The elastic being slightly stretched when the knee is extended, it will be seen that the crossbar turning round the axis of the knee becomes lowered as the knee flexes, so that the elastic is stretched and consequently opposes flexion; but when the knee is bent to a right angle the axis of the joint, the crossbar and the wooden rod are in the same vertical line; the mechanism is at a dead point just as we have already seen in the Marks knee, and the tension on the elastic presses the leg directly downwards without tending either to flex or to extend it. Leather pads deaden the noise of the impact. Extension is limited, as will be seen by comparing figures 58 and 59, by the vertical wooden rod meeting flat surfaces in the thigh and leg pieces simultaneously. 3. _Extending slings._[6]--To the sling which passes over the shoulder on the side of the artificial limb, is attached a strap which passes down in front of the thigh piece and is attached to the upper third of the leg. [6] This is an old French method used in Fouilloy's appliance, which has, however, only become generally used in the suspending braces of the American appliance. When the patient raises the leg from the ground, the weight of the appliance makes it slip down the stump, tension is thus produced upon this strap and as a result the knee is extended. By an adroit movement of the shoulder this extension can be carried out actively. When the limb rests upon the ground the weight of the body presses the stump down into the bucket, the tension on the strap is released and consequently the knee is free to flex. On pages 44 to 48 will be found figures showing the principal points in this extending brace. The braces, whether they have or have not an extending strap, may be constructed in three ways: _a_. To ease the constant pressure exerted on the shoulders by the strap which is stretched by the weight of the artificial limb, the brace may be made of elastic like the ordinary trousers brace. But the limb they carry is heavy, so they rapidly become overstretched and it is difficult to keep them properly adjusted. _b._ The stretching is naturally diminished if the upper part of the brace is not elastic but an elastic section is inserted in its lower third, in front and behind. _c._ But the patients almost always say that better command of the limb is obtained with inelastic braces. If the strap is wide on the shoulder, the pressure is well borne, and the lower attachment may be made narrower, consisting of a leather thong (Fig. 64). [Illustration: Fig. 60. Fig. 61. Fig. 62. Fig. 63. Fig. 60.--Fouilloy's Braces. Figs. 61 to 65.--Marks' braces. Fig. 61.--General construction of the braces. Figures 62 and 63.--Attachment at the sides of the thigh piece. Figures 64 and 65.--General view of the apparatus as worn.]