the bone they belong to; for by this means, the surface of contact between the two bones of any articulation being increased, their conjunction becomes firmer, and the muscles inserted into them act with greater force, by reason of their axes being further removed from the center of motion.
The softness of the ends of bones may be of some advantage in the womb, and at birth, after which the ossification begins at different points to form epiphyses, before the ossification can extend from the middle to the ends of the bones.
However solid and compact adult bones are, yet they were once cartilages, membranes, nay, a mere jelly. This needs no further proof, than repeated observations of embryos when dissected: And how much more tender must the bones be before that time, when neither knife nor eye is capable to discover the least rudiments of them? By degrees they become more solid, then assume the nature of gristles, and at last ossify; the cohesion of their plates and fibres always increasing in proportion to their increased solidities; as is evident from the time necessary to unravel the texture of bones of people of different ages, or of dense and of spongy bones, or of the different parts of the same bone, and from the more tedious exfoliations of the bones of adults than of children.
The ossification of bones depends principally on their vessels being so disposed, and of such diameters, as to separate a liquor, which may easily turn into a bony substance, when it is deprived of its thinner parts; as seems plain from the observation of the callous matter separated after fractures and ulcers, where part of the bone is taken out: For in these cases, the vessels extending themselves, and the liquors added to them, are gradually formed into granulated flesh; which fills up all the space where the bone is taken from, then hardens, till it becomes as firm as any other part of the bone. This happens frequently, even when the ends of the diseased bone are at a considerable distance from each other.
The induration of bones is also greatly assisted by their being exposed, more than any other parts, to the strong pressure of the great weights they support, to the violent contraction of the muscles fixed to them, and to the force of the parts they contain, which endeavour to make way for their own further growth. By all this pressing force, the solid fibres and vessels of bones are thrust closer; and such particles of the fluids conveyed in these vessels as are fit to be united to the fibres, are sooner and more firmly incorporated with them, while the remaining fluids are forcibly driven out by the veins, to he mixed with the mass of blood. In consequence of this, the vessels gradually diminish as the bones harden. From which again we can understand one reason, why the bones of young animals sooner re-unite after a fracture than those of old; and why cattle that are put too soon to hard labour, seldom are of such large size as others of the same brood who are longer kept from labour.
From the effects of pressure only it is, that we can account for the bones of old people having their sides much thinner, yet more dense and solid, while the cavities are much larger than in those of young people; and for the prints of muscles, vessels, &c. being so much more strongly marked on the surfaces of the former than of the latter, if they belong to people of near the same condition in life.-—Pressure must likewise be the cause, which, in people of equal ages, makes these prints stronger in the bones of those who had much labour and exercise, than they are in people who have led an indolent unactive life.
Having thus considered the bones when single, we shall next shew the different manner of their conjunctions. To express these, anatomists have contrived a great number of technical terms; about the meaning, propriety, and classing of which, there has been a variety of opinions. Some of these terms it is necessary to retain, since they serve to express the various circumstances of the articulations, and to understand the writers on this subject.
The Articulations are most commonly divided into three classes, viz. symphysis, synarthrosis, and diarthrosis.
Symphysis, which properly signifies the concretion or growing together of parts, when used to express the articulations of bones, does not seem to comprehend, under the meaning generally given to it, any thing relating to the form or motion of the conjoined bones; but by it most authors only denote the bones to be connected by some other substance; and as there are different substances which serve this purpose, therefore they divide it into the three following species:
1. Synchondrosis, when a cartilage is the connecting substance: Thus the ribs are joined to the sternum; thus the bodies of the vertebræ are connected to each other; as are likewise the offa pubis.
2. Synneurosis, or syndesmosis, when ligaments are the connecting bodies, as they are in all the moveable articulations.
3. Syssarcosis, when muscles are stretched from one bone to another, as they must be where there are moveable joints.
The second class of articulations, the synarthrosis, which is said to be the general term by which the immoveable conjunction of bones is expressed, is divided into three kinds.
1. The future is that articulation where two bones are mutually indented into each other, or as if they were sewed together. Thus the bones of the head are joined; thus epiphyses are joined to the bones, before their full connection and union with them.
2. Gomphosis is the fixing one bone into another, as a nail is fixed in a board: Thus the teeth are secured in their sockets.
3. Schindylesis, or ploughing, when a thin lamella of one bone is received into a long narrow furrow of another: Thus the processus azygos of the sphenoid, and the nasal process of the ethmoid bone, are received by the vomer.
The third class, or diarthrosis, is the articulation where the bones are so loosely connected as to allow large motion. This is subdivided into three kinds.
