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345

CHAPTER X

PAGE

THE PERIPHERAL NERVOUS SYSTEM.

345 Classification and terminology-Minute structure. 351 Chemical composition.

352 Section and excitation-Law of specific nervous energies

conduction.'

NERVE

Law of isolated

354 Consequences of nerve-section: (1) Paralysis-(2) Loss of excitability-(3) Degeneration--(4) Regeneration-(5) Restoration of function and of excitability.

354 Degeneration: of mixed nerve-of anterior root of posterior root'Trophic centres '-Immediate restoration (?)-Recurrent sensory fibres. *Trophic nerves (?).

357

362

359 Stimuli: Evidence of stimulation-Movement-Sensation- The current of action-Natural and artificial stimuli-Excitability and conductivity. *The constant current: Make and break contractions-Pflüger's law' of contractions-Ritter's tetanus-Formula of contractions on man-Reaction of degeneration.

368

* Electrotonus: Electrotonic currents-Electrotonic alterations of excitability The paradoxical contraction-Experiments on man- Experiments on sensory nerves.

375 Velocity of nervous impulses: In motor nerves of man-of frog-In sensory nerves (?)—In sympathetic nerves (?)—Velocity of negative variation. 377 Conduction in both directions-Paul Bert's experiment-Du Bois-Reymond's experiment-Kühne's experiment.

378 Inequalities of excitability-At different points-Of different fibres. Avalanche theory - The Ritter-Valli 'law'-In motor nerves-in sensory

nerves.

380 Fatigue of nerve-of muscle of the motor end-plate-Inhibitory stimulation.

Classification and terminology.-Nerves and nerve-fibres are classified from several points of view-(a) according to their obvious source and course, as cerebro-spinal and sympathetic; (b) according to their microscopical structure, as medullated and nonmedullated; (c) according to their embryonic origin and distribution, as somatic and splanchnic ; (d) according to their function, as efferent and afferent. And we shall learn shortly that these divisions are subdivided into several varieties according to the kinds of impulses conveyed. As regards the correspondence of the various kinds denoted by these terms, it is to be remarked,

1

(1) that cerebro-spinal nerves are in major part composed of medullated fibres, in minor part of non-medullated fibres, while sympathetic nerves are in major part composed of non-medullated fibres, in minor part of medullated fibres; (2) that both medullated and non-medullated fibres may be either afferent or efferent, there being here no correspondence between difference of structure and difference of function. The interchange of fibres between the two classes of nerves is by the rami communicantes, through which the cerebro-spinal nerves receive grey fibres from the sympathetic system and give to that system white fibres. But, as we have already seen, no hard-and-fast line of distinction is to be drawn between the two systems. Sympathetic nerve-fibres have their centres in the spinal axis, and not in their own ganglia.

A certain amount of correspondence is presumed to exist between the size of fibre and the nature of its function. As a rule, the largest nerve-fibres (15 to 20 μ) are those distributed as motor fibres of skeletal muscle; fibres in the posterior nerveroots and in the posterior columns of the cord are on the average of inferior calibre to fibres in the anterior roots and in the pyramidal tracts. Another difference has already been alluded to under the physiological anatomy of cardiac and vasomotor nerves, where we saw that these are characterised by their very small diameter (2 to 4 μ), and stated that in their course along the peripheral nerves vaso-inhibitory fibres are medullated, while vaso-augmentor fibres are non-medullated.

Medullated or white nerve-fibres owe their name to the medulla or myelin, which is a fatty sheath surrounding the cylinder-axis. From within outwards a fibre of this kind consists of-(1) axiscylinder; (2) medullary sheath or white substance of Schwann; (3) primitive sheath or neurilemma.

The axis-cylinder is the essential or conducting part of the fibre; it is to be regarded as a continuous filament of protoplasm formed (according to Engelmann) by long segments very perfectly joined end to end. The medullary sheath is a string of hollow cylinders of a fatty nature surrounding the axis-cylinder and joined end to end by cement substance at nodes (Ranvier); this is an indication that each internode constitutes a single cell, a view which is borne out by the fact that each such internode possesses one, and only one, nucleus. The neurilemma is to the nerve-fibre what the sarcolemma is to the muscle-fibre, viz. a delicate membranous sheath enclosing its substance. Outside this a second membranous sheath is commonly visible, enclosing solitary

nerve-fibres; this is Henle's sheath, and is not an integral part of the fibre, but a prolongation derived from the connective tissue (perineurium), by which nerve-fibres are held together in bundles.

A 'non-medullated,' 'pale,' or 'grey' fibre is nerve reduced to its simplest expression-a naked axis-cylinder. It is to be regarded as being less highly deve

loped than medullated nerve, for it exists in greatest abundance in visceral nerves, in association with non-striped muscle, and histologieally, in addition to its simpler structure, it is characterised by the presence of numerous nuclei.

Motor and efferent-sensory and afferent are not completely synonymous pairs of terms. All motor nerves are efferent, but all efferent nerves are not motor; some are anti-motor or 'inhibitory' of motion. All sensory nerves are afferent, but all afferent nerves are not sensory' in the exact acceptation of the term 'sensation,'' sensory' being strictly applicable, not to all centripetal impressions, but only to such as excite

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FIG. 155.

Transverse

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lated

fibre.

nerve

FIG. 156.

Longitudinal optical section of

a group of nonmedullated fibres.

consciousness. Many, if not all, afferent channels may, however, on some occasions convey impulses that reach consciousness, while habitually conveying impulses of which we remain unconscious. Normally, we are not conscious of the beating of the heart, nor of digestive actions; in disease we may become painfully conscious of them. On the other hand, all, or at least most, sensory channels may on some occasions convey impulses of which we remain unconscious, though habitually we perceive them. A man may fail to perceive that he has received a serious injury if his perceptive centre is preoccupied by a sufficiently powerful idea or sensation. As regards the distribution of these several kinds of nerves, cerebro-spinal nerves are distributed for the most part to the skin and to voluntary muscles, skin-nerves being mostly sensory in function, muscle-nerves being mostly motor in function. The skin, however, contains glands and a small proportion of unstriped muscle, which receive motor nerve-fibres derived from the

sympathetic, causing contraction or dilatation of its vessels, erection of its hairs, secretion of its glands, shrinking of its pigment-cells; and voluntary muscles, or more precisely their tendons and sheaths, possess also a small proportion of sensory nerves, through which the sensorium is kept informed of their state. Motor nerve-fibres are also distributed to internal organs; the impulses that they convey are, however, independent of the will; by voluntary effort we cannot directly move the involuntary muscle of such organs.

Nerve-fibres may be classified as follows:

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INHIBITORY

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to cardiac muscle (accele

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to arterial muscle (constric

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to muscle of hair

broncho-motor to bronchial muscle

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viscero-motor. to intestinal muscle

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secreto-motor. to gland cells

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? thermogenic ')

and trophic .)

(vaso-inhibitory

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to cardiac muscle (inhibiting)

to arterial muscle (dilating)

(viscero-inhibitory to intestinal muscle

The most prominent manifestation of function subserved by efferent nerves is voluntary muscular movement; there is as yet no proof of direct voluntary inhibition at the periphery; voluntary inhibition of a peripheral action occurs in the centre, whence the action would have been excited in the absence of inhibitory control. Of involuntary motor paths we have proof in the action of nerves upon arterial muscle and upon secretory glands; both these are entirely exempt from direct voluntary influence; all that we can voluntarily effect is to apply a peripheral stimulus that will influence them by a reflex mechanism. Vaso-constriction is a motor effect ; vaso-dilatation is of an inhibitory nature; we do not know, however, whether the seat of inhibition is in the muscular element itself, or in some hypothetical peripheral centre contained in it. It is analogous with the better-known case of

cardiac inhibition by the vagus, but in this case also we do not know whether the motor action is interfered with in the motor element itself, or in an intermediate nerve-element-the ganglion cell. Closely analogous with vasomotor and vaso-inhibitory nerves, but still less completely understood, and, indeed, less well certified by experiment, are the effects upon intestinal movements of impulses passing to them through efferent channels. Intestinal movements may through such channels be excited or arrested ; usually, the result of ragus excitation is increased movement, of splanchnic excitation diminished movement; but such results are not infallibly obtained, being dependent upon a variety of circumstances (see p. 168).

Secreto-motor fibres are subdivided into two varieties: 1, such as accelerate the discharge of water; 2, such as accelerate the discharge of the protoplasm-product, which is to form the essential constituent of the secretion. These latter secreto-motor fibres are distinguished from the former as trophic,' a term which is not free from objection, seeing that such fibres have not been proved to excite cell-nutrition, though they certainly effect the converse, viz. cell-disintegration (see p. 182).

Does muscle possess inhibitory nerves? A muscle can at will be relaxed as well as contracted, and it may and has been asked whether we must attribute the relaxation to an action of arrest in the brain, or to an inhibitory effect in the muscle by special anti-motor nerve-fibres. We shall more conveniently discuss. this second alternative at a later stage (p. 381), in considering the relaxant effects upon muscle of experimental stimulation of its nerve. Meanwhile, as regards voluntary arrest of action, it will be sufficient to repeat that there is as yet no proof of direct voluntary inhibition at the periphery; but that voluntary inhibition of a peripheral action occurs at the centre (cortical or bulbar) whence the action would have been excited in the absence of inhibitory interference.

Afferent nerves are naturally classified in accordance with their obvious functions as olfactory, optic, acoustic, tactile, and common sensory. There are less obvious and less satisfactory reasons for admitting a further classification of nerve-fibres into thermic and pathic, for the proof that impressions of heat, of cold, and of pain are specific and served by separate fibres, distinct from common sensory fibres, is not completely clear and unassailable. The term 'sensory' is in common use, and cannot be advantageously ignored; it is well, therefore, to recognise clearly

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