n. [ L. tetrachordon, Gr. &unr_;, from &unr_; four-stringed; te`tra- (see Tetra-) + &unr_; a chord: cf. F. tétrachorde. ] (Anc. Mus.) A scale series of four sounds, of which the extremes, or first and last, constituted a fourth. These extremes were immutable; the two middle sounds were changeable. [ 1913 Webster ]

n. One who, or that which, retracts. Specifically: (a) In breech-loading firearms, a device for withdrawing a cartridge shell from the barrel. (b) (Surg.) An instrument for holding apart the edges of a wound during amputation. (c) (Surg.) A bandage to protect the soft parts from injury by the saw during amputation. (d) (Anat. & Zool.) A muscle serving to draw in any organ or part. See Illust. under Phylactolaemata. [ 1913 Webster ]