This spring is fixed below (by means of a tenon which allows antero-posterior movement) upon a bracket in the calf which is continuous with the ring through which passes the check cord. The cup-shaped upper end is in contact with a ball which projects from the upper surface of the thigh piece between the two openings for the check cord (Fig. 53). It will be seen that when the knee is flexed the spring, the head of which lies below the axis of the joint, will be compressed at the same time as the check cord is relaxed so that there is an elastic recoil tending to reproduce extension. The ball which rests on the top of the spring is fixed in such a manner as to be in the same horizontal plane as the axis of the knee: that is to say, it is in the same vertical plane as this axis when the knee is flexed to a right angle (Fig. 52). Therefore in this position the spring has no tendency to produce either extension or flexion, that is to say the mechanism is now at dead point, and when the patient is sitting flexion to the right angle is maintained without any effort. [Illustration: _The Marks knee._ FIG. 52. FIG. 53. FIGS. 54 to 57. FIGS. 52 and 53.--O, knee bolt. T, cross piece of wood, situated in the extended position above the knee bolt, in the flexed position behind it. C, bracket fixed halfway up the interior of the calf. A U-shaped cord _a_ passes through a hole in the bracket C and is attached at each end to the cross piece T; it limits extension. The two ends of the word enter the thigh piece by two apertures in the posterior surface, between which is fixed a metal ball which projects 2 cms. The extending spring is the rod _b_ which is fixed to this ball and to a socket in the upper surface of the bracket. Figs. 54 to 57 show the parts of this spring: a tube, a spiral spring, and a rod with cup-shaped head. When the spring is in the tube and the rod in the spring (Fig. 57), it will be seen that pressure upon the head of the rod increases the tension of the spring.] In the knee shown in figures 58 and 59 the _extending mechanism_ is as follows. Directly behind the axis of the joint is a metal crossbar, upon which fits the grooved upper extremity of a piece of wood, the other end of which rests (like a lance) in a pocket which is suspended in the leg piece by an elastic band (the latter being kept stretched to a greater or less extent by a lace which emerges from the calf). [Illustration: FIGS. 58 and 59.--Elastic spring for extending the knee.]