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. In order to study the functions of the spinal cord the physiologist takes a frog from the ditch and with one sweep of a sharp knife beheads it, or, what is the same thing, severs the spinal cord just as it enters the skull. No pain is produced, or at most a momentary pang. The spinal cord is thus isolated from the two brains, and all conscious sensibility is lost. The heart continues to beat, and life may go on for hours, but all the muscles are relaxed and the whole body is without feeling. Hang the creature up, and the long legs dangle passive and motionless. Bring, however, a cup of strong vinegar slowly upward from beneath the leg, and the moment the irritating liquid touches the toes the foot is drawn up. The parallel of this experiment may be repeated in the hospital ward. Cross the dangling, insensitive legs of that man whose back has been broken by some one of the murderous engines of modern civilization, and strike it smartly just below the knee with the edge of the hand, when out flies the leg; or as the patient lies in bed tickle the soles of his feet and see how they are drawn up, although all sensibility has vanished. Such movements as these in the frog or in the man are evidently similar to the wrigglings caused in the ascidian by the contact of an external bit of matter. They are, in a word, reflex: to produce them an impulse has travelled up the sensory or afferent nerves to the gray matter of the spinal cord, and has been reflected back along the efferent or motor nerves to the muscles.