THE SHOULDER-JOINT 



1413 



Shoulder-joint. — The frequency of dislocations here, nearly equal to those of 

 all the other joints put together, calls attention to the points contributing to make 

 the joint alike insecure and safe. Strength is given by (1) the intimate blending 

 of the short scapular muscles, especially the subscapularis with the capsule; (2) 

 the coraco-acromial vault; (3) atmospheric pressure; (4) the long tendon of the 

 biceps; (5) the elasticity of the clavicle; (6) the mobility of the scapula. The 

 weakness of the joint is readily explained by its free mobility, the want of corre- 

 spondence between the articular surfaces, its exposure to injury, and the length of 

 the humeral lever. The rent in the capsule is usually anterior and below, and to 

 this spot the head of the humerus must be made to return. While dislocations are 

 usually primarily subglenoid, owdng to the above part of the capsule being the 

 thinnest and least protected, they take usually a secondarily forward direction. 



Fig. 1131. — Posterior View of the Scapular Muscles and Triceps. 



Supraspinatus 



Infraspinatus 

 Teres minor 



Teres major - 



Site of appearance of radial nerve 

 at the back of the arm 



Long head of triceps 



Lateral head 



Medial head 



Anco 



The X mark indicates 

 where the radial 

 nerve leaves the long 

 head of the triceps 

 and passes under the 

 outer head to gain 

 the groove. 



as the triceps prevents the head passing backward. In addition to the above 

 features of the lower part of the capsule, laxity is here also a marked feature, to 

 allow of free abduction and elevation. This movement will be accordingly much 

 checked by any inflammatory matting of this part of the capsule. 



The best incision for exploring the joint is one commencing midway between the coracoid 

 and acromion processes and carried downward parallel with the anterior fibres of the deltoid. 

 The cephaUc vein and biceps tendon are to be avoided. If drainage is needed, it must be 

 supplied by a counter-incision behind. This may be made along the posterior border of the 

 deltoid, part of its humeral attachment being detached if necessary. The axillary (circumflex) 

 nerve must be avoided in the upper part of the incision. 



