CAP. IV. MOTUS ARTIC. FLEXIV. SPHÆRIC. &c.

evident from the fact that the motion of any bone constituting a joint is made around the end of the other bone with which it forms the articulation; that end is made a fulcrum upon which the other bone is moved. If the end of that bone is completely quiescent, then the motion of the other connected bone carried around in the same plane will be almost circular. But if the aforementioned fulcrum is not fixed, but wavers and moves (Fig. 1. TAB. II.), the motion of the articulated bone that follows is not circular, but straight or curved in various ways. Regarding the first, it is clear that with the humerus upper arm bone A E quiescent, the ulna forearm bone or cubitus elbow/forearm A B moves over the joint and fulcrum A, and consequently a round motion arises.

But although the motion of the joints is circular and round, it is not conspicuous where the center of the revolution of the joints and bones consists, which is why it must be assigned and found by reason. For if the bones constituting the joints were merely indivisible lines, then their contact would indeed be an indivisible point, which would have the nature of a center and fulcrum. But since bones are bodies having dimension, their extremities cannot be joined and articulated in one point so easily, so that they revolve around the aforementioned point of conjunction. This could indeed be done if the end of one bone were pointed like a cone or a pyramid, and the point of its vertex were stuck and tied into the cavity of another immovable bone; then the point of contact would be the fulcrum and center of revolution. But this would be very inconvenient and fragile; for if the extremity of the cubitus ended in a conical apex, and this were applied into a wider sinuous conical cavity excavated in the end of the humerus, then that eminent sharpness would easily be bruised and broken, nor could the articulation be tied so firmly without it wavering and deviating from the point of contact on this side and that.

Therefore, so that the provident and most wise Nature might avoid these inconveniences, she has engineered an articulation that is easy, safe, stable, resistant, and not at all prone to dislocations. It is of this kind: she formed the last extremities of the bones to be round, making one convex and the other sinuous and concave, so that the contact might not be made at a point, but on an ample surface. Thus, bruising and fracturing are avoided; furthermore, the extremities of the bones can be tied together more easily and firmly, without the danger of dislocation, in varied and manifold motion. But here the center of revolution, or the fulcrum of the semi-diameter around which it is carried, does not appear. For any point in which the mobile bone touches and is supported by the immovable bone is not a quiescent and stable point, and therefore cannot be the center of revolution. As in the articulation of the bone AB (Fig. 2. TAB. II.), there is a sphere or cylindrical eminence ADEF, and on the contrary, the extremity EDC of the bone GD is sinuous and excavated, which precisely receives and embraces the extreme tubercle of the other bone BA. Then, indeed, no quiescent and stable point can be assigned in the bone DG; but each of them, in the motion of the same bone, describes the periphery of a circle, and these circles are unequal and proportionally increasing the closer they come to the end G, and all the aforementioned revolutions are necessarily made around...