Section 103. Figure 7, Sheet 8, Shows the Typical Structure of Nervous

tissues. The nerve fibres there figured are bound together by endoneurium into small ropes, the nerves, encased in perineurium. There is always a grey axis cylinder (a.c.), which may (in medullated nerves), or may not (in non-medullated or grey nerves) have a medullary sheath (s.S.) interrupted at intervals by the nodes of Ranvier (n.R.). Nuclei (n.) at intervals under the sheath indicate the cells from which nerve fibres are derived by a process of elongation. The nerves of invertebrata, where they possess nerves, are mostly grey, and so are those of the sympathetic system of vertebrata, to be presently described, g.c., g.c. are ganglion cells; they may have many hair-like processes, usually running into continuity with the axis cylinders of nerve fibres, in which case they are called multi-polar cells, or they may be uni- or bi-polar.