thorax

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The part of the human body between the neck and the diaphragm, partially encased by the ribs and containing the heart and lungs; the chest.
2. n. A part in other vertebrates that corresponds to the human thorax.
3. n. The second or middle region of the body of an arthropod, between the head and the abdomen, in insects bearing the true legs and wings.

Century Dictionary and Cyclopedia

1. n. In anatomy and zoology, a part of the trunk between the head or neck and the abdomen or tail, in any way distinguished, as by containing the heart and lungs, by being inclosed with large ribs, or by bearing certain limbs not borne elsewhere. The name is applied both to the walls and to the cavity of this part of the body, but not to the contents of the cavity, and properly not to the thoracic appendages. In all vertebrates the thorax represents several of the segments or somites of the body succeeding the cervical and succeeded by the abdominal or pelvic segments. It is generally defined by the elongation of several ribs and the connection of some or most of these with a breast-bone, the thoracic skeleton thusforming a bony cage or frame which contains and defends the principal organs of circulation and respiration. In invertebrates, however, the thorax is defined upon other considerations. In man and all mammals the thorax is sharply marked off from the rest of the trunk by the lack of developed cervical and lumbar ribs, and its cavity is completely shut off from that of the abdomen by the diaphragm. The human thorax is of conical figure, somewhat like the frustum of a cone, narrowed above, broad below, of greater width than depth, and in cross-section somewhat cardiform or heart-shaped, from the intrusion of the backbone. Its truncated apex presents to the neck; its concave base is formed by the diaphragm. The cavity is divided into a pair of large pleural cavities, right and left, for the lungs, and a third submedian pericardial cavity for the heart. Where the opposite pleural cavities do not quite meet and fit, both before and behind, is an interpleural space, the anterior and posterior mediastinal cavity, or premediastinum and post-mediastinum. Besides the heart and lungs and their respective serous sacs (pericardium and pleura), the thorax contains many other structures, as the thoracic duct and thoracic aorta, many branches of the latter, etc. The thorax of other mammals differs from that of man chiefly in size, shape, degree of movability, etc., but not in actual structure or office.
2. n. In entomology, that part of the body which is situated between the head and the abdomen, and in adult insects alone bears the wings and legs, when there are any. ; In the typical or hexapod insects the thorax is almost always a well-marked region, distinguished from the head in front and from the abdomen behind by bearing the only locomotory appendages which these insects possess in the adult state—namely, one or two pairs of wings and three pairs of legs. The thorax typically consists of three segments or somites of the body, one to each pair of legs, respectively named, from before backward, the prothorax, the mesothorax, and the metathorax, or sometimes the prethorax, medithorax, and post-thorax. The hard crust of each of these segments may and normally does consist of a number of pieces or individual sclerites, on the dorsal or tergal, on the lateral or pleural, and on the ventral or sternal aspects. These sclerites are known as tergites, pleurites, and sternites; they have also other names, and many of the individual sclerites have specific designations. Thus, dorsal sclerites or parts of each segment may be known as pronotum, mesonotum, and metanotum, and so with pleural and sternal sclerites of each thoracic segment. (See sclerite, and cuts under mesothorax and metathorax.) In ordinary descriptive entomology the name thorax has two special restrictions: to the pronotum of coleopterous, hemipterous, and orthopterous insects
3. n. In Crustacea and Arachnida, a part of the body in advance of and in any way distinguished from the abdomen or tail, but usually blended with the head to form a cephalothorax. In ordinary arachnidans, as spiders, and in the higher crustaceans, as crabs, lobsters, shrimps, prawns, and crawfishes, several segments of the body are more or less completely fused in one mass; and the limbs are often so gradually metamorphosed into mouth-parts that even these indicia fail to discriminate a thorax from the head in every case. Generally, however, the bearing of eight or ten legs, developed as ambulatory organs, serves to denote a thorax. In many or most of the lower or entomostracous crustaceans a thorax is indistinguishable from the abdomen as well as from the head, and the character of its appendages does not always decide the case. See Decapoda, Tetradecapoda, Thoracipoda, thoracetron.
4. n. A breastplate, cuirass, or corselet; more especially, the cuirass or corselet worn by the ancient Greek warriors, corresponding to the lorica of the Romans. It consisted of a breastplate and a backpiece fastened by buckles, and was often richly ornamented.

Wiktionary

1. n. the middle of three distinct divisions in an insect, crustacean or arachnid body
2. n. the region of the mammalian body between the neck and abdomen as well as the cavity containing the heart and lungs

GNU Webster's 1913

1. n. (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 vertebræ, the ribs, and the sternum, and which the heart and lungs are situated; the chest.
2. n. 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. n. 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.
4. n. (Antiq.) A breastplate, cuirass, or corselet; especially, the breastplate worn by the ancient Greeks.

WordNet 3.0

1. n. the part of the human torso between the neck and the diaphragm or the corresponding part in other vertebrates
2. n. part of an insect's body that bears the wings and legs
3. n. the middle region of the body of an arthropod between the head and the abdomen