annexed parts, yet we may observe a considerable difference among them in their magnitude, figure, situation, substance, connection, uses, &c. Of these we shall only mention two, viz. that some bones are broad and flat, while others are long and round.
The broad bones have thin sides, by the plates being soon and equally sent off to form the lattice-work; which therefore is thicker, and nearly of an equal form all through. By this structure, they are well adapted to their uses, of affording a large enough surface for the muscles to rise from, and move upon, and of defending sufficiently the parts which they inclose.
The round bones have thick strong walls in the middle, and become very thin towards their ends, which is owing to very few plates separating at their middle; where, on that account, the cancelli are so fine and small that they are not taken notice of: But such bones are said to have a large reservoir of oil in this place. Towards their ends the lattice-work becomes very thick, and rather more complete than in the other sort of bones.—These round bones having strong forces naturally applied to them, and being otherwise exposed to violent injuries, have need of a cylindrical figure to resist external pressure, and of a considerable quantity of oil to preserve them from becoming too brittle. Besides which, they are advantageously provided with thick sides towards their middle, where the greatest forces are applied to injure them; while their hollowness increases their diameter, and consequently their strength, to resist forces applied to break them transversely.
Many bones have protuberances, or processes, rising out from them. If a process stands out in a roundish ball, it is called capet, or head.—-if the head is flatted, it obtains the appellation of condyle.A rough unequal protuberance is called tuberosity.When a process rises narrow, and then becomes large, the narrow or small part is named cervix, or neck.Long ridges of bones are called spines.Small processes as terminate in a sharp point, have the general name of coronæ, or coronoid, bestowed on them, though most of them receive particular names from the resemblance they have, or are imagined to have, to other substances, e.g. mastoid, styloid, &c.Such processes as form brims of cavities, are called supercilia,
Processes serve for the advantageous origin and insertion of muscles, and render the articulations firm and liable.
In children these processes are real epiphyses, or distinct bones, which are afterwards united to the other parts; such are the styloid processes of the temporal bones, processes of the vertebræ, trochanters of the thigh, &c.
0n the surfaces of a great many of the bones there are cavities, or depressions: If these are deep, with large brims, authors name them cotylæ; if they are superficial, they obtain the designation of glenæ, or glenoid. These general classes are again divided into several species:—-Of which pits are small roundish channels sunk perpendicularly into the bone;—furrows are long narrow canals, formed in the surface;—nitches or notches, small breaches in the bone;—sinuosities, broad, but superficial depressions without brims;—fossæ, large deep cavities, which are not equally surrounded by high brims;—sinuses, large cavities within the substance of the bones, with small apertures;—firamena, or holes, canals that pierce quite through the substance of the bones.When this last sort of cavity is extended any long way within a bone, the middle part retains the name of canal, and its ends are called holes.
The cavities allow the heads of bones to play in them; they lodge and defend other parts; they afford safe passage to vessels, muscles, &c.
To far the greater number of bones, whose ends are not joined to other bones by an immoveable articulation, there are smaller ones annexed, which afterwards become scarce distinguishable from the substance of the bone itself. These are called epiphyses, or appendices. Some bones have one, others have two, three, or four of these appendices annexed by the means of cartilages, which are of a considerable thickness in children, but by age become thinner.
Several processes (e.g. trochanters of the thigh, spine of the scapula, &c.) have epiphyses; and processes frequently rise out from epiphyses; for example, at the lower end of the femur, ulna, tibia, &c.
The epiphyses are united chiefly to such bones as are destined for frequent and violent motion; and for this purpose they are wisely framed of a larger diameter than
