five-thousandth of a degree centigrade. Tetanus of frog's muscle gives from fourteen to eighteen-thousandths of a degree centigrade. No doubt each contraction of the muscles of higher animals, such as those of man, produces or is associated with more heat, but still the amount for each individual contraction is not much. But by the accumulation of small amounts, a large amount is formed, and there can be no doubt that a large proportion of the heat of our bodies is derived from the muscles.
That muscles produce heat is consistent with our daily experience. The more actively we work our muscles the hotter we become. When we wish to become warm we run, or leap, or dance, and in doing so we exercise our muscles.
We must now take a more scientific view of this matter. Muscles liberate energy as mechanical energy and as heat. The mechanical energy does work by moving one part of the skeleton upon the other, or by lifting a weight, as when I lift this book. Heat is another mode of energy, and we have seen that it is also liberated. Now a steam-engine does the same kind of thing. It expends or
