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The Collaborative International Dictionary of English v.0.48 (gcide)

Thorax \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 vertebrae, the ribs,
and the sternum, and which the heart and lungs are
situated; the chest.
[1913 Webster]

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.
[1913 Webster]

2. (Zool.)
(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.
[1913 Webster]

3. (Antiq.) A breastplate, cuirass, or corselet; especially,
the breastplate worn by the ancient Greeks.
[1913 Webster]