Letter 40 of December 28, 1683 Figure 3 | Arcana Naturae Detecta
Outline of very thin blood vessels with their branches lining the inside of guts of oxen and cows.
Ommetrek van de seer dunne bloet-vaatgens met haar takjens van binnen tegen de darmen van ossen en koeijen sit.
Alle de Brieven / Collected Letters translates darmen as intestines. However, Sewel's 1735 dictionary had two words for intestines, ingewand or gedarmte and one word for darm, gut, which seems closer to what Leeuwenhoek called it.