The isolated semispinalis cervicis muscle of a 3 to 10 days old chick has been found to provide a very useful assay method for depolarizing substances. There are three main advantages of this preparation over that of the rectus abdominis of the frog: (a) quick relaxation without any artificial stretching of the muscle, (b) greater sensitivity to depolarizing drugs, and (c) no falling off in sensitivity for at least 2 hr.
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