Robert Hooke showed the Royal Society microscopical observations of muscle from a lobster's claw

April 7, 1678

Birch, History, vol. III, p. 396, 28 March 1677/8 (O.S.) in London:

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 fibres sought for, though as much magnified and inlightened as was necessary, did not appear till by the adding a small drop of water the irregular refractions on the outside of the fibre 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 fibres 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.