Outward luxation may be spontaneous or accidental. It is termed spontaneous, when produced by relaxation of the ligaments of attachment or by irregular muscular action; accidental, when resulting directly from any external mechanical cause. Violent contraction of the triceps cruralis, by lifting the patella beyond its normal limit of travel, helps, or at least permits, the patella to be displaced outwardly at the moment when the muscle relaxes. Pathological relaxation of the ligaments and muscles, by allowing the patella to descend too far on the trochlea, also renders displacement possible, hence spontaneous luxation sometimes occurs even while the animal is at rest in the stable. This luxation is certainly only of a temporary character, or perhaps only of the nature of subluxation, and is often reduced by mere muscular contraction when the animals are forced to move. Should the hind limb slip in a backward direction the angle of the joint becomes more obtuse and the lips of the trochlea are turned downwards, thus greatly favouring lateral displacement of the patella, which under these circumstances is no longer immobilised on the trochlear pulley; displacement outwards then occurs, constituting the condition termed luxation. Various forms of mechanical violence, like blows, collisions of the stifle with the jambs of doors, falls, etc., may also bring about this luxation. =The symptoms= of fully-developed accidental luxation are characteristic. Immediately the accident occurs, the limb is immobilised in a state of complete extension; neither the stifle nor the hock joint can be flexed, and only the fetlock joint retains any degree of mobility. Movement is very difficult. The hind limb appears rigid, as though formed of one bone. The pastern is directed backwards and dragged along the ground, and when weight is placed on the limb the anterior surface of the pastern may almost be in touch with the ground. The limb is advanced, but the foot cannot be placed properly on the ground. Locally the patella is found to be outside the external lip of the trochlea, and its internal ligaments are extremely tense. If the luxation is spontaneous and of muscular origin, or a consequence of relaxation of the tendons, it is usually noticeable immediately the animal leaves the stall. The animal cannot move without great difficulty. It grows steadily worse with the lapse of time, because the synovial membrane becomes irritated and chronic arthritis is set up. The symptoms are identical with those of traumatic luxation, but are only temporary. =Diagnosis.= The position of the limb and the displacement of the patella are sufficiently distinctive to render diagnosis fairly easy, and to allow of this accident being differentiated from luxation of the femoro-tibial joint or hooking of the patella in the ischio-tibial muscle.