PECTORALIS MAJOR AND MINOR. 205 



rosity of the humerus. The tendon of this muscle forms a part of 

 the capsule of the joint, glides over a large bursa which separates 

 it from the base of the coracoid process, and is lined by a prolonga- 

 tion of the synovial membrane of the articulation. 



Relations. By its anterior surface with the serratus magnus, the 

 coraco-brachialis, deltoid, and with the axillary vessels and" nerves. 

 By its posterior surface with the scapula, the subscapular vessels 

 and nerves, and the shoulder joint. 



Action. It rotates the head of the humerus inwards, and is a 

 powerful defence to the joint. When the arm is raised, it draws 

 the humerus downwards. 



Posterior scapular Region. 

 Supra-spinatus, Teres minor, 



Infra-spinatus, Teres major. 



The Supra-spinatus muscle, (supra, above; spina, the spine) arises 

 from the whole of the supra-spinous fossa, and is inserted into the 

 uppermost depression on the great tuberosity of the humerus. The 

 tendon of this muscle cannot be well seen without cutting away the 

 acromion process with a saw. 



Relations. By its upper surface with the trapezius, the clavicle, 

 acromion, and coraco-acromial ligament. From the trapezius it is 

 separated by a strong fascia. By its lower surface with the supra- 

 spinous fossa, the supra-scapular vessels and nerve, and the upper 

 part of the shoulder joint, forming part of the capsular ligament. 



The Infra-spinatus (infra, beneath ; spina, the spine,) is covered 

 in by a layer of tendinous fascia, which must be removed before the 

 fibres of the muscle can be seen, the deltoid muscle having been 

 previously turned down from its scapular origin. It arises from the 

 whole of the infra-spinous fossa, and from the fascia above-mentioned, 

 and is inserted into the middle depression upon the greater tuberosity 

 of the humerus. 



Relations. By its posterior surface with the deltoid, latissimus 

 dorsi and integument. By its anterior surface with the infra-spinous 

 fossa, superior and dorsal scapular vessels, and shoulder joint ; its 

 tendon being lined by a prolongation from the synovial membrane. 

 By its upper border it is in relation with the spine of the scapula, 

 and by the lower with the teres minor, w r ith which it is closely 

 united. 



The Teres minor muscle (teres, round) arises from the middle 

 third of the inferior border of the scapula, and is inserted into the 

 lower depression on the great tuberosity of the humerus. The ten- 

 dons of these three muscles, with that of the subscapularis, are in 

 immediate contact with the joint, and form part of its ligamentous 

 capsule, thereby preserving the solidity of the articulation. They 

 are therefore the structures most frequently ruptured in dislocation 

 of the shoulder-joint with violence. 



Relations. By its posterior surface with the deltoid, latissimus 

 dorsi and integument. By its anterior surface with the inferior bor- 



