At the infero-lateral margin of the minor pectoral muscle the strong Gerdy’s ligament (= the lower part of the clavicopectoral fascia) connects the minor pectoral muscle to the axillary fascia
When the needle tip penetrates Gerdy’s ligament it enters the the lower part of the axillary space between the Gerdy’s ligament antero-laterally and the serratus anterior muscle covering the lateral thoracic wall medially in the axilla
Gerdy’s ligament (cyan arrows), minor pectoral muscle (green asterix), major pectoral muscle (red asterix), serratus anterior (magenta asterix), pleura (red arrows), ribs III and IV (r3 and r4)