As we advance in the examination of these articular genera, the extent of their motion diminishes. This has less opposition in many directions than the preceding, and less circumduction which always supposes an extensive opposition. Here this opposition is always limited to one direction only, to that of flexion and extension, for example.
We find this articular genus especially in the middle of the limbs, as at the elbow, the knee, the middle of the fingers in the articulations of the phalanges. Though the bone which composes them, inferiorly can move by itself but in one direction, yet it borrows from the loose motions of the superior articulation of the limb, so as to be able to be turned in every way.
The articular surfaces are found here as in the preceding genus, at the extremity of the bone, having the same axis as the bone; but they differ, 1st, in this that there are many eminences and cavities fitted to each other, an arrangement, which, by permitting the motion in one direction, prevents it in the others. Very commonly there are two kinds of round prominences, called condyles, which roll from before behind, or from without within, &c. upon two analogous cavities, that are separated by an eminence, which is received in the space between the condyles, as we see in the femoro-tibial, phalangeal articulations, &c. 2d. The breadth of the surfaces also distinguishes this genus from the preceding; this breadth insures its solidity, and prevents luxations, which besides are more to be feared when they happen here where more ligaments must be broken than elsewhere.
There is always in this genus greater extent of motion on one side, than on the opposite. In general flexion has always more extended limits than extension; observe in fact the condyles of the femur, of the phalanges, &c. they are extended much further in the first than the second direction; why? because all our principal motions are those of flexion, and the motions of extension are as it were but to moderate the first, and have for their object only to bring back the limb to the position from which it can be bent again. Hence why the number, and the strength of the fibres are much greater in the flexors than in the extensors; why the great vascular and nervous trunks are always on the side of flexion, as we see in the thigh, the leg, the fore-arm, the phalanges, &c. There is always something which limits the motion of extension, as the olecranon in the humero-cubital articulation, the crucial ligaments in the femoro-tibial articulation.
Though in the genus which we are describing, there is no well marked motion of circumduction, yet when the leg or the fore-arm are in flexion, they can move laterally and even in the form of a cone, but not in a very evident manner. In extension this is impossible, because the lateral ligaments being much stretched, do not yield enough to allow the bone to incline from one side to the other.
Fourth Genus.
Every kind of opposition and circumduction is wanting in this genus, which presents rotation alone, as we see in the articulations of the ulna with the radius, and the atlas with the odontoid process. Sometimes it is a concave surface rolling upon a convex one, as at the lower end of the radius, and at the odontoid process; sometimes it is a convex surface moving upon a concave one, as at the head of the radius; there is always a kind of ligament which completes the concave surface, and which thus forms a ring turning upon the bone, or in which the bone turns.
Luxations are here very difficult, because the rotation being made upon the axis of the bone, the ligaments are hardly more distended on one side than the other, and are hence broken with difficulty, whatever may be the extent of the motion. The inferior part of the radius forms a slight exception to this rule, because it is upon the ulna, and not exactly upon its own axis, that the bone turns in this place.
There is no rotation in the leg as in the fore-arm, because, as we have seen, that of the thigh which is very extensive, supplies the place of it; which the humerus could hardly do for the fore-arm, as we know when this last is anchylosed.