5g. Articulation of the Manubrium and Body of the Sternum

FIG. 316– Lateral view of first and seventh ribs in position, showing the movements of the sternum and ribs in A, ordinary expiration; B, quiet inspiration; C, deep inspiration.
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 attachments of their anterior extremities, and it is convenient therefore to consider separately the movements of the ribs of the three groups—vertebrosternal, vertebrochondral, and vertebral.
FIG. 317– Diagram showing the axes of movement (A B and C D) of a vertebrosternal rib. The interrupted lines indicate the position of the rib in inspiration.
FIG. 318– Diagram showing the axis of movement (A B) of a vertebrochondral rib. The interrupted lines indicate the position of the rib in inspiration.
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