Lippincott's Magazine of Popular Literature and Science, Vol. 26, November, 1880

by Various

TWO PAPERS.—I.

A new-born babe has been defined as a lump of possibilities: it is a mass of rudiments, the unfolding of which constitutes its growth. But protoplasm, which, unlike the babe, is a structureless substance, of undefined form and apparently void, is the basis of all life, and contains within itself the germs of an infinite development. The microscopist sees in it only a transparent mass, the chemist so many atoms of carbon, hydrogen, oxygen and nitrogen. But while seemingly the same in its chemical constitution, it differs immeasurably in its properties—always distinguished, however, by the soul of unrest that is within it, by its never-ceasing, self-sustained and self-generated movements. Protoplasm makes the saliva which moistens the morsel of bread; protoplasm bides its time in the new-laid egg; protoplasm weaves the tissue of the most highly endowed brain: its agency, in short, is visible in all the meanest and the supremest acts of life. No difference can be detected between its highest and its lowest forms. Yet difference must exist. The most primitive forms of life consist of simple specks of protoplasm, mere masses of organic slime, in which the particles enjoy all the life-functions share and share alike. As the babe develops into the man, so in ascending the scale of life, from the protoplasm of the infusorial amœba to that of a Newton's brain, it is step by step along a constant development and separating of dormant powers. There is nothing in the man that was not in rudiment in the babe, and so also does the amœba in its microscopic speck represent the life-power of a universe.

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.

To the primitive organic slime, which is the lowest form of life, consciousness can hardly come. The ascidian with its distinct though simple nervous system is a different and much higher being. In its microscopic world some floating particle touches it roughly: the ever-sensitive end of the afferent nerve thrills; the vibrations pass along the nerve-fibres to the central nerve-mass and provoke it into action: forthwith it sends down the efferent nerve a strong impulse which travels out until it reaches a mass of contractile tissue known as a muscle and causes it to shorten. In the ascidian, however, the limits of nerve-life are very narrow—no eye to see, no ear to hear, no separation of one kind of sensation from another, if indeed there be sensation at all, for we do not know whether the ascidian lives like the plant, to itself unknown, or whether it enjoys a twilight of semi-consciousness, feeling dimly its own existence and that of surrounding objects. The movements which it makes do not seem to emanate from consciousness, but always to originate in the contact of some foreign body with the exterior of its organism. The central mass of nervous protoplasm appears never to flash of its own volition nervous energy to the muscle, but only when aroused by an impulse from without. All movements seem to be reflex—that is, produced by an impulse passing up from the surface along the afferent nerve to the central mass, and thrown or reflected back, as it were, along the motor nerve to the muscle.

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.

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.

It is possible that in the ascidian the impulse which causes a reflex movement may be dimly perceived, and therefore awaken some sort of consciousness, but the man whose spinal cord is broken by accident or disease feels nothing below the injury, even if his feet be burnt to a crisp. It is plain that any faculty of consciousness which existed in the central nerve-mass of the polyp has been taken away from the spinal cord of the vertebrate, to be concentrated and perfected within the skull; or, in other words, that in the man, and also in the frog, consciousness and ordinary reflex movements have been separated, the one pertaining to the higher, the other to the lower, nerve-centres.