One belongs to all the bones which serve for locomotion, to some of those destined to internal functions, as the ribs, the lower jaw, &c. The other is especially met with in the bones, the union of which forms the cavities designed to defend the organs; this we see in the head, the pelvis, &c. _Moveable Articulations. Observations upon their Motions._ I divide moveable articulations into four kinds, the characters of which are borrowed from the different motions they execute. To understand this division, it is necessary previously to know the motions of the articulations in general. These motions can be included under four species, which are, 1st, opposition; 2d, circumduction; 3d, rotation; 4th, sliding. 1st. The motion of opposition is that which is made in two opposite directions, for example, from flexion to extension, from adduction to abduction, and vice versa. This motion is extensive or limited; extensive when it is made in all directions, first in the four named above, then in all the intermediate ones; limited, when it only takes place from flexion to extension, from adduction to abduction, &c. The thigh in its articulation with the pelvis enjoys an extensive motion of opposition. The tibia in its articulation with the femur has a limited motion of opposition. 2d. Circumduction is the motion in which the bone describes a kind of cone, the summit of which is in its superior articulation, and the base in the inferior; so that it is found successively in flexion, adduction, extension and abduction, or in abduction, extension, adduction and flexion, according to the motion by which it begins, and that moreover it goes through all the intermediate directions. Hence we see that circumduction is a motion composed of all those of opposition, and in which the bone, instead of moving from one direction to an opposite one, as in the preceding case, moves from one direction to another nearest to it, describing thus by its extremity a circle which is the base of a cone of which I have spoken, and which is so much the greater as the bone is so much the longer. We easily understand that among the bones, those only whose motion of opposition is loose, enjoy that of circumduction. 3d. Rotation is wholly different from the preceding motion. In this there was locomotion, a moving of the bone from one place to another; here it remains always in the same place; it only turns upon its axis. The humerus and the femur enjoy this motion which is simple. 4th. Sliding belongs to all the articulations. It is an obscure motion, by which two surfaces go in an opposite direction, by sliding as it were upon each other. In all the other motions, this is met with; but it often exists without them.