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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
joints of the series. In the lower two the ligament sometimes completely obliterates the cavity, so as to convert the articulation into an amphiarthrosis.

The Costoxiphoid Ligaments (ligamenta costoxiphoidea; chondroxiphoid ligaments).—These ligaments connect the anterior and posterior surfaces of the seventh costal cartilage, and sometimes those of the sixth, to the front and back of the xiphoid process. They vary in length and breadth in different subjects; those on the back of the joint are less distinct than those in front.

Synovial Membranes.—There is no synovial membrane between the first costal cartilage and the sternum, as this cartilage is directly continuous with the manubrium. There are two in the articulation of the second costal cartilage and generally one in each of the other joints; but those of the sixth and seventh sternocostal joints are sometimes absent; where an interarticular ligament is present, there are two synovial cavities. After middle life the articular surfaces lose their polish, become roughened, and the synovial membranes apparently disappear. In old age, the cartilages of most of the ribs become continuous with the sternum, and the joint cavities are consequently obliterated.

Movements.—Slight gliding movements are permitted in the sternocostal articulations.

Interchondral Articulations (articulationes interchondrales; articulations of the cartilages of the ribs with each other) (Fig. 315).—The contiguous borders of the sixth, seventh, and eighth, and sometimes those of the ninth and tenth, costal cartilages articulate with each other by small, smooth, oblong facets. Each articulation is enclosed in a thin articular capsule, lined by synovial membrane and strengthened laterally and medially by ligamentous fibers (interchondral ligaments) which pass from one cartilage to the other. Sometimes the fifth costal cartilages, more rarely the ninth and tenth, articulate by their lower borders with the adjoining cartilages by small oval facets; more frequently the connection is by a few ligamentous fibers.

Costochondral Articulations.—The lateral end of each costal cartilage is received into a depression in the sternal end of the rib, and the two are held together by the periosteum.
 
5g. Articulation of the Manubrium and Body of the Sternum
 
  The manubrium is united to the body of the sternum either by an amphiarthrodial joint—a piece of fibrocartilage connecting the segments—or by a diarthrodial joint, in which the articular surface of each bone is clothed with a lamina of cartilage. In the latter case, the cartilage covering the body is continued without interruption on to the cartilages of the facets for the second ribs. Rivington found the diarthrodial form of joint in about one-third of the specimens examined by him, Maisonneuve more frequently. It appears to be rare in childhood, and is formed, in Rivington’s opinion, from the amphiarthrodial form, by absorption. The diarthrodial joint seems to have no tendency to ossify, while the amphiarthrodial is more liable to do so, and has been found ossified as early as thirty-four years of age. The two segments are further connected by anterior and posterior intersternal ligaments consisting of longitudinal fibers.

Mechanism of the Thorax.—Each rib possesses its own range and variety of movements, but the movements of all are combined in the respiratory excursions of the thorax. Each rib may be regarded as a lever the fulcrum of which is situated immediately outside the costotransverse articulation, so that when the body of the rib is elevated the neck is depressed and vice versa; from the disproportion in length of the arms of the lever a slight movement at the vertebral end of the rib is greatly magnified at the anterior extremity.
  The anterior ends of the ribs lie on a lower plane than the posterior; when therefore the body of the rib is elevated the anterior extremity is thrust also forward. Again, the middle of the body of the rib lies in a plane below that passing through the two extremities, so that when the body is elevated relatively to its ends it is at the same time carried outward from the median plane of the thorax. Further, each rib forms the segment of a curve which is greater than that of the rib immediately above, and therefore the elevation of a rib increases the transverse diameter of the thorax in the plane to which it is raised. The modifications of the rib movements at their vertebral ends have already been described (page 302). Further modifications result from the

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