Page:Life in Motion.djvu/103

Rh but when the weight is removed the muscle readily returns to its former length.

Let us examine more closely the behaviour of the muscle when it is extended by gradually increasing weights, and let us compare it with this band of india-rubber. We will fix the two, side by side, against a board on which we can mark the amount of extension in each case. Notice that with gradually increasing weights the india-rubber extends so that the amount of extension is directly proportional to the weight; that is to say, for each equal increment, the india-rubber is extended to an equal amount. Compare this with the muscle. You observe the first weight stretches it so much; if the weight is now doubled we do not get twice the amount of extension, but less than half; if the weight is again increased by one third, we get only a little more stretching; and so on, each increase on stretching becoming less and less. Thus, with the india-rubber, if we note the amount of stretching on the board by a number of vertical lines, each line representing a uniform increase in the weight, and if we join the end of these lines we get a straight line. On the other hand, by performing the same