These articulations at the front of the thorax may be divided into four sets, viz.: The intersternal joints, or the union of the several parts of the sternum with one another. The costo -chondral joints, or the union of the ribs with their costal cartilages. The chondro-sternal joints, or the junction of the costal cartilages with the sternum. The interchondral joints, or the union of five costal cartilages (sixth, seventh, eighth, ninth, and tenth) with one another.

The Intersternal Joints

The sternum being composed, in the adult, of three distinct pieces - the manubrium, body, and the xiphoid process - has two articulations, viz., the superior, which unites the manubrium with the body (gladiolus), and the inferior, which unites the body with the xiphoid.

The Superior Intersternal Articulation

Class. - False Synchondrosis.

The lower border of the manubrium and the upper border of the body of the sternum present oval-shaped, fiat surfaces, with their long axes transverse, and covered with a thin layer of hyaline cartilage. An interosseous fibro-cartilage is interposed between the bony surfaces: it corresponds exactly in shape and intimately adheres to them. At each lateral border this fibro-cartilage enters into the formation of the second chondro-sternal articulation.

In consistence, it varies, being in some oases uniform throughout, in others softer in the center than at the circumference, and in others again an oval-shaped synovial cavity is found toward its anterior part. When such a cavity exists in the fibro-cartilage this joint has a remote resemblance to the diarthroses, and is classed, with the sacro-iliac joint and the symphysis pubis under similar conditions, as diarthro-amphiarthrosis.

The periosteum passes uninterruptedly over the joint from one segment of the sternum to the other, forming a kind of capsular ligament [membrana sterni]. This capsule is strengthened, especially on its posterior aspect, by longitudinal ligamentous fibers as well as by the radiating and decussating fibers of the chondro-sternal ligaments.

In some instances, the fibro-cartilage is replaced by short bundles of fibrous tissue which unite the cartilage-coated articular bony surfaces.

The Inferior Intersternal Articulation Class. - False Synchondrosis.

The gladiolus is joined to the xiphoid cartilage by a thick investing mem- brane, by anterior and posterior longitudinal fibers, and by radiating fibers of the sixth and seventh chondro-sternal ligaments. The costo-xiphoid ligament also connects the xiphoid with the anterior surface of the sixth and seventh costal cartilages, and thus indirectly with the gladiolus; and some fine fibro-areolar tissue also connects the xiphoid with the back of the seventh costal cartilage.

The junction of the xiphoid with the sternum is on a level somewhat posterior to the junction of the seventh costal cartilage with the sternum. The union is a synchondrosis, each bone being covered by hyaline cartilage which is connected with the intervening fibro-cartilage plate.

The Costo-chondral Joints

Class. - Synarthrosis.

The extremity of the costal cartilage is received into a cup-shaped depression at the end of the rib, which is somewhat larger than the cartilage. The two are joined together by the continuity of the investing membranes, the periosteum of the rib being continuous with the perichondrium of the cartilage.

The Sterno-costal Articulations

Class. - Diarthrosis.

Subdivision - Ginglymus.

These articulations are between the lateral borders of the sternum and the ends of the costal cartilages. The union of the first rib with the sternum is synchondrodial, and therefore forms an exception to the others. From the second to the seventh inclusive, the articulations have the following ligaments, which together form a complete capsule:

Radiate (anterior) sterno-costal. Superior sterno-costal.

Posterior sterno-costal. Inferior sterno-costal.

The radiate (anterior) sterno-costal ligament is a triangular band composed of strong fibers which cover the medial half-inch of the front of the costal cartilage, and radiate upward and downward upon the front of the sternum. Some of the fibers decussate across the middle line with fibers of the opposite ligament. At its upper and lower borders, it is in contact with the superior and inferior ligaments respectively.

The posterior sterno-costal ligament consists of little more than a thickening of the fibrous envelopes of the bone and cartilage, the joint being completed behind by a continuity of perichondrium with periosteum.

The superior and inferior ligaments are strong, well-marked bands, which pass from the upper and lower borders respectively of the costal cartilage to the lateral edges of the sternum. The sixth and seventh cartilages are so close that the superior ligament of the seventh is blended with the inferior of the sixth rib.

Deeper than the fibers of these ligaments are short fibers passing from the margins of the sternal facets to the edges of the facets on the cartilages; they are most distinct in the front and lower part of the joint, and may encroach so much upon the synovial cavity as to reduce it to a very small size, or almost obliterate it. This occurs mostly in the case of the sixth and seventh joints, especially the latter.

The interarticular ligament is by no means constant, but is usually present in the second joint on one, if not on both sides of the same subject. It consists of a strong transverse bundle of fibers passing from the ridge on the facet on the cartilage to the fibrous substance between the manubrium and body; sometimes the upper part of the synovial cavity is partially or entirely obliterated by short, fine, ligamentous fibers.

The costo-xiphoid ligament is a strong flat band of fibers passing obliquely upward and laterally from the front surface of the xiphoid cartilage to the anterior surface of the sternal end of the seventh costal cartilage, and most frequently to that of the sixth also.

Synovial membranes. - -The union of the first cartilage with the sternum being synchondrodial, it has no synovial membrane; the second has usually two, separated by the interarticular ligament. The rest usually have one synovial membrane, which may occasionally be subdivided into two.

The arterial supply is derived from perforating branches of the internal mammary; and the nerves come from the anterior branches of the intercostals.

Movements. - -Excepting the first, the chondro-sternal joints are ginglymoid, but the motion of which they are capable is very limited. It consists of a hinge-like action in two directions: first, there is a slight amount of elevation and depression which takes place round a transverse axis, and, secondly, there is some forward and backward movement round an obliquely vertical axis. In inspiration, the cartilage is elevated, the lowest part of its articular facet is pressed into the sternal socket, and the sternum is thrust forward so that the upper and front edges of the articular surfaces separate a little; in expiration, the reverse movement takes place. Thus, the two extremities of the costal arches move in their respective sockets in opposite directions.

This difference results necessarily from the fact that the costal arch moves upon the vertebral column, and, having been elevated, it in its turn raises the sternum by pushing at upward and forward.

The costo-xiphoid ligament tends to prevent the xiphoid cartilage from being drawn back- ward by the action of the diaphragm.

The Interchondral Articulations

Class. - Diarthrosis. Subdivision. - Arthrodia.

A little in front of the point where the costal cartilages bend upward toward the median line the sixth is united with the seventh, the seventh with the eighth, the eighth with the ninth, and the ninth with the tenth.

At this point, each of the cartilages from the sixth to the ninth inclusive is deeper than elsewhere, owing to the projection downward from its lower edge of a broad blunt process, which comes into contact with the cartilage next below. Each of the apposed surfaces is smooth and they are connected at their margins by ligamentous tissue, which forms a complete capsule for the articulation, and is lined by a synovial membrane. The largest of these cavities is between the seventh and eighth; those between the eighth and ninth, and ninth and tenth, are smaller, and are not free to play upon each other in the whole of their extent, being held together by ligamentous tissue at their anterior margins. Sometimes this fibrous tissue completely obliterates the synovial cavity.

The arteries are derived from the musculo-phrenic, and the nerves from the intercostals.

Movements. - -By means of the costal cartilages and interchondral joints, strength with elasticity is given to the wall of the trunk at a part where the cartilages are the only firm structures in its composition; while a slight gliding movement is permitted between the costal cartilages themselves, which takes place round an axis corresponding to the long axis of the cartilages. By this means, the outward projection of the lower part of the thoracic wall is increased by deep inspiration.