Wrote Letter 20 of 1677-05-14 (AB 32) to Henry Oldenburg
Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren. A Latin translation was published in Hooke's posthumous Philosophical Experiments and Observations (1726) titled (in English) "Mr. Lewenhoek's Letter to Mr. Oldenburg, received from Dr. Crowe, Aug. 14, 1682. Of the Bibres of Muscles, Dura Mater, Brain, and Moxa."
Leeuwenhoek wrote this letter to Henry Oldenburg, complying with his request of February 22, about muscles. This letter followed up on observations he had reported in June 1674.
He also wrote about the structure of the brain and the movement of fluids in fruits. He ended with detailed observations of some moxa given to him by Constantijn Huygens. As usual, he asked for feedback.
The photo below of Leeuwenhoek's handwritten manuscript is from the first of the four volumes of Leeuwenhoek's original letters in the archives of the Royal Society. The photo was taken by Mike Peel < MikePeel.net > and is available on Wikimedia Commons.
The smaller piece of paper labelled 22a has the title of the letter as it appeared in Philosophical Transactions no. 136 and the beginning of the English translation.