Like all other events in vital chemistry, the generation of light by protoplasm is due to a process of fermentation. The luminous organs may be crushed, and the mixture of fermentable substance and ferment extracted with water. The extract is luminous. If an extract is prepared rapidly, and evaporated to dryness _in vacuo_, the residue glows when moistened with water. That two substances are present in the extract, one (luciferin) fermentable, the other (luciferase) a ferment, is proved by the following experiment: A certain quantity of extract is divided into two portions. One part (A) is allowed to glow until its capacity for emitting light is exhausted. The other portion (B), as soon as it is separated, is heated to 55° to kill the ferment. B still contains luciferin; A contains luciferase, although all its luciferin has been used up. Recombined, the extract is luminiferous. CHAPTER XI THE NERVOUS SYSTEM Twenty-five years ago a new process was introduced for colouring the elements which by their combination make up the nervous system. With its aid anatomists discovered the inadequacy of their conceptions of nerve-cells. It was already known that a nerve-fibre—that is to say, its essential part, its core—is a part of a cell, the body and other parts of which are situate within the brain or spinal cord, or in one of their dependents, a ganglion. But the new method showed the nerve-cell as more elaborate in form than anything which had been imagined hitherto; and since the word “cell” was often loosely used when the cell-body alone was referred to, it seemed worth while to give the unit of structure a new name. The term “neurone” was introduced to emphasize its functional individuality. The nervous system is an association of neurones.