PRACTICAL CONSIDERATIONS : THE SHOULDER-JOINT. 279 



these provisions, luxations of the shoulder would be even more frequent than 

 they are. 



The head of the bone may leave the joint-cavity at other points than the in- 

 ferior. If the force is so applied as to drive the head of the bone against the cap- 

 sule at the anterior portion, a direct subcoracoid luxation may result ; if against the 

 posterior portion, a subspinous. The latter is very rare, and the former is also rare 

 as a primary luxation. 



The further mechanism of luxations, their deformities and anatomical diagnosis, 

 will be considered after the muscles, which are such important factors in producing 

 and modifying them, have been described (page 582). 



Disease of the shoulder-joint may be of any variety. In spite of the frequent 

 strains to which the joint is subjected and its wide range of movement, the diseases 

 produced by traumatism are not exceptionally frequent. This is probably because 

 of ( I ) its ample covering of muscles protecting it from the effects of cold and damp. 

 (2) The mobility of the scapular segment of the shoulder-girdle lessening greatly 

 the efifect of traumatisms. (3) The laxity of its capsular and synovial elements, 

 which, though it favors luxation, permits a moderate effusion to occur without harm- 

 ful tension. (4) The influence of the weight of the upper extremity in the usual 

 position of the body in resisting by gravity the destructive pressure of joint surfaces 

 against each other, caused by muscular spasm after injury or during disease. (5) The 



Spine of scapula 



Fig. 293. 



Supraspinatus 



Reflection of capsule 



Deltoid 



Reflection of capsule 



Infraspinatus Head of scapula Teres minor 



Section through right shoulder-joint with arm in abduction. 



ease with which the joint may be immobilized without irksome confinement of the 

 patient. 



These circumstances, especially the latter, account also for the facts that tuber- 

 culous disease of the joint and epiphysitis involving the joint are not so common as 

 in other joints, and that the results are exceptionally good, operative interference 

 being required with comparative rarity. 



Synovial distention causes a uniform rounded swelling of the shoulder, but it 

 can best be recognized by the touch in the bicipital groove, where one synovial 

 diverticulum runs, and in the axilla, where part of the capsule is exposed beyond 

 the margin of the subscapular muscle. The diverticula beneath the tendons of that 

 muscle and (more rarely) of the infraspinatus are usually involved, pain when the 

 arm is rotated being a resultant symptom. 



The -subdeltoid bursa does not usually communicate with the joint. It may 

 be the subject of independent disease. When it is inflamed the position of ease 

 will be one which relaxes the deltoid (abduction of the arm), and rotation or 

 pressure upward will be painless. In disease of the subacromial bursa, abduc- 

 tion and upward pressure are painful because the sac is then pinched between 



