History of the Intellectual Development of Europe, Volume II (of 2) / Revised Edition

by John William Draper

Two elementary forms of nerve structure.[346] In any nervous system there are two portions anatomically distinct. They are, 1st, the fibrous; 2d, the vesicular. It may be desirable to describe briefly the construction and functions of each of these portions. Their conjoint action will then be intelligible.

Structure of a nerve fibre.1st. A nerve fibre consists essentially of a delicate thread—the axis filament, as it is called—enveloped in an oil-like substance, which coagulates or congeals after death. This, in its turn, is inclosed in a thin investing sheath or membranous tube. Many such fibres bound together constitute a nerve.

Function of a nerve fibre is conduction.The function of such a nerve fibre is indisputably altogether of a physical kind, being the conveyance of influences from part to part. The axis filament is the line along which the translation occurs, the investing material being for the purpose of confining or insulating it, so as to prevent any lateral escape. Such a construction is the exact counterpart of many electrical contrivances, in which a metallic wire is coated over with sealing-wax or wrapped round with silk, the current being thus compelled to move in the wire without any lateral escape. Of such fibres, some convey their influences to the interior, and hence are called centripetal; some convey them to the exterior, and hence are called centrifugal. No anatomical difference in the structure of the two has, however, thus far been discovered. As in a conducting wire the electrical current moves in a progressive manner with a definite velocity, so in a nerve filament the influence advances progressively at a rate said to be dependent on the temperature of the animal examined. It seems in the cold-blooded to be much slower than in the hot. It has been estimated in the frog at eighty-five feet per second; in man at two hundred feet—an estimate probably too low.

The fibres thus described are of the kind designated by physiologists as the cerebro-spinal; there are others, passing under the name of the sympathetic, characterized by not possessing the investing medullary substance. In colour they are yellowish-gray; but it is not necessary here to consider them further.

Structure of a nerve vesicle. [347] 2nd. The other portion of the nervous structure is the vesicular. As its name imports, it consists of vesicles filled with a gray granular material. Each vesicle has a thickened spot or nucleus upon it, and appears to be connected with one or more fibres. If the connexion is only with one, the vesicle is called unipolar; if with two, bipolar; if with many, multipolar or stellate. Every vesicle is abundantly supplied with blood.

Function of a nerve vesicle.As might be inferred from its structure, the vesicle differs altogether from the fibre in function. I may refer to my "Physiology" for the reasons which have led to the inference that these are contrivances for the purposes of permitting influences that have been translated along or confined within the fibre to escape and diffuse themselves in the gray granular material. They also permit influences that are coming through many different channels into a multipolar vesicle to communicate or mix with one another, and combine to produce new results. Moreover, in them influences may be long preserved, and thus they become magazines of force. Combined together, they constitute ganglia or nerve centres, on which, if impressions be made, they do not necessarily forthwith die out, but may remain gradually declining away for a long time. Thus is introduced into the nervous mechanism the element of time, and this important function of the nerve vesicle lies at the basis of memory.

It has been said that the vesicular portion of the nerve mechanism is copiously supplied with blood. Indeed, the condition indispensably necessary for its functional activity is waste by oxydation. Arterial vessels are abundantly furnished to insure the necessary supply of aerated blood, and veins to carry away the wasted products of decay. Also, through the former, the necessary materials for repair and renovation are brought. Physiological condition of nerve action is nerve waste. There is a definite waste of nervous substance in the production of a definite mechanical or intellectual result—a material connexion and condition that must never be overlooked. Hence it is plain that unless the repair and the waste are synchronously equal to one another, periodicities in the action of the nervous system will arise, this being the fundamental [348] condition connected with the physical theories of sleep and fatigue.

The statements here made rest upon two distinct forms of evidence. In part they are derived from an interpretation of anatomical structure, and in part from direct experiment, chiefly by the aid of feeble electrical currents. The registering or preserving action displayed by a ganglion may be considered as an effect, resembling that of the construction known as Ritter's secondary piles.

It will not suit my purpose to offer more than the simplest illustration of the application of the foregoing facts. When an impression, either by pressure or in any other way, is made on the exterior termination of a centripetal fibre, the influence is conveyed with a velocity such as has been mentioned into the vesicle to which that fibre is attached, and thence, going forth along the centrifugal fibre, may give rise to motion through contraction of the muscle to which that fibre is distributed. Reflex action of the nervous system. An impression has thus produced a motion, and to the operation the designation of reflexion is commonly given. This reflexion takes place without consciousness. The three parts—the centripetal fibre, the vesicle, and the centrifugal fibre—conjointly constitute a simple nervous arc.

Gradual complexity of the nervous system.A repetition of these arcs, each precisely like all the others, constitutes the first step toward a complex nervous system. Their manner of arrangement is necessarily subordinated to the general plan of construction of the animals in which they occur. Thus, in the Radiates it is circular; in the Articulates, linear, or upon an axis. But, as the conditions of life require consentaneousness of motion in the different parts, these nerve arcs are not left isolated or without connexion with each other. As it is anatomically termed, they are commissured, nerve fibres passing from each to its neighbours, and each is thus brought into sympathy or connexion with all the others.