17. The body passes from one place to another, on the least motion, and sometimes, as Bell observes, the patient, on changing his position during sleep, is awakened by severe pain, in consequence of the foreign body being moved by this change. It happens, in certain cases, that it disappears, and lies concealed for some time, in the back part of the joint. During this period the joint performs all its functions with freedom and ease. Desault made this remark, in the case of a captain of dragoons, from whom, for the first time in his practice, he extracted one of these bodies, and who, for six months previously, had been able to perform all the motions of the joint freely, without pain. This person, experiencing no uneasiness, considered himself perfectly cured, when the body suddenly reappeared, in consequence of a hasty extension of the leg. 18. If the body, when projecting under the integuments, be gently compressed, it yields to the pressure, changes its situation, and, according to the impression it has received, moves either to the internal or the external side of the joint, or reciprocally from one side to the other, passing also behind the rotula, behind the inferior ligament, or sometimes behind the tendon of the extensor muscles. In these alternate displacements, it may in some cases be turned round, in such a manner that its anterior surface will take the place of its posterior one, and then resume its primitive situation. Desault met with an instance, in which the patient himself was in the habit of turning the body round in this manner. 19. Bell, in conformity to the distinction of articular concretions into cellular and solid, attributes to each division its peculiar signs. In the first case, the pains, being rather obtuse than sharp, are constant; in the second, they are extremely acute, but disappear and return at intervals. Supposing the division to be a real one, cases of the last description certainly occur much more frequently than those of the first. § V. OF THE TREATMENT. 20. From what has been said it follows, 1st, that these cartilages floating through the joints, do mischief mechanically (14), by coming into contact with the articular surfaces: 2dly, that to obviate this mischief, it is necessary either to prevent their contact, by fixing the bodies in a spacious part of the joint, and thus doing constantly what nature does on certain occasions, or to extract them through an opening made into the articular cavity. 21. Hence, art can have recourse to but two methods of cure, all hope of discussing these tumours by external applications being, as Bell observes, entirely extinguished.