14 THE SUPERIOR EXTREMITY 



below. It very rarely communicates with the joint, but owing 

 to its position, pathological conditions of the bursa may be 

 mistaken for articular affections. The sub-acromial bursa 

 serves as a soft pad between the humerus and the acromion, 

 but when it is distended with fluid or when the walls are 

 thickened by chronic inflammation (Codman's bursitis), 

 movements of the humerus become painful, especially the 

 movement of abduction, which crushes up the bursa against 

 the acromion. Pus from an abscess of this bursa gravitates 

 either to the anterior or to the posterior border of the deltoid. 



The Deltoid (p. 3) continues and maintains the movement 

 of abduction of the humerus, after it has been initiated by the 

 supra-spinatus. In complete abduction the humerus can be 

 raised to an angle of nearly 180, but the deltoid is responsible 

 for only half of that amount. The rest is effected by rotation 

 of the scapula under the action of the trapezius and the serratus 

 anterior (p. 31). The clavicular fibres are flexors and medial 

 rotators of the humerus ; the posterior fibres have the opposite 

 actions. The muscle overlies the shoulder- joint and the tendons 

 in relation to it, and is supplied by the axillary (circumflex) 

 nerve (C. 5 and 6), which enters its deep surface. 



The Deep Fascia on the Dorsal Scapular Muscles. The 

 deep fascia is strong and dense over the supra-spinatus and 

 infra-spinatus muscles, and is attached to the borders of the 

 scapular fossae. It is weakened above the superior transverse 

 scapular (supra-scapular) ligament, where the transverse scapular 

 vessels enter the fossa, and also at a point half-way down the 

 axillary border, where the circumflex scapular (dorsalis scapulae) 

 vessels pass on to the dorsum. 



Pus originating in the infra-spinous fossa is prevented from 

 passing backwards by the strong fascia. It follows the circumflex 

 scapular (dorsalis scapulae) vessels and gravitates down to 

 point at the lower part of the posterior fold of the axilla. In 

 the supra-spinous fossa it generally passes through or over the 

 scapular (supra-scapular) notch and points, near the coracoid 

 process, under the anterior border of the deltoid ; or, it may 

 gravitate into the infra-spinous fossa : or, very rarely, it may 

 make its way through the supra-spinous fascia and the trapezius 

 and point in the angle between the clavicle and the spine of the 

 scapula. 



The Supra-spinatus arises from the supra-spinous fossa, and is inserted 

 into the upper surface of the greater tubercle of the humerus. It passes 



