The incubation, or the time it takes for the disease to develop, varies, but usually is from three to six months. There is a recorded case where the person began to show symptoms of the disease thirteen days after having received a severe wound on the head. The incubation period is seldom longer than six months. The symptoms of the disease in the human being vary within narrow limits. There are three classic symptoms usually encountered, and these are fear, apprehension or excitement, together with deglutitory (swallowing) spasms, terminating in general paralysis. The patient remains conscious of his agony to the end, but the period of illness is of short duration, lasting from one to three days. The bites of rabid dogs cause ninety per cent of the cases in man and animals. The cat is the next important factor in spreading the disease and about six per cent of the cases are caused by this animal. For other cases four per cent come from bites of horses, wolves, foxes, etc. The wolf in Russia, or other animals like it, may be the chief cause there; but dogs cause ninety per cent, taking all the cases found. Man, dog, cat, horse, cattle, sheep, goat, hog, deer, etc., are subject to the disease either naturally or experimentally. The disease is confined commonly to dogs, because the dog naturally attacks animals of his own species and thus keeps the disease limited mainly to his own kind. Naturally the dog follows this rule, but on the other hand, in the latter stages of the disease he usually goes to the other extreme and even attacks his own master, etc. The dogs that are the most dangerous and do the greatest damage are of the vicious breeds. The rabbit or guinea pig is used for demonstration in the laboratory. Guinea pigs respond to the virus more rapidly than do other animals and therefore they are especially useful in diagnostic work. Rabbits, however, on account of the convenient size and ease with which they are operated upon, are usually the choice in the production of material used in treating patients.