CAP. IV. MOTUS ARTICULORUM

a center, since they are made around a quiescent end, which surely does not exist in the bone DG, but in I, the middle of the sphere or cylinder ADF. Thus, if one understands a straight line drawn from the end G, penetrating the intermediate tubercle ADF, and passing through the center I of the said tubercle, this entire line will move, except for only a single point of it. This, therefore, will be the center and fulcrum around which the revolution of the semi-diameter and the bone is effected. Therefore, the center and fulcrum of this articulation will be outside the mobile bone DG, specifically in the center I of the tubercle of the other immovable bone.

On the contrary, if the bone DG is quiescent and the bone BA must be carried around, its center and fulcrum will not exist in the extreme contact DC, but in the center or intermediate point I of the tubercle itself. From this it is deduced that the center or fulcrum of the humerus or femur exists precisely in the middle of that tubercle which is inserted and tied into the sinuous cavity of the immovable scapula shoulder blade or hip. And in these two joints, the extremity of the mobile semi-diameter and its center is prominent and extended. On the contrary, the center of the semi-diameter of the circumduction of the cubitus exists outside the cubitus in the middle, specifically in the tubercle of the quiescent humerus, to which it is tied and around which it revolves; and the same must be said of the remaining similar articulations.

It must also be noted that the motion of the joints is sometimes spherical, sometimes in one plane of some circle, and many times exists on a conical surface. Let the general rule be: whenever the motion of one bone can be made in any direction around a single fixed point, then the motion will be spherical—that is, to the right, to the left, up, down, forward, and backward. But whenever the motion must be made around two poles or around an axis, the motion and circumduction will necessarily be effected either on a flat circular surface or on a conical surface. An example of the first will be the motion of the humerus, whose extremity tied to the scapula is spherical and globular, in which, because of the sphericity of the tubercle itself, the humerus can be flexed and carried around in any direction, because the straight lines from the center of that tubercle to its surface are equal in all directions. Therefore, contacts can be made just as well from the spherical surface of the tubercle in any direction, and that sphere can be carried around with the greatest ease in any part within the concentric cavity of the scapula. It does not happen thus in the motion of the cubitus around the humerus, and in the motion of the tibia shinbone around the genu knee, because, indeed, the middle of the revolution is not a point, but an axis extended between the two poles of a cylinder. For the lowest extremity of the humerus and femur is not spherical, but cylindrical, excavated with some grooves which are like so many pulleys or tracks, which make for firmness so that dislocations do not occur in the motions. But in these small cylinders, the motions must necessarily be made equidistant to the circles of the same cylinder, and not to the right or to the left toward its poles, which indeed depends on the nature of the cylindrical figure, in which there is no intermediate point that is equidistant from all points of its surface, unless the points are taken in the periphery of some circle equidistant to the bases of the same cylinder; which circles, since they are all parallel to each other and to the bases, necessarily admit the revolutions of the bone...