Axillary nodes are classified according to their positions relative to the pectoralis minor muscle. Level 1 are lateral, Level 2 are deep, Level 3 are medial. Pectoralis Minor divides the axillary artery into three parts.
The pectoralis minor muscle is covered anteriorly (superficially) by the clavipectoral fascia. The medial pectoral nerve pierces the pectoralis minor and the clavipectoral fascia. In attaching to the coracoid process, the pectoralis minor forms a 'bridge' - structures passing into the upper limb from the thorax will pass directly underneath. 
Pectoralis Minor inserted into Humerus; in about 15% of limbs (G.A.Seib) part of the tendon of the Minor passes over the coracoid process and between the two limbs of the coraco-acromial ligament to re-inforce the coraco-humeral ligament.
The origin is from the second, third and fourth or fifth ribs. The tendon of insertion may extend over the coracoid process to the greater tubercle. It may be split into several parts. Absence of this muscle is rare but happens with certain uncommon diseases.