PECTORALIS. SUBCLAVIUS. 229 



Relations. By its external surface with the fibres of origin of the 

 platysma myoides, the mammary gland, the superficial fascia and inte- 

 gument. By its internal surface, on the thorax, with the clavicle, the 

 sternum, the costal cartilages, intercostal muscles, subclavius, pectoralis 

 minor, and serratus magnus ; in the axilla, with the axillary vessels 

 and glands. By its external border with the deltoid, from which it is 

 separated above by a cellular interspace lodging the cephalic vein and 

 the descending branch of the thoracico-acromialis artery. Its lower 

 border forms the anterior boundary of the axillary space. 



The pectoralis major is now to be removed by dividing its fibres 

 along the lower border of the clavicle, and then carrying the incision 

 perpendicularly downwards, parallel to the sternum, and at about three 

 inches from its border. Divide some loose cellular tissue, and several 

 small branches of the thoracic arteries, and reflect the muscle outwards. 

 We thus bring into view a region of considerable interest, in the mid- 

 dle of which is situated the pectoralis minor. 



The PECTORALIS MINOR arises by three digitations from the third, 

 fourth, and fifth ribs, and is inserted into the anterior border of the 

 coracoid process of the scapula by a broad tendon. 



Relations. By its anterior surface with the pectoralis major and 

 superior thoracic vessels and nerves. By its posterior surface with 

 the ribs, the intercostal muscles, serratus magnus, axillary space, and 

 axillary vessels and nerves. Its upper border forms the lower boun- 

 dary of a triangular space bounded above by the costo-coracoid mem- 

 brane, and internally by the ribs. In this space are found the axil- 

 lary vessels and nerves, and in it the subclavian artery may be tied 

 below the clavicle. 



The SUBCLAVIUS muscle arises by a round tendon from the cartilage 

 of the first rib, and is inserted into the under surface of the clavicle. 

 This muscle is concealed by the costo-coracoid membrane, an extension 

 of the deep cervical fascia, by which it is invested. 



Relations. By its upper surface with the clavicle. By the lower 

 with the subclavian artery and vein and brachial plexus, which sepa- 

 rate it from the first rib. In front with the pectoralis major, the costo- 

 coracoid membrane being interposed. 



Actions. The pectoralis major draws the arm against the thorax, 

 while its upper fibres assist the upper part of the trapezius in raising 

 the shoulder, as in supporting weights. The lower fibres depress the 

 shoulder with the aid of the latissimus dorsi. Taking its fixed point 

 from the shoulder, the pectoralis major assists the pectoralis minor, 

 subclavius, and serratus magnus, in drawing up and expanding the 

 chest. The pectoralis minor, in addition to this action, draws upon 

 the coracoid process, and assists in rotating the scapula upon the chest. 

 The subclavius draws the clavicle downwards and forwards, and 

 thereby assists in steadying the shoulder. All the muscles of this 

 group are agents in forced respiration, but are incapable of acting until 

 the shoulders are fixed. 



