How We Tell Things from Touch, and Feel Heat and Cold and Pain. Last of all, the skin is the principal organ of the sense of touch, and also of the "temperature sense"--the sense of heat and cold--and of the sense that feels pain. All these feelings are attended to by little bulbs lying in the deeper part of the skin and forming the tips of tiny nerve twigs,[20] which run inward to join larger nerve branches and finally reach the spinal cord. There are millions of these little bulbs scattered all over the surface of the skin, but they are very much thicker and more numerous in some parts than in others; and that is why, as you have often noticed, certain parts of the skin are more sensitive than others. They are thickest, for instance, on the tips of our fingers and on our lips, and fewest over the back of the neck and shoulders, and across the lower part of the hips.[21] For a long time, it was supposed that all these little nerve-bulbs in the skin did the same kind of work, because they looked, under the microscope, exactly alike; but it was found that they divide the work up among them, so that some of them give their entire attention to heat, and others to cold, others to touch, and others again to pain. So carefully has the work been mapped out among them that they report to different centres in the brain and spinal cord, so that we now understand why, in diseases which happen to attack one or other of these centres, we may lose our sense of heat and cold, as in that terrible disease, _leprosy_; or our sense of touch, as in _paralysis_; or we may even, in some very rare cases, lose our sense of pain, and yet have all our other senses perfect. FOOTNOTES: [19] Hairs are of value chiefly as protection against cold and wet, although we have got rid of them and substituted clothing for this purpose, except on the top of our heads; but their roots also are very richly supplied with nerves so that they form almost a sort of feelers, or organs of sense. Many animals that move about much in the dark, like cats and bats, for instance, have their lips or faces studded with long, delicate, stiff hairs called whiskers, which act in this way and prevent their bumping into objects in the dark. And it is probable that the bristling of the hair on a dog's back, when he is angry or frightened, is in part for this purpose--to enable him to slip aside and dodge a blow, even after it has touched the ends of the hairs. This great sensitiveness of the hair roots is what makes it hurt so when any one pulls your hair. [20] See the diagram of the skin on page 171.