accessory muscle, a relatively rare anatomical duplication of a muscle that may appear anywhere in the muscular system. The most common sign associated with an accessory muscle is the appearance of a soft tissue mass. Differential diagnosis without an exploratory operation is difficult because of the similar appearance of some tumors or soft tissue masses, such as ganglia. The appearance of the soft tissue mass associated with an accessory muscle may be transient, or it may be constant, depending on the location of the accessory muscle in relation to motion. In many individuals with accessory muscles, specific treatment is not indicated unless the accessory muscle interferes with normal function.
