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The innervation of the muscles of the cheliped of Cambarus clarkii has been investigated both anatomically and physiologically. It is found that at least three of the six principal muscles of the cheliped show a triple innervation. Each of these muscles in the unregenerated claw is innervated by three axons only. Such "completely" innervated muscles are the extensor and flexor of the carpopodite and the flexor of the propodite. The extensor of the propodite and the abductor of the dacty-lopodite have only a double innervation. Both these muscles together are innervated by three axons; one fibre gives branches to each of the muscles, a second to the abductor only, and a third to the extensor. An extension of the last fibre can, in a number of cases, be seen to join two fibres for the adductor of the dactylopodite, making the innervation of this muscle complete. In other preparations the third fibre for the adductor is very thin. It is shown for the flexor of the carpopodite that each of the three fibres of this "completely" innervated muscle has a different function. Stimulation of the thickest fibre gives a fast contraction, stimulation of the second in size a slow contraction and stimulation of the third fibre, the smallest, causes inhibition. Both the fast and the slow contraction can be inhibited, but the slow contraction is more sensitive in this respect (see Fig. 4). It is shown that the axon which the abductor and the extensor of the propodite have in common is of the excitatory type. This makes these muscles two parts of one motor unit, each part having its own inhibitory fibre.

The Triple Innervation of Crayfish Muscle and Its Function in Contraction and Inhibition