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thorax,body division found in certain animals. In humans and other mammals it lies between the neck and abdomen and is also called the chest. The skeletal frame of the thorax is formed by the sternum (breastbone) and ribs in front and the dorsal vertebrae in back. Within the thoracic cavity are the heart, lungs, and esophagus. The chest and abdominal cavities are separated by a muscular structure, the diaphragm. In insects and other arthropods the thorax is a body division consisting of several segments lying between the head and abdomen, to which are attached legs or other appendages.
the upper (in humans) or anterior (in most vertebrates) part of the trunk.
The shape of the thorax is determined by its skeleton—the chest and pectoral girdle—and by the muscles arranged on its surface. In arthropods the thorax is the section between the head and the abdomen. In primitive crustaceans (for example, Phyllopoda) the segments of the thorax are numerous and not constant in number; they bear nonspecialized appendages, equipped simultaneously for swimming, chewing, and breathing (gills). In higher crustaceans the number of segments of the thorax is reduced to between four and eight and becomes constant; part of the segments fuse with the head and their appendages become maxillipedary. In Arachnida and Xiphosura the segments of the thorax bear four pairs of gressorial legs and are usually merged with the head into a cephalothorax. In insects the thorax consists of three segments (prothorax, mesothorax, and metathorax), to each of which is attached a pair of legs; in higher insects one pair of wings each is attached to the mesothorax and metathorax in the adult stage.