The pectoral fascia is very thin over the upper part of the pectoralis major, but thicker in the interval between it and the latissimus dorsi, where it closes in the axillary space and forms the axillary fascia. Axillary fascia, together with the skin, forms the base of the axilla.

This article incorporates text in the public domain from page 436 of the 20th edition of Gray's Anatomy (1918)