\Tho"rax\, n. [L., fr. Gr. ?.]
1. (Anat.) The part of the trunk between the neck and the
   abdomen, containing that part of the body cavity the walls
   of which are supported by the dorsal vertebr[ae], the
   ribs, and the sternum, and which the heart and lungs are
   situated; the chest.
Note: In mammals the thoracic cavity is completely separated
      from the abdominal by the diaphragm, but in birds and
      many reptiles the separation is incomplete, while in
      other reptiles, and in amphibians and fishes, there is
      no marked separation and no true thorax.
2. (Zo["o]l.)
   (a) The middle region of the body of an insect, or that
       region which bears the legs and wings. It is composed
       of three united somites, each of which is composed of
       several distinct parts. See Illust. in Appendix. and
       Illust. of {Coleoptera}.
   (b) The second, or middle, region of the body of a
       crustacean, arachnid, or other articulate animal. In
       the case of decapod Crustacea, some writers include
       under the term thorax only the three segments bearing
       the maxillipeds; others include also the five segments
       bearing the legs. See Illust. in Appendix.
3. (Antiq.) A breastplate, cuirass, or corselet; especially,
   the breastplate worn by the ancient Greeks.