The surgeon is familiar with the manifestations of every variety of the human emotions in the various stations of life, from infancy to senility, in health and in disease. Not only does he come into intimate contact with the emotions displayed by the victims of disease and of accidents, but he also observes those manifested by the relatives and friends of the families of his patients. Moreover, he is unhappily forced to notice the emotional effect upon himself when he is waging an unequal battle against death-- the strain and worry at a crisis, when a life is in the balance and a single false move may be fatal, is an experience known only to the operating surgeon. For the data for this paper, therefore, in which I shall for the most part limit my discussion to the strongest of all emotions--FEAR--I have drawn largely from my personal experience as a surgeon, as well as from an experimental research in which I have had the valuable assistance of my associates, Dr. H. G. Sloan, Dr. J. B. Austin, and Dr. M. L. Menten. I believe it can be shown that it is possible to elicit the emotion of fear only in those animals that utilize a motor mechanism in defense against danger or in escape from it. For example, the defense of the skunk is a diabolic odor which repels its enemies; the skunk has no adequate equipment for defense or escape by muscular exertion, and the skunk therefore shows little or no fear. Again, certain species of snakes are protected by venom; they possess no other means of defense nor have they adequate motor mechanisms for escape and they show no fear. Because of their strength other animals, such as the lion, the grizzly bear, and the elephant, show but little fear (Fig. 6). Animals which have an armored protection, such as the turtle, show little fear. It is, therefore, obvious that fear is not universal and that the emotion of fear is felt only by those animals whose self-preservation is dependent upon an uncertain adequacy of their power of muscular exertion either for defense or for flight (Fig. 7). What are the principal phenomena of fear? They are palpitation of the heart, acceleration of the rate and alteration of the rhythm of the respiration, cold sweat, rise in body temperature, tremor, pallor, erection of the hair, suspension of the principal functions of digestion, muscular relaxation, and staring of the eyes (Fig. 12). The functions of the brain are wholly suspended except those which relate to the self-protective response against the feared object. Neither the brain nor any other organ of the body can respond to any other lesser stimulus during the dominance of fear.