In his letter of 2 March 1717 (letter XXXII) addressed to Abraham van Bleyswyk and first published in this edition, Leewenhoek provided the first morphologic description of nerve fibers accompanied by illustrations. “One of the most interesting features relating to the letter of 2 March 1717 is the accompanying illustration (fig.2, fig 2) for it is probably the first attempt to represent the cross section of a peripheral nerve. Individual nerve fibers can be seen, and the central stroke contained in each is meant to indicate the collapsed central tube, which he assumed was present; or, as we should put it today, the axis cylinder surrounded by the myelin sheath. It seems likely therefore that Leeuwenhoek observed the myelinated nerve fiber although his interpretation was incorrect; and having discovered no cavities in the nerve itself, he found in the individual fibers the hollowness that the then current theory of nerve function demanded. The thin section of the nerves which he cut was probably the first ever” (Clarke and  O’Malley, The human brain and spinal cord. 2nd. Ed. [1996] 30-35, citing the Latin edition). The following year this edition was translated into Latin in 1719 and issued by the same publisher as Epistolae physiologicae super compluribus naturae arcanis; ubi variorum animalium atque plantarum fabrica, conformatio, proprietates atque operationes, novis & hactenus inobservatis experimentis illustrantur & oculis exhibentur; item peculiaries & hactenus incognitae rerum quarumdam qualitates explicatur: ut sequens pagina docet: hactenus numquam editae (Delft: Adriaan Beman, 1719.) Digital facsimile of the 1718 Dutch edition from the Biodiversity Heritage Library at this link. Digital facsimile of the 1719 Latin edition from the Hathi Trust at this link.