"Whatever may be said by speculative philosophers to the contrary, the advancement of metaphysics is through the study of physiology. What sort of a science would optics have been among men who had purposely put out their own eyes? What would have been the progress of astronomy among those who disdained to look at the heavens? Yet such is the preposterous course followed by the so-called philosophers. They have given us imposing doctrines of the nature and attributes of the mind in absolute ignorance of its material substratum. Of the great authors who have thus succeeded one another in ephemeral celebrity, how many made themselves acquainted with the structure of the human brain? Doubtless some had been so unfortunate as never to see one! Yet the interpreNecessity of that wonderful organ was the basis of all their tation of speculations. In voluntarily isolating themselves from every solid fact which might serve to be a landmark to them, they may be truly said to have sailed upon a shoreless sea from which the fog never lifts. The only fact they teach us with certainty is, that they know nothing with certainty. It is the inherent difficulty of their method that it must lead to unsubstantial results. What is not founded on a material substratum is necessarily a castle in the air."

structure.

man depend

Considering thus that scientific views of the nature of man can only be obtained from an examination Intellectual of his nervous system, and that the right inter- relations of pretation of the manner of action of that system on his nervous depends on the guiding light of comparative system. anatomy and physiology, I shall, in the following exposition, present the progress of discovery on those principles.

In those low tribes of life which show the first indica

automatic.

tions of a nervous system, its operation is purely The rudimenmechanical. An external impression, as a touch, tary nervous made upon animals of that kind, is instantly system is answered to by a motion which they execute, and this without any manifestation of will or consciousness. The phenomenon is exactly of the same kind as in a machine of which, if a given lever is touched, a motion is instantly produced.

Two elemen

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

ture.

1st. A nerve fibre consists essentially of a delicate thread Structure of a the axis filament, as it is called-enveloped nerve fibre. 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

The function of such a nerve fibre is indisputably altogether of a physical kind, being the conveyance nerve fibre is of influences from part to part. The axis conduction. 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 feetan 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.

2nd. The other portion of the nervous structure is the vesicular. As its name imports, it consists of Structure of a vesicles filled with a gray granular material. nerve vesicle. 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.

As might be inferred from its structure, the vesicle differs altogether from the fibre in function. I may Function of a refer to my "Physiology" for the reasons which nerve vesicle. 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.

Physiological

is nerve waste.

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 condition of wasted products of decay. Also, through the nerve action former, the necessary materials for repair and renovation are brought. 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 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 Reflex action fibre is attached, and thence, going forth along the centrifugal fibre, may give rise to motion vous system. through contraction of the muscle to which that fibre is distributed. 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.

of the ner

Gradual com

tem.

A repetition of these arcs, each precisely like all the others, constitutes the first step toward a complex plexity of the nervous system. Their manner of arrangement nervous sys- 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.

The next advance is a very important one, for it indicates the general plan on which the nervous system ance of spe- is to be developed: it is the dedication of cial ganglia. special nerve arcs to special duties. Thus, in

First appear

the higher articulates and molluscs, there are such combinations expressly for the purpose of respiration and deglutition. Their action is altogether of the reflex kind; it takes place without consciousness. These ganglia are commissured for the sake of sympathetic action, and frequently several of them are coalesced for the sake of package.

This principle of dedication to special uses is carried out in the introduction of ganglia intended to be affected by light, or sounds, or odours. The impressions of those agencies are carried to the ganglion by its centripetal fibres. Such ganglia of special action are most commonly coalesced together, forming nervous masses of conspicuous size; they are always commissured with those for ordinary motions, the action being reflex, as in the preceding case, though of a higher order, since it is attended with consciousness.

mechanisms.

Such being the elementary construction of a nervous system, it is plain that animal tribes in which They are it exists in no higher degree of complexity must automatic be merely automata. In this remark many insects must be included, for the instinct they display is altogether of a mechanical kind, and, so far as they are concerned, without design. Their actions are uniformly alike; what one does under given circumstances, under the same circumstances another will certainly do. They are incapable of education, they learn nothing by experience, and the acts they are engaged in they accomplish as well at the first trial as ever after.

Evidence to

gations.

Of parts like those described, and of others of a higher order, as will be presently seen, the most complex nervous system, even that of man, is composed. It might, perhaps, be expected that for the deter- be used in mination of the duty of each part of such these investicomplex system the physiologist must necessarily resort to experiment, observing what functions have been injured or destroyed when given portions have been removed by his knife. At the best, however, evidence of that kind must be very unsatisfactory on account of the shock the entire system receives in vivisections, and accordingly, artificial evidence can, for the most part, be used only in a corroborative way. But, as Cuvier