thoraxn.[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, the ribs, and the sternum, and which the heart and lungs are situated; the chest.()()2. (Zol.) 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.()3. (Antiq.) A breastplate, cuirass, or corselet; especially, the breastplate worn by the ancient Greeks.()