The warm-blooded animals, birds and mammals, have the remarkable power of keeping the body temperature constant (at 98-99 degrees Fahrenheit, in man, somewhat higher in birds), in spite of great variations in the external temperature to which the body is exposed, and in spite of great variations in the {142} amount of heat generated in the body by muscular exercise. Sweating and flushing of the skin are reactions to heat, and prevent the body temperature from rising; paling of the skin, shivering and general muscular activity are responses to cold and prevent the body temperature from falling. Shrinking from great heat or cold are also instinctive, while seeking shelter from the heat or cold is a preparatory reaction that is not definitely organized in the native constitution of man, but gives rise to a great variety of learned reactions, and plays a considerable part in life. Shrinking from injury. The "flexion reflex" of the arm or leg, which pulls it away from a pinch, prick or burn, is the type of a host of defensive reactions--winking, scratching, rubbing the skin, coughing, sneezing, clearing the throat, wincing, limping, squirming, changing from an uncomfortable position--most or all of them instinctive reactions. With each goes some sort of irritating sensation, as pain, itching, tickling, discomfort; and a conscious impulse to get rid of the irritation is often present. When the simpler avoiding reactions do not remove the irritating stimulus, they are repeated more vigorously or give way to some bigger reaction tending towards the same result. The climax of the avoiding reactions is flight or running away. Akin to flight are cowering, shrinking, dodging or warding off a blow, huddling into the smallest possible space, getting under cover, clinging to another person; and most or all of these, too, are instinctive reactions. With flight and the other larger danger-avoiding reactions there is often present, along with the impulse to escape, the stirred up organic and conscious state of _fear_.