164 THE ARTICULATIONS OF THE UPPER LIMB. 



to the upper articulation ; and when the same movement is alternately performed in the 

 joints of opposite sides a horizontal oblique or grinding motion is produced. The fibres of the 

 external lateral ligament remain tight in opening the mouth, owing to the descent of the 

 condyle when it passes forwards on to the articular eminence. 



II. THE ARTICULATIONS OP THE UPPER LIMB. 



THE SCAPULO-CIiAVICULAR ARCH. 



The supporting arch of the upper limb has only one point of attachment to the 

 skeleton of the trunk, namely, at the sterno-clavicular articulation, the scapula being' 

 connected with the trunk only by muscles. 



The clavicle articulates at its inner end with the first piece of the sternum, and 

 is connected by ligaments also to its fellow of the opposite side and to the first rib. 

 At its outer end it is united to the scapula. 



STERNO-CLAVICULAR ARTICULATION. The articular surface of the inner end of 

 the clavicle is considerably larger than the opposing surface of the sternum, and the 

 investing layer of both surfaces is composed mainly of fibro-cartilage. The joint is- 

 surrounded by a fibrous capsule, which is of considerable thickness in front and 

 behind, forming anterior and posterior ligaments, but very thin at the lower and 

 outer part, between the inner end of the clavicle and the first rib-cartilage. Between 

 the two bones an interarticular fibro-cartilage is interposed. 



The anterior sterno-clavicular ligament, broad and consisting of parallel 

 fibres, passes from the front of the inner extremity of the clavicle, downwards and 

 inwards, to the anterior surface of the manubrium. 



The posterior sterno-clavicular ligament, on the posterior aspect of the 

 joint, is of similar conformation to the anterior ligament, but is not so strongly 

 marked. 



The inter clavicular ligament is a dense fasciculus of fibres passing between 

 the sternal ends of the clavicles. It dips downwards in the middle, where it is- 

 attached to the interclavicular notch of the sternum. 



The interarticular fibro-cartilage, nearly circular in form, and thicker 

 above and at its margins than at the centre, is interposed between the articular 

 surfaces of the two bones. Superiorly it is attached to the upper part of the inner 

 extremity of the clavicle, and inferiorly to the cartilage of the first rib. In the 

 latter situation it is thin and prolonged outwards, beneath the lower border of the 

 clavicle. 



Synovial cavities. There are two synovial cavities in this articulation, one 

 on each side of the interarticular fibro-cartilage. The outer one is the more exten- 

 sive, and is continued a short distance below, between the clavicle and first costal 

 cartilage. 



The interarticular fibro-cartilage is occasionally defective or wanting, in which case there* 

 is only one synovial cavity. The interclavicular ligament and interarticular fibre-cartilages- 

 together represent the episternal bone of monotremes and lizards. 



The costo-clavicular or rhomboid ligament may be regarded as an accessory 

 ligament of the sterno-clavicular articulation. It is attached inferiorly to the 

 cartilage of the first rib near its sternal end, and passes obliquely upwards, 

 backwards and outwards, to be fixed to a rough impression on the under surface of 

 the clavicle near the sternal end. The ligament is sometimes hollow, containing a 

 bursa. 



SCAPULC-CLAVICULAR ARTICULATION. At its outer end the clavicle is connected 

 to the acromion and coracoid processes of the scapula. 



