MUSCLES OF THE PECTOKAL EEGION. 



371 



or other of the above-named slips is occasionally present, crossing the floor of the axilla in the 

 interval between the latissimus dorsi and the pectoralis major. They take origin from the costal 

 cartilages, ribs, or borders of the" pectoralis major (chondroepitrochlearis, axillary arches, costo- 

 coracoideus), or from the border of the latissimus dorsi (dorsoepitrochlearis, axillary arches, costo- 

 coracoideus). Their insertion is variable. The chondroepitrochlearis and dorsoepitrochlearis are 

 inserted into the fascia of the arm, the medial intermuscular septum, or the medial epicondyle 

 of the humerus. The axillary arches are inserted into the border of the pectoralis major, the 

 fascia of the arm, or the coracobrachialis or biceps muscle. The costocoracoideus, arising 

 from the ribs or the aponeurosis of the obliquus externus, or detaching itself from the border 

 of the pectoralis major or latissimus dorsi, is attached to the coracoid process, alone or along 

 with one of the muscles attached to that bone. These variable slips of muscle are supplied 

 by the medial anterior thoracic nerve, the medial cutaneous nerve of the arm, or the intercosto- 

 brachial. 



M. Pectoralis Minor. The pectoralis minor is a narrow, flat, triangular 

 muscle. It arises, under cover of the pectoralis major, from (1) the surfaces 

 and superior borders of the third, fourth, and fifth ribs near their anterior ends, 

 and (2) from the fascia covering the third and fourth intercostal spaces between 

 those ribs. It may have an additional origin from the second rib (Fig. 414, p. 468) ; 

 and that from the fifth rib is often absent. 



Directed obliquely upwards and laterally, it is inserted by a short, flat tendon 

 into the lateral half of the anterior border and upper surface of the coracoid 

 process (Fig. 333, p. 372), and usually also into the conjoint origin of the biceps 

 brachii and coracobrachialis. 



It enters into the formation of the anterior wall of the axilla, and gives attach- 

 ment along its superior border to the costo-coracoid membrane. It crosses the 

 axillary vessels and the cords of the brachial plexus, and is pierced by the medial 

 anterior thoracic nerve. 



Either in part or wholly the pectoralis minor may pass over the coracoid process of the 

 scapula, separated from it by a bursa, to be inserted into the coraco-acromial ligament, or the 

 acromion process ; or piercing the coraco-acromial ligament, it may be attached to the capsule of 

 the shoulder-joint (coraco-humeral ligament). 



Pectoralis Minimus. This is a slender slip, rarely present, which extends between the first 

 costal cartilage and the coracoid process. 



I Nerve-Supply. The pectoralis minor is innervated like the pectoralis major by both anterior 

 thoracic nerves. The lower division of the lateral nerve (0. 5. 6. 7.) communicates with the 

 medial anterior thoracic nerve (C. 8. T. 1.) over the axillary artery. Both nerves pierce and 

 supply the pectoralis minor, and end in the pectoralis major. 



Action. The main use of the pectoralis minor is to draw the shoulder forwards. It is thus 

 a chief assistant of the serratus anterior muscle. 



M. Subclavius. The subclavius muscle arises from the superior surface of the 



Coraco-clavic- 



ular ligament 



(trapezoid 



part) 



Pectoralis major (origin) 



isertion) __ 



Subclavius (insertion) 



Conoid ligament 

 FIG. 331. MUSCLE-ATTACHMENTS TO THE RIGHT CLAVICLE (Inferior Surface). 



costal cartilage in front of the costo-clavicular ligament, and from the upper 

 surface of the sternal end of the first rib (Fig. 414, p. 468). 



It is inserted into a groove in the middle third of the inferior surface of the 

 clavicle (Fig. 331). 



The muscle is invested by the fascia which forms the costo-coracoid membrane, 

 and is concealed by the clavicle and the clavicular origin of the pectoralis major. 



Nerve-Supply. The nerve to the subclavius is a fine branch of the brachial plexus 

 (C. 5. 6.), which arises above the clavicle, and passes anterior to the subclavian artery to reach 

 the muscle. 



Action. It acts as a depressor of the clavicle; or, the shoulder girdle being fixed, it is 

 i I capable of raising and fixing the first rib, in inspiration. 



