the resting muscle breathes. It is a little living thing, taking in oxygen and giving out carbonic acid. When it is obliged to work hard it breathes faster, that is to say, it produces more carbonic acid, and uses up more oxygen. This is exactly what is consistent with experience. Active muscular exercise, as in running, causes an increased consumption of oxygen, and an increased elimination from the blood of carbonic acid.
I imagine some of you may be asking the question. Will a muscle contract only in an atmosphere containing oxygen? Let us appeal to experiment. In this jar, is a muscle still capable of contracting, and yet there is practically no air here, as it has been nearly all removed by an air-pump. In this other jar you see a muscle contracting in an atmosphere of nitrogen, and in this third jar a similar muscle contracting in an atmosphere of hydrogen. You observe this fourth one contracting even in a jar of carbonic acid. It is quite true a muscle will not live long in these circumstances, but even in an atmosphere containing no oxygen a muscle will go on producing carbonic acid. Now as carbonic acid is
