Section 101. The primary elements in the tissue of the nervous system are three; nerve fibres, which are simply conducting threads, telegraph wires; ganglion cells, which are the officials of the system; and neuroglia, a fine variety of connective tissue which holds these other elements together, and may also possibly exercise a function in affecting impressions. A message along a nerve to a ganglion cell is an afferent impression, from a cell to a muscle or other external end is an efferent impression. The passage of an impression may be defined as a flash of kataboly along the nerve, and so every feeling, thought, and determination involves the formation of a certain quantity of katastases, and the necessity for air and nutrition. Section 102. Unlike telegraph wires, to which they are often compared, nervous fibres usually convey impressions only in one direction, either centrally (afferent or sensory nerve fibres), or outwardly (efferent or motor nerve fibres). But the so-called motor nerve fibres include not only those that set muscles in motion, but those that excite secretion, check impulsive movements, and govern nutrition. 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. Section 104. The simplest example of the action of the nervous system is reflex action. For instance, when the foot of a frog, or the hand of a soundly sleeping person, is tickled very gently, the limb is moved away from the irritation, without any mental action, and entirely without will being exercised. And when we go from light into darkness, the pupil of the eye enlarges, without any direct consciousness of the change of its shape on our part. Similarly, the presence or food in the pharynx initiates a series of movements-- swallowing, the digestive movements, and so on-- which in health are entirely beyond our mental scope.