FASCIAE AND MUSCLES OF THE SHOULDER 373 



the whole shoulder to be brought forward by a movement at the steriio-clavicular joint. The 

 movement of stretching forward the arm as in fencing is due to this action of the muscle. 

 Further, by its relation to the inferior angle of the scapula, the serratus anterior causes (along 

 with the trapezius) a rotation of the scapula, resulting in a tilting upwards of the glenoid 

 cavity, and so facilitating the upward movement of the arm above the head. Acting from the 

 shoulder on the ribs the serratus becomes a powerful muscle of inspiration. 



Action of Muscles on the Sterno -Clavicular and Acromio -Clavicular Joints. The 

 muscles just considered (along with the sterno-cleido-mastoid and omo-hyoid muscles) act for 

 the most part in the sterno-clavicular and acromio-clavicular joints. 



A. Sterno- Clavicular Joint. The movements at this articulation are vertical, horizontal, 

 and rotatory, and the muscles concerned may be tabulated as follows : 



Sterno-Clavicular Joint. 



B. Acromio- Clavicular Joint. Movements at this joint are associated with rotation of the 

 scapula. By the combined action of such muscles or the trapezius and serratus anterior (inferior 

 fibres), the inferior angle of the scapula is drawn or thrust forwards, the body of the scapula 

 is rotated, and the glenoid cavity is tilted upwards, so facilitating the upward movement of 

 the arm above the horizontal level. 



In forced inspiration, the sterno-mastoid, trapezius, levator scapulae, rhomboidei, sub- 

 clavius, omo-hyoid, serratus anterior, pectoral muscles, and latissimus dorsi, acting together, raise 

 and fix the shoulder girdle ; while those of them which have costal attachments subclavius, 

 pectoral muscles, serratus anterior, and latissimus dorsi simultaneously elevate the ribs and 

 expand the thorax. 



Lateral flexion and rotation of the vertebral column in the neck is effected partly by 

 the action of the trapezius, levator scapulae, and rhomboid muscles (with the shoulder fixed). 

 The latissimus dorsi and pectoralis major act in climbing in a similar way, raising up the 

 trunk towards the shoulder. 



Action on the Upper Limb. By reason of their insertion into the humerus the pectoralis 

 major and latissimus dorsi muscles assist the movements of the upper limb. Acting together, the 

 two muscles depress the shoulder, and draw the arm to the side of the body, at the same time 

 rotating the humerus medially. The two parts of the pectoralis major have slightly different 

 actions on the humerus. The clavicular part of the muscle (portio attollens) draws the arm 

 medially and upwards ; the sterno-costal part of the muscle (portio deprimens) draws it medially 

 and downwards. The latissimus dorsi acting alone, besides rotating the limb, draws it medially 

 and backwards, as in the act of swimming. 



FASCIAE AND MUSCLES OF THE SHOULDER. 



The deep fascia covering the scapular muscles presents no feature of special 

 importance. Attached to the clavicle, acrornion, and scapular spine, it is thin over 

 the deltoid muscle. Below the deltoid it is thicker ; it encases and gives origin to 

 the infraspinatus muscle, and is continuous with the fasciae of the axilla and the 

 back. 



Muscles. 



The muscles proper to the shoulder comprise the deltoid, supraspinatus, infra- 

 spinatus, teres minor, teres major, and subscapularis. 



M. Deltoideus. The deltoid, a coarsely fasciculated multipennate muscle, has 

 an extensive origin from (1) the front of the clavicle in its lateral third (Figs. 327, 

 p. 366, and 331, p. 371) ; (2) the lateral border of the acromion ; (3) the inferior edge 



the free border of the spine of the scapula (Figs. 329, p. 368, and 333, p. 372) ; 

 and (4) from the deep fascia covering the infraspinatus muscle. Its origin 

 embraces the insertion of the trapezius. 



The fibres of the muscle converge to the lateral aspect of the body of the 



