1. The processes of tendon healing within the canine flexor digital sheath following aseptic division and as nearly atraumatic repair as possible are described.
2. Under these conditions only the sheath tissues provide the active healing elements in the healing of a flexor digital tendon within its sheath. The tendon is united by a scar produced by fibroblasts derived from the sheath. These fibroblasts lay down collagen which by 128 days is indistinguishable from that of the tendon proper.
3. Microscopically, wounds before twenty-eight days appear too immature for active mobilization. By thirty-five days they look as though they would function satisfactorily.
4. Nutrition of the fibroblasts derived from the sheath and the nutrition of the scar they produce may be provided by the loose filmy adhesions which persist between the sheath and tendon scar. There is no evidence that these adhesions interfere with the function of the healed tendon.