In a loose and general way every one knows that the Nervous System is a dominant agent in all sentient processes; although not by any means the only agent, yet, because of its predominance, it is artificially accepted as the only one. With the greater complexity of this system, there is observed a corresponding increase in the variety of sentient phenomena. The labors of anatomists have secured a tolerably exact plan of the topographical distribution of this system; a somewhat chaotic mass of observation and inference passes as a description of its elementary structure. The labors of physiologists have succeeded to a small extent in localizing certain functions in certain organs of this system. But imperfect as our knowledge of the elementary structures is, our knowledge of the functions is still more so. I wish I could say otherwise, and that I could ask my readers to accept with confidence what teachers confidently propound. The attitude of scepticism is always repulsive; the sceptic is seldom received without disfavor, because he throws on us the labor of investigation there where we wish for the confidence of knowledge. Yet it is only by facing the facts that we can hope one day to solve the great questions. 2. The nervous system has, in our artificial view of it, two divisions: the Peripheral, which connects the organism with the external world; and the Central, which connects each part of the organism with all the other parts. Although the system is constituted by various tissues--neural, connective, vascular, and elastic--it receives its characteristic designation from nerve-fibrils, nerve-fibres, and nerve-cells; just as the muscular system receives its designation from contractile cells and fibres. This neural tissue assumes three well-marked forms: 1°, _nerves_, which are bundles of fibres and fibrils, enclosed in a membranous sheath; 2°, _ganglia_, which are clusters of cells, fibres, and fibrils, sometimes enclosed in a sheath, sometimes not; 3°, _centres_, which are artificial divisions of the neural axis, serving as points of union for different organs. In the Invertebrata the neural axis is the chain of ganglionic masses running along the ventral side, and giving off the nerves to organs of sense, and to the muscles. It may be seen represented in Fig. 1. [Illustration: Fig. 1.--_Nervous system of a beetle._ The small round masses, or _ganglia_, are seen to be connected by longitudinal fibres, and from the ganglia issue fibres to the limbs, organs of sense, and viscera.]