Figs. 206 and 207.—The elbow ratchet and mode of action of its lock.
A lock is thus provided which can be manipulated with the other hand.
In the particular pattern illustrated, traction is made by an elastic cord fixed to the centre of the back of the wrist and ending above in a leather strap pierced with holes which fix on the button of the lever. This arrangement allows of the adjustment necessitated by the gradual stretching of an elastic which is subjected to continuous tension.
A steel spring of this length (the whole length of the forearm) would be too heavy if it were sufficiently powerful. If it is desired to use this method the two ends of a powerful spring should be fixed, one to the button on the lever, the other to the catch on the forearm with which the ratchet engages.
The spring should always be in tension. As the distance between the joint on the arm and any point on the forearm increases as the elbow extends, it is better for the ratchet, with notches on its upper edge, to be engaged when the button points upwards and free when it points downwards. In the opposite arrangement, which is often used, the tension is considerable without being useful when the forearm is vertical, and the mechanism soon wears out.
3. Hand and Other Appliances.—The hand attached to the end of the forearm has a spring thumb which may be passive or automatic. In the latter case, if the stump is long enough to allow considerable movements of the arm, the cord works in the way described on [page 87], for amputation of the forearm. If the stump is short, traction must be exerted by movement of the shoulders, rounding the back.
The hand with a mobile wrist is never used with these amputations except in certain expensive appliances, in which in addition the four fingers may be articulated, as described on [page 101]. For the ordinary limb these delicate mechanisms are devoid of practical utility.
It is easy to replace the hand with interchangeable appliances, but when the patient has to do hard work this is not a satisfactory method.