General discussion about bones and joints

We affirm: among the bones there are those that act as a foundation in relation to the body, and the body is built on them. These are, for example, the vertebrae of the spinal ridge. It is the basis for the body on which the body is built, just as a ship is built on a beam that is laid first. And some bones serve as armor and protection in relation to the body, such as the parietal bone, while others serve as a weapon that reflects shocks and damage. These are, for example, bones called “spikes,” which sit on the dorsal vertebrae like needles. There are bones that are like filling in the spaces between the joints These are, for example, the sesamoid bones between the joints of the fingers. Other bones are suspended from parts of the body that should hang This is a bone similar to the letter lambda, which is connected with the muscles of the larynx, tongue and other muscles.

The set of bones serves as support and support for the body. Those of these bones that are needed only for support and protection and are not needed to set the limbs in motion are solid, although they also have holes and crevices that cannot be dispensed with. And in those bones that are also needed for movement, the size of the voids is increased, and the void in the middle of the bone is created as a single one, so that the body of the bone does not need scattered food sources - this would make the bone loose, while its body, on the contrary, is dense. The food of these bones, that is, the brain, is collected in itself, like a filling. The benefit of increasing the void is that the bone becomes lighter, and the benefit of unifying the void is that the body of the bone remains dense. The density of the bone body is useful in that the bone does not break from sudden movements, and the usefulness of the marrow contained in the bone is that it nourishes it, as we explained in detail earlier, and constantly moisturizes it, so that the bone does not crumble from movements that dry it out. Thus, the bone, although there is emptiness in it, is similar to solid. The emptiness is small when there is a greater need for hardness, and great when there is a greater need for lightness.

Tubular bones are created this way for the sake of the mentioned matter of nutrition in addition, this is also necessary because something must pass through them, as, for example, the smell inhaled with the air through the ethmoid bone passes through, and also because of the excessive secretions of the brain that are driven through them.

All the bones are mutually adjacent and converge, so that there is no large gap between any bone and the one that follows it. However, between some of them there is a slight gap, which is filled with “additives” - cartilaginous or cartilage-like. They are created for the useful function that is inherent in cartilage between those bones in which this useful function should not be taken into account, the articulations are created without “additives”. This is, for example, the lower jaw.

There are different types of adjacencies between bones. Some of them are connected by means of a pliable joint, others are connected by a tight joint, although not motionless, while others are connected by a fixed joint wedged, sewn or glued.

A compliant joint is one in which one of the bones easily moves, while the other bone does not move. This is, for example, the articulation of the wrist and forearm.

The joint is tight, but not motionless - such when the movement of one of the bones is difficult and small in scope such is the articulation between the wrist and metacarpus, or the articulation between the two bones of the metacarpus. As for a fixed joint, this is a joint in which neither of the two bones can move alone at all such, for example, is the articulation of the sternum.

A wedged joint occurs when one bone has a protrusion, and the other has a depression, into which this protrusion is wedged so much that it cannot move in it. Such, for example, are teeth in their sockets. A stitched joint is one where both bones have grooves and teeth, like a saw. The teeth of one bone are located in the grooves of another, like coppersmiths joining sheets of copper. This joining  is called “seaming” and “sewing”. This is how the bones of the skull are connected.

A glued joint can be glued longitudinally, or it can also be glued transversely. These are the lower vertebrae of the spinal ridge the upper vertebrae are not connected by fixed joints.