At the beginning of this letter, as he often did, Leeuwenhoek gave some context for the observations he was about to report to Robert Hooke.
In your letter of April 18th 1678 you say that about a month ago you showed, in the Society, the structure and composition of a muscle; you will find some mention of this in the enclosed treatise, but not of what it was.
The letter to Leeuwenhoek did not survive, but the March 28, 1678, meeting of the Royal Society to which it referred was reported in Birch's History (vol. III, pp. 396 and 397).
Mr. Hooke shewed an observation of the figure of the small and imperceptible parts of a muscle, which he had discovered by the help of a microscope. The muscle, which he had made choice of for examination, was that of a lobster's claw, the fabric of which was such, that all the motion must necessarily be made in the fibrous part thereof; since first the tendon is nothing else but a bone, and so not capable of shrinking or stretching; and secondly, the other end thereof is fastened immediately to the inside of the shell.
In this observation notice was taken, that the small fibers sought for, though as much magnified and enlightened as was necessary, did not appear till by the adding a small drop of water the irregular refractions on the outside of the fiber were removed; after which it was very plainly visible, that the whole fibrous part of the muscle examined consisted of an indefinite number of exceeding small strings extended strait between the inside of the shell and the tendinous bone in the middle; which were so small, that five hundred of them would scarce exceed the bigness of an hair.
Each of these small fibers or strings was conceived to be seen of the shape and figure of a wreathed pillar, or a stick naturally grown wreathed by the twisting of a string of ivy. Others supposed it of other shapes. But the determination thereof was left till another time.
In his response here four years later, Leeuwenhoek wrote:
I will only tell you, (for fear of anticipating your opinion), that the muscles which I showed were those of crabs, lobsters and shell-fish, principally those of their large claws. I am sure that, if you examine these, you will find a structure that will interest you, and I have no doubt but your opinion and observations will greatly confirm mine, as I shall be glad to hear after you have examined them.
Sir, since your request I have often examined the muscles of large lobsters, caught off the coast of Norway and carried hither, but however many observations I have made, I have not been able to ascertain their exact structure.
While he often mentioned other research that spurred his own, in this letter he acknowledged that the inspiration went in both directions.
In the above-mentioned last letter you say among other things: Your discoveries both in this letter and in the previous one are very considerable, but I am not a little pleased to see from your last letter that you have discovered in the muscle of the flesh the same that I discovered long ago in the muscles of fish, especially of lobsters, crabs and shrimps, about which I informed you about 4 years ago.