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The fate of the radial nerve in the human brachialis muscle, a subject of dispute regarding its frequency of occurrence and nature of innervation, was studied in eight pairs of this muscle. A constant occurrence of radial supply to the brachialis muscle was found. With the exception of two cases which were discarded on account of defects, all brachialis of the sample definitely showed motor as well as sensory innervations by the radial nerve in addition to the major supply by the musculocutaneous nerve. The number and regional distribution of spindle capsules were also determined in all cases. A complete analysis in one of them showed that 7% of the spindle population in the entire muscle were supplied by the radial nerve and that their distribution was confined to the deep lateral quadrant for a n extent about half of the muscle length. The probable dorsal origin of this muscle was discussed.

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