As to circumduction, the length of the neck of the thigh is an obstacle to it. In fact, let us remark that this motion is much the more easy, when it is performed by a rectilinear lever, because then the axis of the motion is the axis of the lever; that on the contrary, if the lever is angular, the motion becomes so much the more difficult because the axis of the motion is not that of the lever; and in general we can say, that the difficulty of the motion is in the direct ratio of the distance of the two axes. This being settled, let us observe that the axis of the motion of circumduction of the thigh is evidently a straight line, obliquely directed from the head to the condyles, and distant consequently above from the axis of the bone, the whole length of the neck. Now, from what has just been said, it is evident, that the difficulty of circumduction will be in the direct ratio of the length of the neck, and consequently very great. In the humerus, on the contrary, the neck being very short, the axis of the bone and that of the motion are almost the same; hence the facility and extent of the circumduction. We might fix precisely the relation of these motions by this proportion; the circumduction of the humerus is to that of the femur, as the length of the neck of the humerus is to the length of the neck of the femur; which shows us how much more difficult the circumduction of the femur is than that of the humerus. To know this, it is sufficient in fact to know the excess of the length of the neck of the first over that of the second. It is easy to perceive the advantages of this very great extent in the circumduction of the superior limbs destined to seize, and of the limits placed by nature to that of the inferior limbs destined to standing and locomotion. We understand also why luxations are more easy in the first than in the second. The displacement almost always takes place in fact, in one of the simple motions, the succession of which forms the compound motion of circumduction, for example, in elevation or depression, in adduction or abduction, &c. Now all these motions being carried much further in the humerus than in the femur, the surfaces are more easily separated. _Second Genus._ This genus differs from the first by the absence of the motion of rotation. Opposition and circumduction are alone met with in it. We find examples of it in the temporal maxillary, sterno-clavicular, radio-carpal, meta-carpo-phalangeal articulations, &c. The want of rotation evidently supposes, from what has been said above, the absence of an osseous head, the axis of which would make, as in the preceding genus, an angle with the axis of the body of the bone. Thus in all the bones of the articulations that I have just mentioned, the articular surface is at the extremity even of the bone, and not upon the side; the axis is the same in both cases. They form a rectilinear lever, instead of an angular one.