Thorax.—The thorax, pectus or chest is a cavity, the walls of which are formed of bone and of cartilage. Its skeleton consists of the sternum in front, the twelve thoracic vertebrae behind, and the twelve ribs, with their corresponding cartilages, on each side.
Sternum.—The sternum or breast bone is an elongated bone which inclines downward and forward in the front wall of the chest. It consists of three parts—an upper, called manubrium or presternum ; a middle, the gladiolus or mesosternum ; and a lower, the ensiform process or xiphisternum. Its anterior and posterior surfaces are marked by transverse lines, which indicate not only the subdivision of the entire bone into three parts, but that of the mesosternum into four originally distinct segments. Each lateral border is marked by seven de pressed surfaces for articulation with the seven upper ribs ; at each side of the upper border of the presternum is a sinuous depres sion, where the clavicle articu lates. The xiphisternum remains cartilaginous up to a late period of life.
Ribs.—The ribs or costae, twelve on each side of the thorax, consist not only of the bony ribs, but of a bar of cartilage con tinuous with the anterior end of each bone, called a costal carti lage, so that they furnish ex amples of a cartilaginous skeleton in the adult human body; in aged persons these cartilages usually become converted into bone. The upper seven ribs are connected by their costal cartilages to the side of the sternum, and are called true ribs; the lower five do not reach the sternum, and are named false, and of these the two lowest, from being comparatively unattached in front, are called floating. All the ribs are articulated behind to the thoracic ver tebrae, and as they are symmetrical on the two sides of the body, the ribs in any given animal are always twice as numerous as the thoracic vertebrae in that animal. They form a series of osseo cartilaginous arches, which extend more or less perfectly around the sides of the chest. A rib is an elongated bone, and as a rule possesses a head, a neck, a tubercle and a shaft. The head usually has two articular surfaces, and is connected to the side of the body of two adjacent thoracic vertebrae ; the neck is a constricted part of the bone, uniting the head to the shaft ; the tubercle, close to the junction of the shaft and neck, is the part which articulates with the transverse process of the vertebra. The shaft is com
pressed, possesses an inner and outer surface, and an upper and lower border, but from the shaft being somewhat twisted on itself, the direction of the surfaces and borders is not uniform through out the length of the bone. The ribs slope from their attach ments to the spine, at first outward, downward and backward, then downward and forward, and where the curve changes from the backward to the forward direction an angle is formed on the rib. The angle and the tubercle are at the same place in the first rib and in each succeeding rib the angle is a little farther from the tubercle than in the last.
The surface of the first rib which is not in contact with the lung is directed upward, forward and outward while that of the second rib is much more outward; the eleventh and twelfth ribs are rudi mentary, have neither neck nor tubercle, and are pointed ante riorly. The ribs increase in length from the first to the seventh or eighth, and then diminish to the twelfth; the first and twelfth are therefore the shortest ribs. The first and second costal carti lages are almost horizontal, but the others are directed upward and inward.
In its general form the chest is like a barrel which is wider below than above. It is rounded at the sides and flattened in front and behind, so that a man can lie either on his back or his belly. Its upper opening slopes downward and forward, is small in size, and allows the passage of the windpipe, gullet, large veins and nerves into the chest, and of several large arteries out of the chest into the neck. The base or lower boundary of the cavity is much larger than the upper, slopes downward and backward, and is oc cupied by the diaphragm (q.v.), which separates the chest from the abdomen. The transverse diameter is greater than the antero posterior, and the antero-posterior is greater laterally, where the lungs are lodged, than in the mesial plane, which is occupied by the heart.