bids us, if we value our eyes, to turn them away; if we put our hand against a sharp point it is a signal not to go on pressing in that direction; an unpleasant taste often indicates a poisonous matter; and a bad smell teaches that we are breathing impure air.
The case is better understood by analogy if we refer to the hand of a workman. If rough work is done the soft and delicate hand is pained; if, however, the work is persevered in, the hand ceases to be so sensitive, the skin grows thick and horny, so that rough surfaces, and even hot coals, can be handled with impunity; but at the same time the delicate sense of touch has become grievously impaired.
Owing to the wonderful adaptability of Nature, if the warning given by a sensation is persistently neglected, after a time it ceases to be given. The pain has become deadened, but injury results none the less. For example, a bell-ringer going for the first time to his work finds the clang and clash of his bells almost insupportable; after a time, however, he grows accustomed to it, and at last hardly feels it at all. He has gained immunity from pain, but he has gained it at the expense of injury to the nerves of hearing—he has become deaf.
Besides the senses just now referred to, there are others of equal importance. Such, for instance, is the sense of hunger. If sensations of hunger are neglected, appetite is lost; on the other hand, if the stomach's warning of "I have had enough" is unheeded, the nerves of that organ after a time get wearied, and no longer send their important mes-
