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what is a nerve? This little white cord, the sciatic nerve of a frog, that we have been experimenting with, consists of numerous delicate fibres called nerve-fibres, each about the one-twelve-hundredth of an inch in breadth. Fig. 49.—Medullated nerve-fibres from the sciatic nerve of a frog. 1, 2, 3, fresh, in a solution of common salt. 3, fibre with a constriction at r. 4, fibres as affected by water. 5, as acted on by alcohol. 6, fresh; c, segments. 7, 8, hardened; a, axis-cylinder; b, swelling; k k′, nuclei; m, white substance; r, constriction; s, white substance shrinking from neurilemma. As a rule, each individual fibre has an external sheath called the neurilemma. Inside this we find a cylinder of matter of a fatty nature, known as the white substance of Schwann, and in the cylinder, a core of another substance called the axis-cylinder. These substances dur-