STRING-HALT.--This term is applied to a peculiar involuntary movement of one or both hind limbs that is characterized by a sudden, purposeless flexion of the hock-joint (Fig. 45). Horses that are slightly affected may show this movement of the hind limbs when first exercised. Other horses may be "string-halted" when backed, turned, walked, or trotted, and fail to drive out of it. The cause of true "string-halt" is not known. _The treatment recommended is surgical._ This consists in cutting the tendon of the peroneus muscle. The seat of the operation is a little below the hock and on the external face of the cannon. [Illustration: FIG. 45.--String-halt.] SPAVIN.--A spavin is a chronic inflammation of the articular faces of the hock bones, ligaments and synovial membranes. The inflammation may result in the formation of a bony enlargement on the inner surface of the region, and a union between the small bones forming the lower portion of the hock, and the upper extremities of cannon and lower hock bones (Figs. 46 and 47). [Illustration: FIG. 46.--A large bone spavin.] The _predisposing causes_ are of the greatest importance. A spavin is one of the unsoundnesses of horses that may be transmitted to the offspring. Young colts that have heavy bodies and are fed a fattening ration are predisposed to it. Crooked hind limbs, small hocks and quarters that are heavily muscled are predisposing factors. The _external causes_ are strains caused by slipping, turning quickly, rearing, pulling heavy loads and kicks. Horses three or four years of age if given work that favors hock strain, such as excavating cellars, may develop a spavin. [Illustration: FIG. 47.--Normal cannon bone and cannon bone showing bony enlargement and lower hock bones united to superior extremity; this occurs in bone spavin.]