a nerve globule to an element of another variety, or simply to another nerve globule.

These nerve fibres seem to be only a physiological supplement to the globule from which they originate; every excitation of the fibre is retained by the globule, and vice versû the fibre disconnected from its globule undergoes a degeneration (fatty) more or less complete.

2. Life of the Nervous System.This physiological whole (globule and its prolongations) lives and is nourished: the nerve centres, composed practically of globules, needs an enormous quantity of material, and gives back to the surrounding media (by means of the blood) a large quantity of refuse matter. The mass of nerve fibres (nerves) consumes likewise some materials, and produces refuse matter; they in other words are fed; they are very vascular, and when the supply of blood is shut off, phenomena resembling decomposition may be observed.

It will be noticed, farther on, that the materials consumed by the muscles during their activity are principally hydro-carbons (sugars and fats) and also albuminoids in small quantity. On the other hand, the nerve element seems to require albuminoid substances; and the more intense is the nerve work, the greater will be the amount of refuse material, from the combostion of the albuminoids (especially urea), in the excretions, in the urine, and in the products of the liver. According to Biasson (1868) the amount of urea excreted by man varies according to the amount of cerebral activity. Again, Oscar Liebreich has shown that, in animals who have been made to die by pain, after cutting the sensitive roots of one side of the spinal cord, this side (reduced to inertia) would consume less protagon than the other side. Protagon, whose composition is not yet defined, seems to be a compound of fatty phosphates and neurine, and serves for the nutrition of the nervous system, to which it is carried by the blood globule. According to Austin Flint, Jr., the excrementitial product formed by the disassimilation of the brain and of the nerves, at the expense of protagon, is represented by cholesterine, separated from the blood by means of the liver, and then thrown into the intestinal canal. This view is based upon a number of experiments, which show, moreover, that the excretion of cholesterine is in direct ratio with the nervous activity. The common expression, "to feel bilious," seems justified by one of the elements of the bile, viz., cholesterine.

These acts of nutrition produce in the nerves a disengagement of forces, which are brought to light by electrical currents; this phenomenon, though not directly observed in the nerve globules, is very evident in the peripheral nerves.

In the state of rest certain currents are constantly traversing nerves, going from the surface to the interior, and acting as if the nerve fibres were the seat of two enclosed elements, the extremity being positive and the centre negative. In fact, whenever by means of a galvanometer, a communication is made between the external surface and the surface of the section of a nerve, a current is observed to pass from the periphery towards the centre. This electrical phenomenon, called the electro-motory force of the nerve, disappears or becomes feeble whenever the fibre is subjected to an irritation, or whenever it acts as a conductor, or in fact whenever it performs its proper function; a disappearance of the electro-motor power is called negative oscillation. It has been surmised that at this moment nutrition is arrested, and with this ensues the normal current of a state of rest. The deduction can easily be drawn in what way the fatigue of the nerve may be brought about, and why an irritation too long maintained may cause destruction, which latter may also be accompanied with pain.

But, on the other hand, direct experiment shows that the nerve in functional activity does more,- there is produced a development of heat, the existence of which Schiff has just demonstrated in the nerve-centres, influenced by fear, or excitement of the senses, or from every cause which may produce cerebral activity. It may be that the negative oscillation indicates that electricity of the nerve in a state of repose is transformed into heat in the active state. (In regard to this see farther on an analogue of the negative oscillation, in the study of the muscles, and also the transformation of one force into another force.)

3. Action of the Nervous System. What constitutes the special function of the nerve apparatus, both fibre and cell?

B

This consists essentially in a phenomenon called reflex. When a nerve fibre is irritated, this irritation is transmitted to globules more or less distant, and from the latter to the peripheral parts. Most generally this irritation is upon a tactile corpuscle or

Fig. 10.

Diagram of a simple reflex action.*

*1, Surface (epithelium). 2, Muscle. A, Centripetal fibre. B, Central nerve cell. C, Centrifugal fibre. A, B, C, form the nerve arc, which presides over the reflex action: the diastaltic arc of Marshall Hall. A represents the eisodic fibre ; B, the central excito-motor; and C, the exodic fibre.

some analogous organ (adjuncts of the peripheral surfaces); it is transmitted by a centripetal fibre to a central globule, which reflects this by a centrifugal fibre to another organ more or less peripheral, as, for instance, a muscle whose contraction may be thus effected, or to a gland which then pours out its secretion.

Thus fibres perform their function of carrying the excitation towards a globule, or of transmitting it from the globule to the periphery; hence the names centripetal or sensitive given to the former nerves, and centrifugal or motory to the latter. This name should indicate merely that this is the sense in which the function of the fibre is manifested to us, but no essential difference between centripetal and centrifugal filaments are intended, as we shall soon see that direct experiments demonstrate the contrary.

The office of the globule is to favor the transmission of the excitation from one to another fibre; oftentimes, indeed, the first globule reflects its action, by commissural fibres, upon one or several other globules which can turn the action in a different direction again, either directly upon a centrif ugal fibre, properly so called, or upon some fresh nerve globules: the globular elements can even absorb or enfeeble the action, or even store it up, as it were, in a latent state, and send it off only at another time, when influenced by new excitations. Hence we see that reflex centres present very complicated phenomena, becoming at one time centres of diffusion, and again of co-ordination of movements, of memory, etc.; these centres can be also the seat of sensation for the peripheral excitations.

Leaving out of mind the central phenomena that are difficult of analysis, we see that the office of the nerves is essentially that of conduction. Now what constitutes conduction, and what is the peculiar phenomenon by which it is characterized? For a long time this was supposed to resemble and partake of the nature of the electric current; but at the present time it is proved that the nerve influx has nothing to do with electricity. In the first place its rapidity of propagation has been calculated to be 28 to 30 metres to the second, a very different rate from that of the electric current, and even this varies with the temperature of the nerve; according to Helmholtz, in frog's nerve cooled to a temperature of the freezing point of water (0° c.), the rapidity of the nerve agent is but one-tenth of what it is at 15° or 20° higher. Again, when the nerve performs its functions, in

stead of producing electricity, there is, on the contrary, negative oscillation (as has been before remarked), that is a weakness or disappearance of the normal current of repose.

In a nerve displaying activity, there appears to be a sort of molecular vibration which is propagated from point to point at the rate of 28 to 30 metres to the second. This molecular vibration extends both ways along the nerve; when the stimulus is applied midway, its existence is evident only at the nervous extremity, where an organ suitable for its reception may be found; as, for instance, towards the central end for sensitive nerves, and to the surface or periphery for the motor nerves. Thus it may be noticed that the terms centripetal and centrifugal depend upon the dif ferent connections, and that both can conduct, indifferently, either way (Vulpian).

4. Excitants of the Nervous System. Those excitants which can set in motion the functions of the nerves are numerous. Some of these are chemical, such as acids, ammonia, &c.; these agents, it will be seen, excite likewise the muscles, but in this case they need not be so concentrated as in the former. Others may be in the nature of mechanical or physical excitants; as, for instance, a blow, electricity or heat. Electricity seems to excite the nerves only by the sudden changes it produces in their molecular condition; thus a current applied to a nerve affects its action, only when it begins or terminates its passage through the nerve; during its passage no action is evident. order to excite nerves, sudden electrical discharges must be applied, and this is the reason for the employment of an induced current, frequently interrupted. At each interruption, there ensues an excitation of the nerve. In normal physiological conditions, the external excitors are brought to bear upon the ends of the so-called sensitive nerves; certain of the peripheral organs of this class (organs of special sense) exist where particular agents (light, sound, heat, odors, &c.), give rise to special excitations.

In

Finally, the central organs act as physiological excitants in the reflex order, where they only transmit previously received excitations, and in the phenomena called voluntary (which are doubtless a more or less complex form of reflex actions). This is due to the power which the nerve globules possess of storing up certain excitations (memory), whose manifestations they allow only at a given time. We may perhaps

suppose that the central nerve globules, by the simple effect of their nutrition, and without excitation coming from outside the body, are capable of setting free forces which act upon the fibres; this property has been called automatism of the nerve centres (will, muscular tone?)

5. Excitability of the Nerve Elements.-The excitability of the nerve elements, especially if a nerve used for experimental researches, may vary under many circumstances. Heat increases this up to a certain point: cold diminishes it. Certain medicinal agents, as, for instance, strychnine, have the power of exciting the reflex properties of the nervous centres; others, like the bromide of potassium, enfeeble these properties. Woorara (curare), on the other hand, seems to act especially upon the motory terminations of the nerves, and there to arrest the power of transmission, for it is hardly reasonable to suppose that it would act upon the motory nerves, and not upon the sensory nerves; this would show that these two kinds of nerves have no different characters.

Electricity acts at the same time both as an excitant and as a modifying agent of excitability to a nerve; in fact, when a current is applied to a nerve, excitability is increased at the negative pole, and diminished at the positive pole, a phenomenon more especially described under the head of electro-tonus.

But the excitability of a nerve is especially dependent on its nutrition. Every nerve tube separated from a central living organ undergoes fatty degeneration and ceases to be excitable at the end of a few days. Absolute rest produces the same effect, for the function is necessary to the maintenance of life and of nutrition; per contra, the exaggerated excitations produce momentarily the weakening of a nerve, which must needs recover its strength by rest, and we have remarked that excitation of the nerve modifies temporarily the phenomena of nutrition.

II. GENERAL PHYSIOLOGY OF THE NERVE CENTRES.

For a long time the point of departure of the nervous system was a matter of ignorance: the size and position of the brain led the ancient physiologists to consider that as the principal centre of the nerve-substance: the spinal cord was to them but a collection of nerves ending at the brain. The minute study (histology) of the gray axis of the spinal cord