as much as twenty per cent in the form of effective mechanical energy. The muscle, however, has this great advantage over any engine, that the heat it produces supplies one of the conditions of its very existence, the maintenance of a certain uniform temperature. Muscle-substance will only work within a limited range of temperature, and as the body is constantly losing heat by radiation, conduction, evaporation of sweat, and by other means, heat must be supplied. This comes mainly from the muscles. It would not be correct to say, however, that one of the final purposes of a muscle is to produce heat. It is not a heat-producing machine. The primary function of a muscle is to contract, and thus to do effective work; but by producing heat at the same time it becomes possible for the muscle -substance to do its work in the best possible conditions. This is only another illustration of that wise economy that we see in most, if not in all, of the arrangements of nature.
After considering the points discussed in this lecture, you will readily understand how it is that a muscle becomes tired. It becomes fatigued after continuous work. Fatigue
