22 THE SUPERIOR EXTREMITY 



immediately to the medial side of the coracoid process, and 

 is directed upwards towards the root of the neck, whilst the 

 wider part or base of the space looks downwards. The 

 medial wall, formed by the thorax, is of greater extent than 

 the lateral wall which is formed by the arm. It follows, 

 therefore, that the anterior and posterior walls converge as 

 they proceed laterally, and, because the posterior wall is 

 longer, from above downwards, than the anterior, the posterior 

 border of the base is lower than the anterior. 



Before beginning the dissection of the space, the dissector should have 

 a general knowledge of its boundaries and of the manner in which the 

 contents are disposed in relation to the boundaries. 



Boundaries of the Axilla. The four walls of the axilla are 

 (i) anterior, (2) posterior, (3) medial, and (4) lateral. The 

 anterior wall is formed by the two pectoral muscles, the 

 subclavius and the fascia which surrounds the pectoralis 

 minor and attaches its upper border to the clavicle and its 

 lower border to the floor of the axilla, and to the deep fascia 

 on the medial side of the arm. The pectoralis major forms 

 the superficial stratum, and is spread out over the entire 

 extent of the anterior wall. The pectoralis minor lies 

 posterior to the pectoralis major, and takes part in the 

 formation of the middle third of the anterior boundary. The 

 fascia which fills the gap between the pectoralis minor and 

 the clavicle is called the costo-coracoid membrane ; it splits 

 above to enclose the subclavius muscle, and along the lower 

 border of that muscle it is stronger than elsewhere. The 

 lower border of the anterior wall is the anterior fold of the 

 axilla. It is formed by the lower border of the pectoralis 

 major, strengthened, medially, by the lower border of the 

 pectoralis minor, which projects beyond the major near the 

 side of the thorax. 



The posterior wall of the axilla is formed by the lateral 

 part of the subscapularis muscle, by a portion of the latissimus 

 dorsi and its tendon, and by the teres major muscle. The 

 subscapularis covers the costal surface of the scapula. The 

 latissimus dorsi winds from the back, round the medial part 

 of the lower border of the teres major to gain its anterior 

 surface; thus the lower border of the posterior wall, which 

 constitutes the posterior fold of the axilla, is formed in its 

 medial part by the latissimus dorsi, and laterally by the 

 inferior margin of the teres major. 



