Page:Life in Motion.djvu/116

96 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.

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