We have then in the biceps a thick fleshy muscular belly placed in the front of the arm and fastened by tendons, at one end to the shoulder-blade, and at the other to the fore-arm. What would happen if when the arm is straight and the shoulder-blade fixed, the biceps were suddenly to grow very much shorter than it was? Evidently the same thing that happened when you pinched up and shortened the string which, if you look back you will see, we supposed to be placed very much as the biceps with its tendons is placed. =The radius and ulna would be pulled up, the fore-arm would be bent on the arm.= Now tendons have no power of shortening themselves, but muscular substance does possess this remarkable power of suddenly shortening itself. Under certain circumstances each soft muscular fibre of which the muscle is made will suddenly become shorter, and thus the whole muscle becomes shorter, and so pulls its two tendinous ends closer together, and if one end be fastened to something fixed, and the other to something moveable, the moveable thing will be moved. This way that a muscle or a muscular fibre has of suddenly shortening itself is called a =muscular contraction=. =All muscles, all muscular fibres, have the power of contracting.= Now a mass of substance like the biceps might grow shorter in two ways. It might squeeze itself together and become smaller altogether, it might squeeze itself as you would squeeze a sponge into a smaller bulk. Or it might change its form and not its bulk, becoming thicker as it became shorter, just as you might by pressing the two ends together squeeze a long thin roll of soft wax into a short thick one. It might get shorter in either of these two ways, but it does actually do so in the latter way; it gets thicker at the same time that it gets shorter, and gets nearly as much thicker as it gets shorter. And that is why, when you put your hand on the arm which is being bent, you feel something rise up. =You feel the biceps getting thicker as it is getting shorter in order to bend the arm.= The shortening does not last for ever. Sooner or later the muscle lengthens again, getting thinner once more, and so returns to its former state. The lengthened condition of the muscle is the natural condition, the condition of rest. The shortening or contraction is an effort which can only be continued for a certain time. The contraction bends the arm, and as long as the muscle remains shortened the arm keeps bent; but as the muscle lengthens, the weight of the hand and fore-arm, if there is nothing to prevent, straightens the arm out again.