In addition to these familiar uses of the muscles there is a use which is just as important but about which we are apt to think less. This is their use in connection with posture, the taking and holding of particular bodily positions. Posture is unlike other muscular activities in several things. In the first place there is a steady, but rather feeble, tension which can be held without marked fatigue for long periods; all other forms of muscular contraction become severely fatiguing rather quickly if held steadily. In the second place the nervous control of posture seems to be different in some respects from our ordinary control of our muscles. Finally there is some doubt as to whether the contractions of the muscles themselves are the same. Measurements of the functional metabolism of posture show that it is much less than would be expected if the muscular action were of the ordinary type. This, of course, explains why posture is less fatiguing than other forms of activity. The other two kinds of muscle, heart muscle and smooth muscle, must have a word of description. Heart muscle contracts quickly and powerfully, as does skeletal. It differs from skeletal in not depending on nervous stimulation to make it contract; the heart can be cut clean out of the body and will go on beating for a short time; in cold-blooded animals, like frogs or turtles, for a long time. This could not be true if the heart muscle had to be aroused to activity by nerves. Besides being automatic, heart muscle shows the peculiarity that whenever it contracts all the fibers join. We do not have a varying strength of pull shown by heart muscle as we do in skeletal. As we shall see, it would be a serious disadvantage rather than an advantage if heart muscle were to be like skeletal in this respect.