thorax

n. [ L., fr. Gr. &unr_;. ] 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 ]

☞ 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 ]