Hair oils and greases of all sorts are sanitary nuisances, and mere half-civilized and lazy substitutes for proper brushing and washing. There is no drug known to medicine which will cause hair to grow, or make it thicker or curlier. All "hair tonics" claiming to do this are frauds. Corns, Calluses, and Warts. Our skin not only made our hair, teeth, and nails, but still retains in every part a trace of its nail-making powers, so that under pressure or irritation, it can thicken up into a heavy leather-like substance which we call _callus_. This is naturally and healthfully present in the soles of the feet and the palms of the hands. Savage, or barbarous, races who wear no shoes get the skin of their soles thickened into a regular human leather, almost half an inch thick, and as tough as rawhide. A somewhat similar condition develops in the palms of the hands of those who work hard with spades, axes, or other tools. Any good process carried to excess becomes bad, and this is true of this power of callus formation in the skin; for parts of it which are under constant pressure, like the surface of the toes inside the shoe, and particularly of the outside toes, the little and the big toe, develop under that pressure patches of thickened, horny skin, which we call _corns_. These patches start to grow into cone-shaped projections or buttons; but being prevented from growing outward by the pressure of the shoe, they turn upon themselves and burrow into the skin itself, and we get the well-known ingrowing corn. If there is anything in the human body which we ought to be thoroughly ashamed of, it is corns; for they are caused by our own vanity, and nothing else, in cramping our feet into shoes one or two sizes too small for them. There are a number of things that can be done to relieve the discomfort of the corn, but the only sure way is to remove its cause, namely, the tight shoe. Under other kinds of irritation, the skin has the power of growing curious little button-like buds, or projections, which we call _warts_. These are commonest in childhood, and generally disappear at about twelve or fifteen years of age, when we no longer delight in dirt, and glory in mud pies. They can be produced upon the hands of grown men and women by irritating fluids and substances, such as wet sugar in the case of bakers and confectioners, and various color-stains in dye works. They seldom last for more than a few months, and usually narrow at their base and drop off, when the particular irritation that caused them ceases. On this account it is seldom worth while to try to remove them by burning with acids or cutting them off; and it is best not to pick at, or irritate, or scratch them too much. CHAPTER XVII THE PLUMBING AND SEWERING OF THE BODY