An impression of sound—a musical note, for example, is carried to the brain; the result is a responsive action and excitement extending to the voice, mouth, eyes, head, &c. This multiplex and various manifestation implies a system of connexion among the centres of action, whereby many strings can be touched from one point; a connexion due to the conducting nerves that pass and repass from centre to centre, and from the centres to the muscular apparatus over the body. Supposing the corpora quadrigemina to be a centre for the sense of vision, an impression passing to this centre propagates a movement towards many other centres,to the convoluted hemispheres upwards, to the cerebellum behind, and to the medulla oblongata and spinal cord beneath; and through these various connexions an extensive wave of effects may be produced, ending in a complicated chain of movements all over the framework of the body. Such a system of intercommunication and transmission of power is therefore an essential part of the bodily and mental structure.

23. The experiments of Du Bois Reymond, show that there is a community of nature between the nerve force and common electricity. Electric currents are constantly maintained in the nerves and muscles, their character being changed during sensation and muscular contraction. The direction of these currents has been minutely examined by Du Bois Reymond, and he lays down a number of general principles regarding them. The following are some of his conclusions:

"The muscles and nerves, including the brain and spinal cord, are endowed during life with an electro-motive power.'

'This electro-motive power acts according to a definite law, which is the same in the nerves and in the muscles, the law of the antagonism of the longitudinal and the transverse sections. The longitudinal surface is positive, and the transverse section negative.'

'Every minute particle of the nerves and the muscles must be supposed to act according to the same law as the whole nerve or muscle.' The total currents are, in fact, the combined effect of these currents circulating round the ultimate particles.

'The current in muscles when in the act of contraction, and in nerves when conveying motion, or sensation, undergoes a sudden and great negative variation of its intensity.' 'It has not been ascertained whether, in the act of contraction, the muscular current is only diminished, or wholly vanishes, or whether it changes its direction.'

Thus the proper nerve force-that is to say, the currents in the nerves during sensation and movement—is so far in unison with electricity, that it neutralizes and reverses genuine electrical currents proved to exist in the nerves and muscles in their condition of rest. This is the utmost that can be said in the present state of our knowledge. Even granting that the force conveyed along the nerves during the mental processes were identical with voltaic electricity, the character of the nerve substance would create some points of contrast between the phenomena of vital action and a common voltaic battery. The conducting power of nerve fibre is attended with nervous waste, and the substance has to be constantly renewed from the blood, which is largely supplied to the nerves, although perhaps not so largely as to the vesicles.

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, 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 nerve force is propagated more slowly than an electric current through a wire. The rate has been estimated at about 200 feet a second on an average. (It is to be remarked that a nerve is not a simple conductor, but is supposed to consist of a countless number of molecules, each of which has, playing round it, an electrical current, or currents, which are an obstacle to the simple or direct propagation.) There is always a certain delay in passing through the nerve centres; a reflex movement occupies from 3 to 10 of a second under favourable circumstances, which is more time than would be required for transmitting an influence through the same length of nerve without interruption. When the stimulus is weak, a proportionally longer time is required to produce the corresponding movement. We may suppose that what is called nervous excitement is a quicker rate of the nervous current.

24. It is now an admitted doctrine 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 converted—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 a peculiar force, but a collocation of the forces of inorganic matter in such a way as to keep up a living structure. If our means of observation and measurement were perfect, we might render an 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 make up 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 form 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; at other points it is converted into sensible heat; while 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.

Both

25. The current character of the nerve force, leads to a considerable departure from the ancient 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. 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 that originate or receive the nerve stimulation. The organ of mind is not the brain by itself: it is the brain, nerves, muscles, organs of sense and viscera. 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 circle of mechanism.

The notion that the brain is a sensorium, or inner chamber, where impressions are accumulated, like pictures put away in a store, requires to be modified and corrected. The brain is highly retentive of the impressions made upon it; they are embodied in its structure, and are a part of its growth. They may be reproduced on after occasions, and then what we find is a series of currents and counter currents,

SUPPLY OF BLOOD TO THE NERVES.

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much the same as what existed when the impression was first made. When the mind is in the exercise of its functions, the physical accompaniment is the passing and re-passing of innumerable streams of nervous influence. Whether under a sensation of something actual, or under an emotion or an idea, or a train of ideas, the general operation is still the same. It seems as if we might say, no currents, no mind. The transmission of influence along the nerve fibres from place to place, seems the very essence of cerebral action. This transmission, moreover, must not be confined within the limits of the brain not only could no movements be kept up and no sensation received by the brain alone, but it is uncertain how far 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.

The more immediate source of nervous power is an abundant supply of blood. The arrest of the circulation in the brain, by stoppage of the heart, or by pressure on the head, is followed by loss of consciousness. On the other hand, excessive rapidity of the circulation quickens the thoughts and feelings, in other words, is productive of excitement, which may amount even to delirium. Again, as regards the quality of the blood, excess of carbonic acid, of urea, or of the other impurities removed by the excreting organs, depresses or destroys the mental function; the same effect arising from deficiency of nutritive material. And, obversely, abundance of nourishment, the full exercise of the purifying organs, and the presence of the agents known as stimulants, by affecting the quality of the blood, impart exhilaration and vigour to the mental functions.