342 THE ARTICULATIONS. 



above, to the upper and posterior border of the articular surface of the clavicle ; 

 below, to the cartilage of the first rib, at its junction with the sternum ; and by 

 its circumference, to the anterior and posterior sterno-clavicular and interclavicular 

 ligaments. It is thicker at the circumference, especially its upper and back part, 

 than at it's centre or below. It divides the joint into two cavities, each of which is 

 furnished with a separate synovial membrane. 



Of the two Synovial Membranes found in this articulation, one is reflected 

 from the sternal end of the clavicle over the adjacent surface of the fibro-cartilage 

 and cartilage of the first rib ; the other is placed between the articular surface of 

 the sternum and adjacent surface of the fibro-cartilage ; the latter is the larger of 

 the two. They seldom contain much synovia. 



Actions. This articulation is the centre of the movements of the shoulder, and 

 admits of a limited amount of motion in nearly every direction upward, down- 

 ward, backward, forward as well as circumduction. When these movements 

 take place in the joint, the clavicle in its motion carries the scapula with it, this 

 bone gliding on the outer surface of the chest. This joint therefore forms the 

 centre from which all movements of the supporting arch of the shoulder originate, 

 and is the only point of articulation of this part of the skeleton with the trunk. 

 " The movements attendant on elevation and depression of the shoulder take place 

 between the clavicle and the interarticular fibro-cartilage, the bone rotating upon 

 the ligament on an axis drawn from before backward through its own articular 

 facet. When the shoulder is moved forward and backward, the clavicle, with 

 the interarticular fibro-cartilage, rolls to and fro on the articular surface of the 

 sternum, revolving, with a sliding movement, round an axis drawn nearly vertically 

 through the sternum. In the circumduction of the shoulder, which is compounded 

 of these two movements, the clavicle revolves upon the interarticular fibro-cartilage, 

 and the latter, with the clavicle, rolls upon the sternum." l Elevation of the clavicle 

 is principally limited by the costo-clavicular ligament ; depression, by the inter- 

 clavicular. The muscles which raise the clavicle, as in shrugging the shoulders, 

 are the upper fibres of the Trapezius, the Levator anguli scapulae, the clavicular 

 head of the Sterno-mastoid, assisted to a certain extent by the two Rhomboids, 

 which pull the inferior angle of the Scapula backward and upward, and so raise 

 the clavicle. The depression of the clavicle is principally effected by gravity, 

 assisted by the Subclavius, Pectoralis minor, and lower fibres of the Trapezius. 

 It is drawn backward by the Rhomboids and the middle and lower fibres of the 

 Trapezius, and forward by the Serratus magnus and Pectoralis minor. 



Surface Form. The position of the sterno-clavicular joint may be easily ascertained by feel- 

 ing the enlarged sternal end of the collar-bone just external to the long, cord-like, sternal origin 

 of the Sterno-mastoid muscle. If this muscle is relaxed by bending the head forward, a depres- 

 sion just internal to the end of the clavicle, and between it and the sternum, can be felt, indica- 

 ting the exact position of the joint, which is subcutaneous. When the arm hangs by the side, 

 the cavity of the joint is V-shaped. If the arm is raised, the bones become more closely approx- 

 imated, and the cavity becomes a mere slit. 



Surgical Anatomy. The strength of this joint mainly depends upon its ligaments, and 

 it is to this, and to the fact that the force of the blow is generally transmitted along the 

 long axis of the clavicle, that dislocation rarely occurs, and that the bone is generally broken 

 rather than displaced. When dislocation does occur, the course which the displaced bone takes 

 depends more upon the direction in which the violence is applied than upon the anatomical 

 construction of the joint; it may be either forward, backward, or upward. The chief point 

 worthy of note, as regards the construction of the joint, in regard to dislocations, is the fact 

 that, owing to the shape of the articular surfaces being so little adapted to each other, and 

 that the strength of the joint mainly depends upon the ligaments, the displacement when 

 reduced is very liable to recur, and hence it is extremely difficult to keep the end of the bone in its 

 proper place. 



II. Acromio-clavicular Articulation (Fig. 243). 



The Acromio-clavicular is an arthrodial joint formed between the outer 

 extremity of the clavicle and the upper edge of the acromion process of the 

 scapula. Its ligaments are the 



1 Humphry, On the Human Skeleton, p. 402. 



