304 SYNDESMOLOGY 



I 



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 liga- 

 ment 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 inter chondrales; 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. 



VII. 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 car- 

 tilage. 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 amphiar- 

 throdial 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 



