MOVEMENTS OF THE RIBS. 



161 



STERNAL ARTICULATIONS. The manubrium and body of the sternum are united 

 by a symphysis, the disc being composed of a layer of cartilage above and below, 

 with a central fibro-carfcilaginous stratum, in which a cavity is frequently formed. 

 The articulation is farther supported by anterior and posterior ligamentous fibres, 

 which have chiefly a longitudinal direction, and are most developed on the back of 

 the sternum. The whole sternum is much strengthened by thick periosteum and 

 the radiating fibres of the chondro-sternal ligaments already mentioned. A layer 



Fig. 187. DIAGRAM OP FIRST AND SEVENTH RIBS, IN CON- 

 NECTION WITH THE SPINE AND THE STERNUM, SHOWING 

 HOW THE LATTER IS CARRIED UPWARDS AND FORWARDS 

 IN INSPIRATION. (Gf. D. T. ) 



The expiratory position is indicated by continuous lines, 

 the inspiratory by broken lines. 



of cartilage intervenes between the body and 

 ensiform process so long as they are not united 

 by bone. 



Movements of the ribs. The chief movement 

 of the ribs generally is one of rotation, upwards and 

 downwards, about an axis which is directed obliquely 

 outwards and backwards, as well as mostly somewhat 

 downwards, passing through the costo-central articu- 

 lation and the neck of the rib, and a little in front of 

 the costo-transverse joint. Owing to the downward 

 inclination of the ribs as they extend forwards, their 

 anterior ends when elevated also advance, so that 

 the front wall of the chest is carried upwards and 

 forwards in inspiration, thus increasing the sagittal 



Fig. 188. DIAGRAM OF THE FIFTH THORACIC VER- 

 TEBRA AND COSTAL ARCH, PROJECTED ONTO A 

 HORIZONTAL PLANE, TO SHOW THE AXIS OF MOVE- 

 MENT OF THE RIB, AND THE MANNER IN WHICH 

 THE SAGITTAL AND CORONAL DIAMETERS OF THE 

 THORAX ARE INCREASED. (Gr. D. T.) 



The continuous lines indicate the position in 

 expiration, the broken lines in inspiration. 



diameter of the thorax. At the same time, by 



reason of the obliquity of the axis, the anterior 



and lateral parts of the ribs move outwards, 



giving rise to the so-called eversion of the ribs, 



and the transverse diameter of the cavity is 



increased. This lateral movement of the fore 



part of the rib is accompanied by an opening 



out of the angle between the rib and its costal 



cartilage. The degree of lateral expansion 



is necessarily proportionate to the obliquity 



of the axis of movement. In the first rib the 



axis is nearly transverse, and thus, while a 



considerable degree of elevation is permitted, 



the eversion is but slight. In proceeding 



downwards the transverse processes of the vertebras and the necks of the ribs become 



more inclined backwards, so that the obliquity of the axis increases, and the outward 



movement becomes more extensive. In the upper six ribs, the convex tuberosities of 



which are received into small hollows on the corresponding transverse processes, this rotation 



is the only movement allowed ; but from the seventh to the tenth the costo-transverse joints 



have nearly plane articular surfaces, the vertebral facets being placed at the upper and fore 



part of the extremity of the transverse process, and these ribs move backwards as well as 



