a , STERNO-CLAVICULAR JOINT. 265 
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 ensiform 
cartilage, 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, outwards, and backwards 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 asternal 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 outwards 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 
axial skeleton only at one point, viz. the sterno-clavicular joint. 
ARTICULATIONS OF THE CLAVICLE. 
THE STERNO-CLAVICULAR JOINT. 
The sterno-clavicular joint (articulatio sterno-clavicularis) is an example of an 
arthrodial diarthrosis. The articular surfaces concerned in its formation present the 
following appearances :— 
1. The sternal end of the clavicle is somewhat triangular in outline, having its 
-mInost prominent angle directed downwards and backwards. The anterior and 
posterior sides of the triangle are slightly roughened for the attachment of 
hgaments, while the base or “inferior side is smooth and rounded, owing to the 
prolongation of the articular surface to the inferior aspect of the bone. In the 
antero-posterior direction the articular surface tends to be concave, while vertically 
it is shiehtly convex. 
2. An articular facet, situated on the superior lateral angle of the manubrium 
sterni, but in a plane slightly behind the supra-sternal notch, articulates with the 
clavicle. This facet is considerably smaller than the clavicular facet with which it 
articulates. 
3. The superior surface of the first costal cartilage close to the sternum also 
participates to a small extent in the articulation. 
It should be noted that the articular surfaces of the clavicle and sternum are 
covered mainly by fibro-cartilage. 
A capsular ligament is well marked on all sides except inferiorly, where it is 
very thin. 
The anterior sterno-clavicular ligament (Fig. 207) forms part of the capsule, and 
consists of short fibres which extend obliquely downwards and inwards from the 
anterior aspect of the sternal end of the clavicle to the adjoining anterior surface 
of the sternum and the anterior border of the first costal cartilage. 
The posterior sterno-clavicular ligament also forms part of the capsule, and 
