In the separation of the protoplasm for various specific duties very early is a part of it set aside to constitute a so-called nervous system. The simplest form of this system occurs in the sac-like infusoria known as ascidians. This nervous apparatus is composed of a grayish central mass and certain white fibres which radiate from this mass to the outer surface of the sac that constitutes the body of the ascidian. The central mass contains much of very active protoplasm: it is for the ascidian the chief source of all nervous power and action, and is the simplest form of a so-called "nerve-centre." In the higher forms of life the nerve-centre is very complex, but is really formed by superadding small centres, like to the single central nerve-mass of the ascidian, one above the other, and binding them together with innumerable threads of white matter. The white threads seen in the ascidian are the earliest expressions of the white cords of the higher animals which every one knows as nerves. It is the function or office of the gray nerve-centre to originate waves of nervous influence, whilst to the white threads is given the duty of conducting such waves or impulses. The two parts may be well compared to the cells and the conducting wires of a galvanic battery. There are always at least two sets of the nerve-threads or conductors--one leading from the surface of the body to the nerve-centre, and the other from the centre to the exterior of the organism. Those fibres which start from the surface carry impulses to the centre, and are known by the scientist as afferent nerves: in the higher animals they conduct to the centres of consciousness impulses which give rise to sensations; hence they are often known as _sensory_ nerves. The nerve-threads which carry impulses away from the centre to the outer regions of the body are known as efferent nerves, because through them motion is produced as _motor_ nerves.