THE following case represents the fourth example of fracture of a sesamoid bone of the thumb to be recorded in the literature. E. B., male, white, aged twenty-six years, civil engineer, reported at the Surgical Out-patient Department of the University Hospital (Case record 6i996) on March 2, 19I5, with the history of having fallen three days previously in such manner that the left thumb was traumatized against the granite Belgian block pavement.This hand had never been injured previously. Clinical examination revealed moderate swelling of the left thumb, ecchymosis along the thenar eminence, and " wincing" tenderness both at the head and at the base of the first metacarpal. A clinical diagnosis was made of a " chipping " fracture of the metacarpal head. Skiagram (Fig. i) revealed a fracture of the ulnar sesamoid bone opposite the head of the first metacarpal, the smaller fragment being separated distally, and there being a definite, dentate line of fracture. There was also an oblique fracture at the base of the metacarpal, separating the ulnar corner and entering the carpo-metacarpal joint at its middle. The thumb was immobilized in extension and slight abduction by a spica sodium silicate bandage.Re-examination after the reading of the skiagram brought out two additional facts. In the first instance, the patient was loosejointed, and could hyperextend both thumbs normally at the metacarpo-phalangeal joint. In the second place, there was definite " wincing " tenderness, localized to the ulnar sesamoid bone. After four weeks the immobilizing dressing was removed, and massage instituted. While the clinical diagnosis of "chipping" fracture of the metacarpal head was inaccurate, yet it was close enough to lead to the detection of the fracture of the sesamoid bone. In order to parry the question of a congenitally bipartite sesamoid, a skiagram of the right thumb was taken (Fig. 2), but no abnormality of the sesamoids was reevealed. This fact, together with the history of a fall upon the thumb, the clinical localization of " wincing tenderness to the sesainoid, shown *Read before the Philadelphia Academy of Surgery, April;5, 1915. 297