26 Anatomy Applied to Medicine and Surgery. 



be the result of the contraction of the muscles controlling 

 the humerus, especially the deltoid and the latissimus dor- 

 si, the former extending the arm, and the latter, while 

 opposing to some extent this action of the deltoid, draw- 

 ing the humerus backwards and rotating it as above. An- 

 other explanation is that the cavity o>f the joint contains 

 the greatest amount of fluid when the arm is in the above 

 position, and hence that the arm assumes this position, in 

 synovitis, to lessen the irritation caused by pressure on the 

 sensitive nerves of the synovial membrane of the joint. 

 The laxity of the capsule of the shoulder joint permits the 

 effusion, in synovitis, to produce distention, appreciable 

 externally, just below the acromion process, and, internal- 

 ly, well up in the axilla. On placing the fingers of one 

 hand on the swelling below the acromion and those of the 

 other on the swelling in the axilla, fluctuation is evident, 

 i.e., a wave may be propelled from one hand to the other. 

 This assists in diagnosing synovitis of the shoulder joint 

 from inflammation of the subacromial bursa, since in the 

 latter, while there would be swelling under the acromion, 

 there would be none in the axilla, and, therefore, no fluc- 

 tuation between these points could be elicited. 



Bursae. Besides the subacromial bursa, already re- 

 ferred to, there is one beneath the subscapularis, and near 

 its insertion, which communicates, as a rule, with the joint, 

 hence, in synovitis with much effusion, this bursa is dis- 

 tended and can be felt well up in the axilla along the pos- 

 terior or scapular wall. Another bursa lies under the ten- 

 don of the infra-spinatus, and, when it communicates with 

 the joint, which it occasionally does, produces a swelling, 

 posteriorly, below the spine of the scapula, when the joint 

 is filled with fluid. Lastly, under similar circumstances, 

 a prominence appears in front of the upper part of the 

 arm, because of the projection of the synovial membrane 



