28 THE UPPER EXTREMITY 



detached from the superior border of the clavicle and the sternal part of the 

 muscle must be pulled towards the medial plane. 



Articulatio Sternoclavicularis. The sterno-clavicular joint 

 is a diarthrodial joint formed by the sternal end of the 

 clavicle, the lateral part of the superior border of the 

 manubrium sterni, and the superior surface of the sternal 

 end of the cartilage of the first rib. It helps to increase 

 the range of the forward, backward, and upward movements 

 of the arm. The clavicle is attached to the sternum and the 

 first rib by a strong fibrous capsule. Within the capsule is 

 an articular meniscus which separates the joint cavity into two 

 parts. It is attached to the superior border of the sternal 

 end of the clavicle, to the superior surface of the first rib, and 

 to the anterior and posterior parts of the capsule. On the 

 lateral aspect of the capsule there is a strong accessory 

 ligament, the costo-clavicular ligament, which lies behind the 

 origin of the subclavius muscle and passes upwards, back- 

 wards, and laterally from the first rib to the costal tubercle on 

 the lower surface of the clavicle. In the capsule itself there 

 are three thickened bands, an anterior, a posterior, and a 

 superior, and as some of the fibres of the latter pass from 

 one clavicle to the other, it is called the interclavicular 

 ligament. 



Dissection. Pull the sternal head of the sterno-cleidomastoid muscle 

 towards the medial plane. Cut through the anterior part of the capsule of 

 the joint close to the sternum. Pass the knife behind the capsule, avoid- 

 ing the anterior jugular vein which runs laterally behind the upper border 

 of the joint, and detach the fibres of origin of the sterno-hyoid muscle 

 which spring from the back of the capsule. Cut through the posterior 

 ligament and pull the clavicle laterally. The meniscus is now exposed. 

 Detach it from the first rib, then carry the knife laterally below the clavicle 

 and cut through the lower part of the capsule and the costo-clavicular 

 ligament. The clavicle can now be displaced sufficiently upwards and 

 laterally to bring the whole of the brachial plexus into view. Before 

 studying the plexus, the dissector should note that behind the sterno- 

 clavicular joint there are the lower fibres of the sterno-hyoid and sterno- 

 thyreoid muscles which intervene on the right side between the capsule of 

 the joint and the bifurcation of the innominate artery into its right common 

 carotid and subclavian branches, and on the left side between the joint 

 and the left common carotid artery. 



Plexus Brachialis (Brachial Plexus). This important 

 plexus is formed by the anterior primary divisions of the four 

 lower cervical nerves and the greater part of the large anterior 

 branch of the first thoracic nerve. Above, the plexus is 

 further reinforced by a small twig of communication which 

 passes from the fourth to the fifth cervical nerve, whilst below, 



