The disease is not associated with any constant cerebral lesion, though it may supervene in the course of other nervous disorders, and therefore may own an exciting cause in existing lesions of the brain. The immediate cause must however be held to be functional, and this is in keeping with its most common form in man (hysterical), and with the hypnotic form which is observed both in man and animals. This latter may be looked on as a form of induced or hypnotic sleep, in which the retention of the position given to a limb or part is the most prominent symptom. In all cases there is an impaired condition of the sensory functions of the cerebral convolutions, and an insusceptibility of the motor centres to the control of the will, or the reflex stimulus. Hering has recorded the disease in the horse, Landel in the ox, and Leisering in the prairie wolf. The hypnotic form has been shown in cats, chickens, and Guinea pigs. The serpent charming of the Indian dervishes and similar effects on frogs and crayfish have been attributed to hypnotic catalepsy. _Causes._ Strong mental emotions and diseases which profoundly affect the nervous system have been adduced as causes (fear, excitement, chills). Indigestible food has even been charged with causing it. There is undoubtedly, to begin with, a specially susceptible nervous system, and hence it is liable to prove hereditary, and in man to appear as a form of hysteria, or to alternate in the same family with epilepsy, chorea, alcoholism, opium addiction and other neurosis. Hypnotism as a cause is claimed by various writers. Azam says that in the fairs in the South of France, jugglers hypnotize cocks by placing the bill on a board, on which they trace a black line passing between the two feet of the bird. Cadeac adds that Father Kircher, in the 17th century, employed a similar method to put fowls to sleep. Alix put cats to sleep by securing them firmly, and then looking steadily into their eyes. The condition attained varies according to the degree of the sleep, the will being dominated first, and later, consciousness of external objects is lost. Hypnotism, however, appears to be difficult and uncertain in the lower animals, in keeping with the limited development of intelligence and will, as compared with the human being. Cadeac states that the very old and the very young are completely refractory to hypnotizing influences. _Lesions._ No constant pathological changes are found, though different nervous lesions may serve to rouse the disease in a predisposed subject. Fröhner found in the affected muscles granular swelling, fatty degeneration, hæmorrhages, and waxy (amyloid) degeneration of the cardiac muscles, corresponding to what has been found in tetanus; also hæmorrhages on the stomach and intestines.