The emotion of fear is well known, but its purpose is not so often recognized. An emotion brings about internal changes, visceral changes they are called, which enable the organism to act on the emotion,--to accomplish its object. There is only so much energy available at a given moment, stored up in the brain cells, ready for use. In such an emergency as flight every ounce of energy is needed. The large muscles used in running must have a great supply of extra energy. The heart and lungs must be speeded up in order to provide oxygen and take care of extra waste products. The special senses of sight and hearing must be sensitized. Digestion and intestinal peristalsis must be stopped in order to save energy. No person could by conscious thought accomplish all these things. How, then, are they brought about? =Internal Laboratories.= In the wonderful internal laboratory of the body there are little glands whose business it is to secrete chemicals for just these emergencies. When an object is sighted which arouses fear, the brain cells flash instantaneous messages over the body, among others to the supra-renal glands or adrenals, just over the kidneys, and to the thyroid gland in the neck. Instantly these glands pour forth adrenalin and thyroid secretion into the blood, and the body responds. Blood pressure rises; brain cells speed up; the liver pours forth glycogen, its ready-to-burn fuel; sweat-glands send forth cold perspiration in order to regulate temperature; blood is pumped out from stomach and intestines to the external muscles. As we have seen, the body as a whole can respond to just one stimulus at a time. The response to this stimulus has the right of way. The whole body is integrated, set for this one thing. When fear holds the switchboard no other messages are allowed on the line, and the creature is ready for flight. But after flight comes concealment with the opposite bodily need, the need for absolute silence. This is why we sometimes get the opposite result. The heart seems to stop beating, the breath ceases, the limbs refuse to move, all because our ancestors needed to hide after they had run, and because we are in a very real way a part of them. =Old-Fashioned Fear.= There is one passage from Dr. Crile's book which so admirably sums up these points that it seems worth while to insert it at length.