A simple secreting gland. Secreting glands serve to secrete (i.e., separate out some substance from the blood). They vary in size and structure.
Miller, F. E.; Hunt, H. L.; McCormick, F.J.; Burr, B.; & King, M.L. Domestic Medical Practice: A Household Adviser in the Treatment of Diseases, Arranged for Family Use (Chicago: Domestic Medical Society, 1919) 7
Courtesy the private collection of Roy Winkelman