The articular surfaces are in general, as in the preceding case, uniform, without eminences and reciprocal depressions; which would embarrass and even prevent circumduction. In the bone which serves for support, there is a cavity more or less deep; in the bone which is moved, there is an analogous convexity. The corresponding surfaces of the temporal and the inferior maxillary bone, of the bones of the metacarpus and the first phalanges, are examples of this arrangement. This articular mode is the most favourably disposed for circumduction, which is, as we have seen, constantly in the inverse ratio of rotation, and which consequently has the greatest possible facility when the lever is rectilinear, a circumstance that destroys rotation. Yet in many articulations of this genus, circumduction is evidently less extensive than in the humerus and the femur; but this arises from the arrangement of the moving powers which being in much greater number in the articulations of these two bones, compensate for the bad arrangement of the articular surfaces for circumduction. In the genus of articulations of which we are treating, there is always one direction in which the motion of opposition is more easy than in the others; for example, it is elevation and depression in the jaw, flexion and extension in the first phalanges, in the wrist, &c. In general there are two lateral ligaments and the capsule in the direction in which the motions are most limited, the capsule only in that in which they are the most extensive. _Third Genus._ 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.