Such simple reflex movements occur in all, even the highest, animals as the most primitive expressions of nervous energy. The exigences of lives fuller than that of the ascidian demand, however, other movements, whose complexity is continually increasing as the scale of life is traversed. Keeping pace with this elevation of function is a progressive setting apart of certain parts of the nervous system for certain purposes. In other words, as function or office becomes more complicated and more separate and particular, so also does the anatomy or structure of the nervous system. The whole nerve-life of the ascidian dwells in one cell or central point and a few fibres: that of the man is divided up between some millions of similar cells and fibres, each set aside for the performance of only one act or class of actions. Special powers everywhere require special structure. From the nervous system of the ascidian to that of man is a transition like that from the universal workman of a barbarous society to the complex factory of modern civilization, in which one man does only one little thing, but does this perfectly.
The general structure of the nervous system in all vertebrates from the frog to the man may be thus described: The cerebral hemispheres, or _upper brain_, a large double central nerve-mass which has no distinct nerves; the _lower brain_, which is also situated within the cranium, but below the cerebral hemispheres, and is provided with afferent and efferent nerves; and a _spinal cord_, which springs out of the lower brain and has an abundant supply of nerves connecting it with all parts of the body.
Of these great central nerve-tracts the spinal marrow is the simplest in construction. It is the "silver cord" of the old Hebrew poet, and owes its whitish appearance to the innumerable nerve-like fibres which run up and down or across to bind all parts of the cord together or to pass out from it into the nerves. In the central portions of the spinal marrow is a continuous tract of gray matter which is composed of an infinitude of protoplasmic masses closely resembling the central nerve-mass of the ascidian. This gray matter is indeed a series of such nerve-centres, each capable of discharging energy precisely as the central nerve-mass of the polyp does. The white fibres are the conductors of impulses: during life they are continually occupied in carrying messages to and fro between different portions of the spinal gray matter, or up and down--that is, to and from the brain.