228 ACROMIAL REGION. 



The TERES MAJOR muscle arises from the lower thirc. of the inferior 

 border of the scapula, encroaching a little upon its dorsal aspect, and is 

 inserted into the posterior bicipital ridge. Its tendon lies immediately 

 behind that of the latissimus dorsi, from which it is separated by a syno- 

 vial membrane. 



Relations. By its posterior surface, with the latissimus dorsi, scapula 

 head of the triceps and integument. By its anterior surface, with th 

 subscapularis, latissimus dorsi, coraco-brachialis, short head of the biceps 

 axillary vessels, and branches of the brachial plexus. By its upper border, 

 it is in relation with the teres minor, from which it is separated by the 

 scapular head of the triceps ; and by the lower, it forms with the latissimus 

 dorsi, the lower and posterior border of the axilla. 



A large triangular space exists between the two teres muscles, which is 

 divided into two minor spaces by the long head of the triceps. 



Actions. The supra-spinatus raises the arm from the side ; but only 

 feebly, from the disadvantageous direction of the force. The infra-spinatus 

 and teres minor are rotators of the head of the humerus outwards. The 

 most important use of these three muscles is the protection of the joint, 

 and defence against displacement of the head of the humerus, in which 

 action they co-operate with the subscapularis. The teres major combines, 

 with the latissimus dorsi, in rotating the arm inwards, and at the same 

 time carrying it towards the side, and somewhat backwards. 



Acromial Region. 

 Deltoid. 



The convexity of the shoulder is formed by a large triangular muscle, 

 the deltoid (A, delta ; sTdo$, resemblance), which arises from the outer third 

 of the clavicle, from the acromion process, and from the whole length of 

 the spine of the scapula. The fibres from this broad origin converge to 

 the middle of the outer side of the humerus, where they are inserted into 

 a rough triangular elevation. This muscle is remarkable for its coarse 

 texture, and for its numerous tendinous intersections, from which mus- 

 cular fibres arise. The deltoid muscle may now be cut away from its 

 origin, and turned down, for the purpose of bringing into view the 

 muscles and tendons placed immediately around the shoulder joint. In 

 so doing, a large bursa will be seen between the under surface of the 

 muscle and the head of the humerus. 



Relatio?is. By its superficial surface, with a thin aponeurotic fascia, a 

 few fibres of the platysma myoides, the superficial fascia and integument. 

 By its deep surface, with the shoulder joint, from which it is separated by 

 a thin tendinous fascia, and by a synovial bursa ; with the coraco-acromial 

 ligament, coracoid process, pectoralis minor, coraco-brachialis, both heads 

 of the biceps, tendon of the pectoralis major, tendon of the supra-spinatus, 

 infra-spinatus, teres minor, teres major, scapular and external head of the 

 triceps, the circumflex vessels anterior and posterior, and humerus. By 

 its anterior border, with the external border of the pectoralis major, from 

 which it is separated by a cellular interspace, lodging the cephalic vein 

 and descending branch of the thoracico-acromialis artery. Its posterior 

 border is thin and tendinous above, where it is connected with the apo- 

 neurotic covering of the infra-spinatus muscle, and thick below. 



Actions. The deltoid is the elevator muscle of the arm in a direct 



