What does Boxing Day have to do with boxing?
endothelium en·do·the·li·um (ěn'dō-thē'lē-əm)
n. pl. en·do·the·li·a (-lē-ə)
A thin layer of flat epithelial cells that lines serous cavities, lymph vessels, and blood vessels.
A thin layer of flat epithelial cells that lines the lymph vessels, blood vessels, and the inner cavities of the heart. Compare mesothelium.