SOURCE OF NERVOUS POWER. 59 blood vessels through their whole length, although less so than the vesicular matter of the centres. Moreover, in experiments upon the irritability of nerves, it is shown that they lose this quality entirely after being separated from the means of nourishment for a certain length of time—a time not sufficient to produce a radical alteration in the structure of the nerve. If now we compare this liability to waste and exhaustion with the undying endurance of an electric wire, we shall be struck with a very great contrast. The wire is doubtless a more compact, resisting, and sluggish mass; the conduction requires a certain energy of electric action to set it agoing, and in the course of a great distance becomes faint and dies away. The nerve, on the other hand, is stimulated by a slighter influence, and propagates that influence, it may be, with increase, by the consumption of its own material. The wire must be acted on at both ends, by the closure of the circuit, before acting as a conductor in any degree; the nerve takes fire from a slight stimulus like a train of gunpowder, and is wasted by the current that it propagates. If this view be correct the influence conveyed is much more beholden to the conducting fibres than electricity is to the copper wire. The fibres are made to sustain or increase the force at the cost of their own substance. 'The proofs,' say Messrs. Todd and Bowman, 'of the passage of an electric current through the nerve fibres during nervous action must be held to be altogether defective. Not only is experimental evidence wanting to support the electrical theory, but certain facts are admitted which greatly invalidate it.'-Vol. I. p. 233. 24. It is nevertheless manifest that the nervous power is generated from the action of the nutriment supplied to the body, and is therefore of the class of forces having a common origin, and capable of being mutually transmitted,—including mechanical momentum, heat, electricity, magnetism, and chemical decomposition. The power that animates the human frame and keeps alive the currents of the brain, has its origin in the grand primal source of reviving power, the Sun; his influence exerted on vegetation builds up the structures whose destruction and decay within the animal system give forth all the energy concerned in maintaining the animal processes. What is called vitality is not so much a peculiar force as a collocation of the forces of inorganic matter for the purpose of keeping up a living structure. If our means of observation and measurement were more perfect, we might render account of all the nutriment consumed in any animal or human being; we might calculate the entire amount of energy evolved in the changes that constitute this consumption, and allow one portion for animal heat, another for the processes of secretion, a third for the action of the heart, lungs, and intestines, a fourth for the muscular exertion made within the period, a fifth for the activity of the brain, and so on till we had a strict balancing of receipt and expenditure. The nerve force that is derived from the waste of a given amount of food, is capable of being transmuted into any other force of animal life. Poured into the muscles during violent conscious effort, it increases their activity; passing to the alimentary canal, it aids in the force of digestion; in moments of excitement the power is converted into sensible heat; the same power is found capable of yielding true electrical currents. The evidence that establishes the common basis of mechanical and chemical force, heat, and electricity, namely, their mutual convertibility and common origin, establishes the nerve force as a member of the same group. 25. The current character of the nerve force leads to a considerable departure from the common mode of viewing the position of the brain as the organ of mind. We have seen that the cerebrum is a mixed mass of grey and white matter, -the matter of centres and the matter of conduction. Both are required in any act of the brain known to us. The smallest cerebral operation includes the transmission of an influence from one centre to another centre, from a centre to an extremity, or the reverse. Hence we cannot separate the centres from their communicating branches; and if so, we cannot separate the centres from the other organs of the body THE BRAIN NOT A SENSORIUM. 61 that originate or receive nerve stimulus. The organ of mind is not the brain by itself; it is the brain, nerves, muscles, and organs of sense. When the brain is in action, there is some transmission of nerve power, and the organ that receives or that originated the power is an essential part of the mechanism. A brain bereft of the spinal cord and spinal nerves is dead though the blood continues to flow to it; and these nerves, if plucked out of the limbs and other parts where they terminate, would probably not suffice to sustain the currents associated with mental life. It is, therefore, in the present state of our knowledge, an entire misconception to talk of a sensorium within the brain, a sanctum sanctorum, or inner chamber, where impressions are poured in and stored up to be reproduced in a future day. There is no such chamber, no such mode of reception of outward influence. A stimulus or sensation acting on the brain exhausts itself in the production of a number of transmitted currents or influences; while the stimulus is alive, these continue, and when these have ceased the impression is exhausted. The revival of the impression is the setting on of the currents anew; such currents show themselves in actuating the bodily members, the voice, the eyes, the features,-in productive action, or in mere expression and gesture. The currents may have all degrees of intensity, from the fury of a death struggle to the languor of a half-sleeping reverie, or the fitful flashes of a dream, but their nature is still the same. We must thus discard for ever the notion of the sensorium commune, the cerebral closet, as a central seat of mind, or receptacle of sensation and imagery. We may be very far from comprehending the full and exact character of nerve force, but the knowledge we have gained is sufficient to destroy the hypothesis that has until lately prevailed as to the material processes of perception. Though we have not attained a final understanding of this obscure and complicated machinery, we can at least substitute a more exact view for a less; and such is the substitution now demanded of current action for the crude conception of a central receptacle of stored up impressions. Our present insight enables us to say with great The transmission of inplace to place, seems the This transmission, moreover, probability, no currents, no mind. fluence along the nerve fibres from very essence of cerebral action. must not be confined within the limits of the brain: not only could no action be kept up and no sensation received by the brain alone, but it is doubtful if even thought, reminiscence, or the emotions of the past and absent, could be sustained without the more distant communications between the brain and the rest of the body-the organs of sense and of movement. It is true that between the separate convolutions of the brain, between one hemisphere and another, between the convoluted hemispheres and the corpora striata, thalami optici, corpora quadrigemina, cerebellum, medulla oblongata, and spinal cord, influence might be imagined to pass and repass without flowing into the active extremities or to the five senses, and might thus constitute an isolated cerebral life; but it is in the highest degree improbable that such isolation does or can exist. Nervous influence, rising in great part in sensation, comes at last to action; short of this nothing is done, no end served. However feeble the currents may be, their natural course is towards the organs accustomed to their sway. Hence the reason for adopting language, as we have done throughout the present chapter, to imply that the brain is only a part of the machinery of mind; for although a large part of all the circles of mental action lie within the head, other parts equally indispensable extend throughout the body. |