Thisn is a thin section of a cat's muscle. It was stained (some form of silver impregnation that was used at the time) to reveal nerve terminals (which appear black) on muscle fibres (most of them are brown/red, but some muscle fibres were also stained black).

When you look at it at low magnification, the preparation appears fragmented and there are some air bubbles trapped under the coverslip (which appear as appear round bright circles). The most interesting regions to look at in greater detail are to the left (around 9-10 o'clock) and on the right side (around 3 o'clock) of the centre of the section.

If you zoom into these regions you can see nerves that break up to individual nerve fibres as they approach the muscle fibres. When they reach the muscle fibres, they form terminals, so-called neuromuscular junctions.  A single axon terminal can innervate several muscle fibres, the number of fibres that are 'innervated' by a single axon terminal depends on our muscle. In our eyes, there are very few fibres innervated by each terminal but in our gluteus maximus (buttocks) muscles, one axon ternimal will innervate many fibres. Sherrington called the muscle fibres that are innervated by a single axon terminal the motor unit.  

Answer to question on the Oxford Sparks website: the muscle spindle can be found on the right-hand side of the slide. 

Muscle Spind[le]; Muscle Spindle ? Cat. (Pri[mary] Sec[ondary] Motor) -> innervation ?C.S.S.