There are twelve ribs on each side, and all articulate by their poste­rior ends with the bodies of the thoracic vertebrae. The anterior ends of the upper seven ribs are joined directly to the sternum by cartilages. These are the true ribs (costae verae). The next three ribs, eighth, ninth, and tenth, are joined by their cartilages not to the sternum but to the cartilage of the rib next above, and are referred to as false ribs (costae spuriae). The anterior ends of the eleventh and twelfth ribs lie free, and these ribs are called floating ribs (costae fluctuantes). The ribs (costae) are narrow curved plates which in their longest, posterior part are formed of bone, os costale, belonging to the group of long spongy bones, and in the narrower anterior part, of cartilage, cartilago costalis. On the anterior end the bony part of the rib fuses closely with the cartilaginous part. Posterior and anterior ends and a shaft of the rib (corpus costae) between them are distinguished in each bony rib. The posterior end is slightly enlarged to form the head of the rib (caput costae), which has an articular facet separated by a rigde; the rib articulates by means of this facet with the vertebral bodies. The facet of the first, elev­enth, and twelfth ribs does not have a separating ridge. A narrowed part, the neck of the rib (collum costae), immediately succeeds the head; it has a longitudinal crest of the neck of the rib (crista colli costae) on its superior border. The first and last ribs lack this crest. At the junction of the neck and shaft, there is a tubercle of the rib (tuberculum costae), which carries a facet for joining with the articular surface of the transverse process of the corresponding vertebra. The eleventh and twelfth ribs do not have this tubercle because they do not articulate with the last thoracic vertebrae. Lateral to the tubercle, the rib curvature is greatly accentuated and the angle of the rib (angulus costae) is located here posteriorly on the shaft. The angle and the tubercle coincide on the first rib. On the other ribs the dis­tance between them increases down to the eleventh rib, while on the twelfth rib there is no angle. A groove (sulcus costae) is detectable on the inferior border of the inner surface of the middle ribs; it conducts the intercostal ves­sels. The superior surface of the first rib carries the scalene tubercle (tuber­culum m. scaleni anterioris), which is of practical importance because it gives attachment to the scalenus anterior muscle (m. scalenus anterior). Immediate­ly posterior to this tubercle, a small groove, the groove for the subclavian artery (sulcus a. subclaviae), can be seen, into which the subclavian artery fits when it curves over the first rib. Another, flatter groove for the subcla­vian vein (sulcus v. subclaviae) is found to the front of the tubercle. The size and shape of the different ribs are closely associated with the shape of the thoracic cage and their location in relation to it. It should firstly be pointed out that the ribs slope, their anterior ends slanting downward con­siderably. They increase in length from the first to the seventh rib; the se­venth and eighth ribs are the longest. The lower four ribs again diminish in length. Since the thoracic cage is ovoid, the ribs are curved in relation to the plane and their border. Longitudinal convexities form posteriorly in the tho­racic cage on both sides of the spine and concavities (pulmonary sulcus) on the inner surface because the curvature of the posterior part of the ribs is very steep. The extreme first, eleventh, and twelfth ribs differ markedly from the other ribs in shape. Their specific features are indicated above. The X-ray image of the ribs and sternum. Ossification. Radiographs of the sternum demonstrate separate ossification points: in the manubrium (1-2), in the body (4-13), in which the lower points occur before birth and in the first year of life, and in the xi­phoid process (between the ages of 6 and 20). The lower segments of the body fuse at the age of 15-16 years, the upper segment at 25 years of age. The xiphoid process fuses with the body after the age of 30, while the manubrium fuses still later or not at all. In the last case, when the synchondrosis sternalis persists, the radiograph shows a zone of translucency between the shadows of the body and manubrium. One of the ossification points in the sternal body close to the first rib may remain as an accessory bone, os parasternale. The ribs have points of ossification (1) in the region of the angle of the rib; it is the source of ossification of the body except for the anterior end, which remains cartilaginous (costal cartilage); (2) in the head of the rib (epiphysis); and (3) in the tubercle (apophysis). The apophysis appears between the ages of 15-20 and fuse between the ages of 18-25. In adults, the 12 pairs of ribs are clearly seen on a postero-anterior ra­diograph; the anterior parts of the ribs are superimposed on the posterior parts and intersect mutually. To interpret the picture properly, it should be borne in mind that the posterior parts of the ribs are connected to the verte­bral column and run obliquely, downward and laterally. The anterior parts slope downward, but in an opposite direction, medially. The shadow of the anterior ends is abruptly interrupted where the bone tissue is continuous with the cartilaginous tissue. The radiograph demonstrates the heads and necks superimposed on the shaft, as well as the transverse processes of the corre­sponding vertebrae. The tubercles of the ribs and their articulations are also seen close to the transverse processes. Of most practical impoitance among variations of costal development are accessory ribs (seventh cervical and first lumbar rib); the twelfth pair of ribs as a rudimentary structure is marked by wider variability than the other ribs. Two forms of the twelfth rib are distinguished: sabre-like, with a long rib sloping downward, and stiletto-like, with a small, short rib stretching ho­rizontally.