(ARTICULATIONS OF THE CLAVICLE. 317 



ARTICULATIONES STERNI. 

 Primarily the sternum consists of an elongated plate of hyaline cartilage, which 

 omes subdivided into segments by the process of ossification. 

 The four segments of which the body of the sternum is originally composed 

 unite with each other after the manner of typical synchondroses. 



Similarly the xiphoid process and the body ultimately become united. It is 

 not usual to find the joint between the manubrium and the body obliterated by the 

 ossification of the two bony segments. Even in advanced life it remains open, and 

 the joint, which is named the synchondrosis sternalis, partakes of the nature of an 

 amphiarthrosis (Fig. 301), although a joint cavity is not found under any circum- 

 stances in the plate of fibro-cartilage which intervenes between the manubrium 

 and the body of the sternum. 



The membrana sterni, to which reference has already been made, assists in 

 strengthening the union between the different segments of the sternum. 



Movements of the Ribs and Sternum. These movements may be considered either 

 independently of, or as associated with, respiration. 



In the former condition the ribs move in connexion with flexion and extension of the 

 vertebral column, being more or less depressed and approximated in the former, and elevated or 

 pulled apart in the latter case. Considered in connexion with respiration, it is necessary to 

 observe that, to all intents and purposes, the vertebral column and the sternum are rigid structures. 

 Next, we must remember that the heads of all the ribs occupy fixed positions, and similarly 

 the anterior ends of seven pairs of cartilages are fixed to the lateral margins of the sternum. 

 The ribs thus form arches, presenting a large amount of obliquity from behind forwards. There- 

 fore, during inspiration, when the rib is elevated, the arch becomes more horizontal, and the 

 transverse diameter of the chest is increased. At the same time, the anterior ends of the sternal 

 ribs tend to thrust the sternum forwards and upwards ; but the nature of the attachment of the 

 first pair of ribs to the sternum, as well as the attachment of the diaphragm to the xiphoid 

 process, prevents this movement from becoming excessive, and hence the sternum becomes a line 

 of resistance to the forward thrust of the ribs. As a consequence, the ribs rotate upon themselves 

 about an oblique axis which passes downwards, laterally, and posteriorly through the capitular 

 joint and the neck of the rib anterior to the costo-transverse joint. 



In this way increase, both of the antero-posterior and transverse diameters of the thorax, is 

 provided for, although the amount of increase is not equally pronounced in all planes. Thus at 

 the level of the first rib very little eversion is possible, because the axis of rotation is nearly 

 transverse, and therefore any increase in the transverse or antero-posterior thoracic diameters at 

 this level may be disregarded, although a certain amount of elevation of the manubrium sterni 

 and anterior end of the first rib is evident. 



Below the level of the sixth rib elevation and rotation of the rib during inspiration are 

 usually said to be complicated by a certain amount of backward movement, due to the character 

 of the costo-transverse joint, until, in the case of the last two ribs, which are destitute of costo- 

 transverse joints, a movement backwards is almost entirely substituted for elevation. It is 

 probable, however, that the movements of the asternal ribs exactly correspond to those of the 

 sternal series, and that by the contraction of the costal digitations of the diaphragm the 

 anterior ends of the false ribs are provided with fixed positions comparable to those supplied by 

 the sternum to the ribs of the sternal series. 



We may therefore say that during inspiration the ribs move upwards and laterally between 

 their fixed ends, while as a whole the rib rotates, and its anterior end is thrust slightly forwards. 



During expiration these movements are simply reversed. 



THE ARTICULATIONS OF THE SUPERIOR EXTREMITY. 



The bony arch formed by the clavicle and scapula articulates directly with the 

 il skeleton only at one point, viz., the sterno-clavicular joint. 



ARTICULATIONS OF THE CLAVICLE. 



ARTICULATIO STERNOCLAVICULARIS. 



The sterno-clavicular joint is an example of an arthrodial diarthrosis. The 



(""""'cular surfaces concerned in its formation present the following appearances: 

 1. The sternal end of the clavicle is somewhat triangular in outline, having 

 most prominent angle directed inferiorly and posteriorly. The anterior and 



