Compared with the amount of energy actually received from the environment when a sensory cell of the eye or ear is excited, the energy needed to start an artificial impulse in a nerve is relatively enormous; yet a well-known comparison of the energy conveyed to the nerve in a certain experiment with a nerve-muscle preparation from a frog, and the energy expended by the muscle in contracting, brings home to our minds the fact that it is impossible to carry even this item to account. The energy furnished to the nerve from an electric condenser measured 0·001 erg; the energy expended by the muscle reached 100,000 ergs. It is easy to determine the amount of mechanical work which results from a given expenditure of energy. By alternately flexing and extending the joints of his legs, a man lifts his own weight up a hill of a certain height. The work can be measured in foot-pounds or in kilogrammetres. But this by no means accounts for all the energy potential in his food. A still larger amount is expended for the purpose of keeping the body warm, or, not improbably, making it too warm; in either case generating heat which is dissipated into the atmosphere. When a machine is being planned, attention is concentrated upon the problem of how to get the largest result in work for a given quantity of fuel. Fuel costs money. All energy dissipated as heat is wasted. Every ounce saved makes for economy. Engineers therefore speak of the “efficiency” of an engine as the relation between the work actually done and the work which would have been done if no energy had been wasted. In the best steam-engines it stands at about 1 to 10. Since the chief function of muscle is to do mechanical work, physiologists are apt to adopt the engineer’s point of view. But in the case of muscle this is justifiable only in a limited degree. The body of a warm-blooded animal is maintained at a temperature higher than that of the surrounding air. Muscles are the chief producers of heat. If they turned all the energy which they receive into work, they would be inefficient as regards this very important function. Yet even from the engineer’s point of view muscles are more efficient than the best of engines.