1. Our modern treatises on diseases are little else than fabrics artfully constructed of materials confusedly scattered through the writings of the ancients. Many of those materials oftentimes escape our notice, and we find them only, after practice has disclosed them to us, in the chambers of the sick. Thus, Hippocrates had an accurate knowledge of spontaneous luxations of the os femoris, and has even left an aphorism expressly on that subject. Yet this disease appeared to be unknown to the physicians who came after him, till John Louis Petit, having met with it in his practice, drew the attention of practitioners to it, by a memoir respecting it, published among those of the Academy of Sciences, in the year 1722. 2. The history of this affection, which was considered afterwards, _ex professo_, in his course on diseases of the bones, has been sanctioned by the assent of all practitioners, to whom it has since very frequently occurred, and who have generally admitted as Louis observes, the doctrine of Petit, respecting the cause on which it seems to depend. 3. Experience bears witness, that usually a fall on the great trochanter, more rarely on the knee, or the sole of the foot, precedes it, and has doubtless some share in producing it. But what is the nature of the primary affection which, rising immediately from this occasional cause, becomes the immediate cause of the luxation? Petit, and with him the practitioners of the present day, have conceived, that the different parts of the joint, being irritated and contused, pour out, in consequence of the injury received, a superabundant quantity of synovial fluid, which, not being absorbed with the same rapidity, accumulates in the articular cavity, distends the capsule, and, by degrees, forces the head of the os femoris from its natural cavity. Hence astringent and tonic remedies, with alum, spirit of wine, &c. are directed to be applied externally to the upper part of the diseased thigh. 4. But this doctrine, and the practice which results from it, seem by no means to accord with our knowledge of anatomy. The truth of this was deeply impressed on the mind of Desault, who had frequent opportunities of witnessing the disease. CASE I. A young woman walking hastily along the street, slipped and made a false step, in which the left thigh, being violently twisted, supported for a moment the whole weight of the body. A severe pain experienced at the moment, obliged her to stop at first, but becoming easier afterwards, permitted her to proceed on her way, and soon ceased entirely. A sensation of weight occurring in the part about fifteen days afterwards, was at first troublesome to the patient in walking. This sensation was afterwards succeeded by a dull, deep-seated pain, accompanied by a swelling in the parts around the joint.