The pyramidal hollow between the upper arm and the chest, confined in front by the pectoral or breast muscles, behind by the shoulder blade and its muscles, and running up to a point beneath the collarbone: The armpit consists of the axillary vessels and nerves that run to the arm, also much fatty tissue, many sweat glands and lymphatic glands which act to remove bacteria and certain proteins from the tissues, transport fat from the small intestine, and supply mature lymphocytes to the blood.

The lymphatic glands in the armpits are important, because in poisoned wounds of an arm, they may become inflamed, resulting in abscess (inflamed tissue and pus); and still more, because in cancer of the breast, they may become infected with cancer, and have to be removed with the breast.

Wounds in an armpit are dangerous for the outer, front, and back walls, because large blood vessels are located there.