THE SHOULDER-JOIXT. 347 



glenoid cavity. It is continuous above with the long tendon of the Biceps, 

 which bifurcates at that point. 



The Synovial Membrane is reflected from the margin of the glenoid cavity over 

 the fibro-cartilaginous rim surrounding it : it is then reflected over the internal 

 surface of the capsular ligament, covers the lower part and sides of the neck of the 

 burner us. and is continued a short distance over the cartilage covering the head 

 of the bone. The long tendon of the Biceps muscle which passes through the 

 capsular ligament is enclosed in a tubular sheath of synovial membrane, which is 

 reflected upon it at the point where it perforates the capsule, and is continued 

 around it as far as the summit of the glenoid cavity. The tendon of the Biceps is 

 thus enabled to traverse the articulation, but it is not contained in the interior of 

 the synovial cavity. The synovial membrane communicates with a large bursal 

 sac beneath the tendon of the Subscapularis, by an opening on the anterior 

 side of the capsular ligament ; it also occasionally communicates with another 

 bursal sac. beneath the tendon of the Infraspinatus, through an orifice at its 

 posterior part. A third bursal sac, which does not communicate with the joint, 

 is placed between the under surface of the Deltoid and the outer surface of the 

 capsule. 



The Muscles in relation with the joint are, above, the Supraspinatus ; below, 

 the long head of the Triceps : internally, the Subscapularis ; externally, the Infra- 

 spinatus and Teres minor ; within, the long tendon of the Biceps. The Deltoid is 

 placed most externally, and covers the articulation on its outer side, as well as in 

 front and behind. 



The Arteries supplying the joint are articular branches of the anterior and 

 posterior circumflex, and suprascapular. 



The Xffi-f > are derived from the circumflex and suprascapular. 



Actions. The shoulder-joint is capable of movement in every direction, forward, 

 backward, abduction, adduction, circumduction, and rotation. The humerus is 

 drawn fnru'ard by the Pectoralis major, anterior fibres of the Deltoid. Coraco- 

 brachialis, and by the Biceps when the forearm is flexed ; backward, by the Latis- 

 simus dorsi. Teres major.- posterior fibres of the Deltoid, and by the Triceps when 

 the forearm is extended : it is abducted (elevated) by the Deltoid and Supraspinatus ; 

 it i* adducted (depressed) by the Subscapularis, Pectoralis major, Latissimus dorsi, 

 and Teres major ; it is rotated outward by the Infraspinatus and Teres minor; 

 and it is rotated imvard by the Subscapularis, Latissimus dorsi, Teres major, and 

 Pectoralis major. 



The most striking peculiarities in this joint are : 1. The large size of the head 

 of the humerus in comparison with the depth of the glenoid cavity, even when 

 supplemented by the glenoid ligament. 2. The looseness of the capsule of the 

 joint. 3. The intimate connection of the capsule with the muscles attached to the 

 head of the humerus. 4. The peculiar relation of the biceps tendon to the joint. 



It is in consequence of the relative size of the two articular surfaces that the 

 joint enjoys such free movement in every possible direction. When these movements 

 of the arm are arrested in the shoulder-joint by the contact of the bony surfaces 

 and by the tension of the corresponding fibres of the capsule, together with that of 

 the muscles acting as accessory ligaments, they can be carried considerably farther 

 by the movements of the scapula, involving, of course, motion at the acroniio- and 

 sterno-clavicular joints. These joints are therefore to be regarded as accessory 

 structures to the shoulder-joint. 1 The extent of these movements of the scapula is 

 very considerable, especially in extreme elevation of the arm, which movement is 

 best accomplished when the arm is thrown somewhat forward, since the articular 

 surface of the humerus is broader in the middle than at either end, especially the 

 lower, so that the range of elevation directly forward is less, and that directly 

 backward still more restricted. The great width of the central portion of the 

 humeral head also allows of very free horizontal movement when the arm is 

 raised to a right angle, in which movement the arch formed by the acromion, the 



1 See p. 344. 



