When the whirl-bone is displaced, it is always found as an unnatural lump upon the outer side of the thigh; it cannot, for three sufficient reasons, be drawn to the inner part of the leg. The inner condyle of the humerus, over which the patella plays, is sufficiently large to oppose any unnatural motion in that direction; the inner ligaments are the weakest, and are, therefore, most readily stretched in the outward direction; the circumstances permit the bone to be displaced from the inside of the leg. Then, moreover, the muscles are altogether more powerful upon the outer side. Any force acts more energetically as debility increases, and, to favor it, there is less resistance in the direction opposite to which the force pulls; for these reasons the bone is invariably luxated upon the outer side of the animal's haunch. [Illustration: THE PATELLA, OR WHIRL-BONE DISPLACED.] The symptoms denoting luxation of the patella are: the leg thrust out behind, and remaining fixed; the horse's entire frame is affected; the head is erect; the muscles quiver; the pastern of the protruded leg is violently flexed; there is an unnatural swelling upon the outer and lower part of the buttock. If the animal be forced to move, it can only imperfectly hop upon three legs; such an accident may occur at any time, and never be repeated. It may, however, become so common as to be mistaken for a species of habit; for luxation of the patella, when by frequency confirmed, will take place upon the slightest possible cause. In stinted colts the most trivial motion will often give rise to this accident; the creature can hardly move without its leg being thrust out behind it. The cure is, in these cases, anything which may flurry the animal. A noise, made by moving the hand quickly and rather energetically from side to side within a hat, the crack of a whip, or any sudden and loud sound, will occasion the bone to return, with apparent ease and the utmost rapidity, to its natural situation. The colt, however, may the next moment exhibit the misfortune which, in young life, can only be cured by kindly treatment and liberal sustenance. Probably the author will best describe the nature of the affection in old animals, by narrating a case which a few years ago happened to himself.