The Collaborative International Dictionary of English v.0.48:

Muscle \Mus"cle\ (m[u^]s"'l), n. [F., fr. L. musculus a muscle,
   a little mouse, dim. of mus a mouse. See Mouse, and cf.
   sense 3 (below).]
   1. (Anat.)
      (a) An organ which, by its contraction, produces motion.
          See Illust. of Muscles of the Human Body, in Appendix.
      (b) The contractile tissue of which muscles are largely
          made up.
          [1913 Webster]

   Note: Muscles are of two kinds, striated and nonstriated. The
         striated muscles, which, in most of the higher animals,
         constitute the principal part of the flesh, exclusive
         of the fat, are mostly under the control of the will,
         or voluntary, and are made up of great numbers of
         elongated fibres bound together into bundles and
         inclosed in a sheath of connective tissue, the
         perimysium. Each fiber is inclosed in a delicate
         membrane (the sarcolemma), is made up of alternate
         segments of lighter and darker material which give it a
         transversely striated appearance, and contains,
         scattered through its substance, protoplasmic nuclei,
         the so-called muscle corpuscles.
         [1913 Webster] The nonstriated muscles are involuntary.
         They constitute a large part of the walls of the
         alimentary canal, blood vessels, uterus, and bladder,
         and are found also in the iris, skin, etc. They are
         made up of greatly elongated cells, usually grouped in
         bundles or sheets.
         [1913 Webster]

   2. Muscular strength or development; as, to show one's muscle
      by lifting a heavy weight. [Colloq.]
      [1913 Webster]

   3. [AS. muscle, L. musculus a muscle, mussel. See above.]
      (Zool.) See Mussel.
      [1913 Webster]

   4. An essential part of something; as, budget cuts have gone
      beyond the fat and are cutting into the muscle of the
      government.
      [PJC]

   5. Bodyguards or other persons hired to provide protection or
      commit violence; as, he doesn't go out without his muscle
      along. [slang]
      [PJC]

   Muscle curve (Physiol.), contraction curve of a muscle; a
      myogram; the curve inscribed, upon a prepared surface, by
      means of a myograph when acted upon by a contracting
      muscle. The character of the curve represents the extent
      of the contraction.
      [1913 Webster]