It would be impossible in this book to describe all the varieties of neurone, for nothing is so characteristic of these elements as their enormous range both in size and form. It may be truly described as having no limits. Each of the two electric organs of Malapterurus is governed by a single neurone. Its cell-body is a fifth of a millimetre or more in diameter—large enough to be seen with the naked eye—and traversed by capillary bloodvessels. The axon of this nerve-cell—its single nerve-fibre—ramifies to supply a separate branch to each of the 2,000,000 chambers of the electric organ, and each branch breaks up into a bunch of twigs within the chamber. Contrast with such a giant cell as this one of the granules of the retina or cerebellum, the smallest cells to be found in the body, yet each a perfect neurone, exquisitely elaborate in form. [Illustration: FIG. 19.—A NERVE-FIBRE CONSISTING OF A, THE UNDIVIDED, FIBRILLATED AXON OF A NERVE-CELL, WITH ITS VARIOUS WRAPPINGS. In segment 1 the wrappings comprise B, a tube of phosphatic fat (myelin), interrupted at H, a node of Ranvier; C, a delicate membrane (sarcolemma); D, connective tissue; E, the rind of the axon; F, a tubular space containing lymph, between the axon and its sheath of myelin; G, nucleus of an enwrapping cell. At I the myelin is seen to be divided into overlapping conical rings. 2, The medullated nerve-fibre, running an isolated course, is merely enclosed in a tube of connective tissue containing lymph. 3, As a “grey” or “non-medullated” fibre, the axon has lost its myelin sheath.]