A certain percentage of individuals are peculiarly susceptible to tobacco, so that it has a special poisonous effect upon the nerves of the heart, causing a rapid pulse and shortness of breath, known as _tobacco heart_. This is not of very common occurrence; but it is exceedingly troublesome when it does occur, and it takes a long time to get over it, even after the use of tobacco has been stopped entirely. Sometimes it leads to permanent damage of the nerves and of the heart. Give your heart plenty of vigorous exercise, but don't make it beat uncomfortably hard. Give it plenty of food, sleep, and fresh air; _avoid poisoning it_, either with the toxins of diseases, or with your own waste-poisons, or alcohol, or tobacco; and it will serve you faithfully till a good old age. CHAPTER XIII HOW AND WHY WE BREATHE Life is Shown by Breathing. If you wanted to find out whether a little black bunch up in the branches of a tree were a bird or a cluster of leaves, or a brown blur in the stubble were a rabbit or a clod, the first thing you would probably look for would be to see whether it moved, and secondly, if you could get close enough without its moving away, whether it were breathing. You would know perfectly well if you saw it breathing that it was alive, and that, if it were not breathing at all, it would probably be dead, or very nearly so. Why is breathing so necessary to life that it lasts practically as long as life does, and when it stops, life stops too? Animals can stop eating for days, or even weeks, and yet live, especially if they were fairly fat when they began to fast. Indeed, some animals, like woodchucks, bears, and marmots, will go to sleep in the fall, and sleep right on through to spring without eating a mouthful. But if any animal or bird is prevented from breathing for three minutes, it will die. Short Storage Supply of Air. There is a difference between the kind of things that you take in when you breathe and the kind of things you take in when you eat or drink. Food and drink are solids and liquids; and the body is a great sponge of one soaked full of the other, so that large amounts of food and water can be stored up in the body. But what you take in when you breathe is, of course, air--which is neither a solid nor a liquid, but a gas, very light and bulky. Of gases the body can soak up and hold only a very small amount; so its storage supply of them will be used up completely in about three minutes, and then it dies if it cannot get more air.