The skin or external integument may at first thought seem to constitute a much greater surface than the more hidden part known as the internal integument, but the latter really represents much the greater area. This is due to the smaller number, size and depth of the openings and duplicatures existing on the skin for absorbent, secretory and excretory purposes, while the internal integument possesses much more numerous and larger duplicatures, with more numerous, larger and longer glandular and absorbent structures. The part of the internal integument that pertains to the respiratory system being excepted, the remainder, reaching from the margin of the lips to the verge of the anus, constitutes, with its appertaining or collatitious viscera, what is designated in this paper ''the gastro-intestinal system.'' By function is to be understood a work or process accomplished in a living part by the operation of chemical, mechanical and vital laws.
JAMA – American Medical Association
Published: Oct 22, 1898