We see why it is at the superior part of the limbs that nature has placed this genus. A double advantage results from this situation. On the one hand, very far from the part of the limb immediately exposed to the action of external bodies, it more easily escapes luxations to which its want of solidity renders it liable. On the other hand, it can by this situation give to the limb the motions of a whole which compensate for those of the inferior articulations, the solidity of which prevents the power of motion in all directions. For example, the two articulations of which I have just spoken, are not only the articulations of the bones that form them, of the humerus and the femur, but also the articulations of the whole limb, which they direct in different directions; thus the anchylosis of these articulations renders the limb completely useless, whilst that of the inferior articulations only destroys partial motions. The kind of motion of this genus of articulation requires a rounded form in the articular surfaces, whether they be concave that receive and convex that are received. This form is in fact the only one that can accommodate itself to extensive opposition, rotation and circumduction united; this is the form of the superior parts of the humerus with the scapula, and the femur with the os innominatum. The bone which moves has a convex surface, that which serves for support a concave one. There are in animals examples of an opposite arrangement: that is to say that a concavity is moved in all directions upon a convexity; but this is not found in man. Though the two limbs have between them the greatest analogy in their motions, yet there are some differences relative especially to their respective uses, which in the one are for seizing and repelling bodies, in the other destined to locomotion. The principal of these differences is, that rotation and circumduction are found in them in an exactly inverse ratio. The mechanical reason and advantages of this arrangement are easily understood. In the femur the length of the neck which is the lever of rotation, gives much extent to this motion, which supplies the pronation and supination that are wanting in the leg; so that every rotation of the foot is a motion of the whole of the limb. In the humerus on the contrary, the neck being very short and bringing the axis of the bone near the centre of the motion, limits rotation, which is less necessary on account of that of the fore-arm; the motion of the hand without or within is never communicated but by a part of the limb.