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5. A transverse section of the cord (Fig. 81, B, and Fig. 82) shows that each half contains two substances a white substance on the outside, and a greyish red substance in

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FIG. 82.-TRANSVERSE SECTION OF ONE-HALF OF THE SPINAL CORD (IN THE LUMBAR REGION), MAGNIFIED.

1, anterior fissure; 2, posterior fissure; 3, central canal; 4 and 5, bridges connecting the two halves (posterior and anterior commissures); 6, posterior column; 7, lateral column; 8, anterior column; 9, posterior root; 10, anterior root of nerve.

a, a, posterior horn of grey matter; e, e, e, anterior horn of grey matter. Through the several columns 6, 7, and 8, each composed of white matter, are seen the prolongations of the pia mater, which carry blood-vessels into the cord from the outside. The pia mater itself is seen over the whole of the cord.

the interior. And this grey matter, as it is called, is so disposed that, in a transverse section, it looks something like a crescent, with one end bigger than the other, and with the concave side turned outwards. The two ends of the crescents are called its horns or cornua (Fig. 82, e e), the one directed forwards being the anterior cornu; the one turned backwards the posterior cornu (Fig. 82, a a). The convex sides of the cornua of the grey matter approach one another, and are joined by the bridge which contains the central canal.

There is a fundamental difference in structure between the grey and the white matter. The white matter consists entirely of nerve-fibres supported in a delicate framework of connective tissue, and accompanied by blood-vessels. Most of these fibres run lengthways in the cord, and consequently, in a transverse section, the white matter is really composed of a multitude of the cut ends of these fibres.

The grey matter, on the other hand, contains in addition, a number of nerve-cells or ganglionic corpuscles, some of them of considerable size. These cells are wholly absent in the white matter.

Many of the nerve-fibres of which the anterior roots are composed may be traced into the anterior cornu, while those of the posterior roots enter the posterior cornu.

6. The physiological properties of the organs now described are very remarkable.

If the trunk of a spinal nerve be irritated in any way, as by pinching, cutting, galvanizing, or applying a hot body, two things happen: in the first place, all the muscles to which filaments of this nerve are distributed, contract; in the second, acute pain is felt, and the pain is referred to that part of the skin to which fibres of the nerve are distributed. In other words, the effect of irritating the trunk of a nerve is the same as that of irritating its component fibres at their terminations.

The effects just described will follow upon irritation of part of the branches of the nerve: except that when a branch is irritated, the only muscles directly affected, and the only region of the skin to which pain is referred, will be those to which that branch sends nerve-fibres. And these effects will follow upon irritation of any part of a nerve from its smallest branches up to the point of it

trunk, at which the anterior and posterior bundles of root fibres unite.

7. If the anterior bundle of root fibres be irritated in the same way, only half the previous effects are brought about. That is to say, all the muscles to which the nerve is distributed contract, but no pain is felt.

So again if the posterior, ganglionated bundle be irritated, only half the effects of irritating the whole trunk is produced. But it is the other half; that is to say, none of the muscles to which the nerve is distributed contract, but intense pain is referred to the whole area of skin to which the fibres of the nerve are distributed.

8. It is clear enough, from these experiments, that all the power of causing muscular contraction which a spinal nerve possesses, is lodged in the fibres which compose its anterior roots; and all the power of giving rise to sensation, in those of its posterior roots. Hence the anterior roots are commonly called motor, and the posterior sensory.

The same truth may be illustrated in other ways. Thus, if, in a living animal, the anterior roots of a spinal nerve be cut, the animal loses all control over the muscles to which that nerve is distributed, though the sensibility of the region of the skin supplied by the nerve is perfect. If the posterior roots be cut, sensation is lost, and voluntary movement remains. But if both roots be cut, neither voluntary movement nor sensibility is any longer possessed by the part supplied by the nerve. The muscles are said to be paralysed, and the skin may be cut, or burnt, without any sensation being excited.

If, when both roots are cut, that end of the motor root which remains connected with the trunk of the nerve be irritated, the muscles contract; while, if the other end be so treated, no apparent effect results. On the other hand, if the end of the sensory root connected with the trunk of the nerve be irritated, no apparent effect is produced, while, if the end connected with the cord be thus served, violent pain immediately follows.

When no apparent effect follows upon the irritation of any nerve, it is not probable that the molecules of the nerve remain unchanged. On the contrary, it would appear that the same change occurs in all cases ; but a

motor nerve is connected with nothing that can make that change apparent save a muscle: and a sensory nerve with nothing that can show an effect but the central nervous system.

9. It will be observed that in all the experiments mentioned there is evidence that, when a nerve is irritated, a something, probably a change in the arrangement of its molecules, is propagated along the nerve-fibres. If a motor or a sensory nerve be irritated at any point, contraction in the muscle, or sensation in the central organ, immediately follows. But if the nerve be cut, or even tightly tied at any point between the part irritated and the muscle or central organ, the effect at once ceases, just as cutting a telegraph wire stops the transmission of the electric current or impulse. When a limb, as we say, goes to sleep," it is because the nerves supplying it have been subjected to pressure sufficient to destroy the nervous continuity of the fibres. We lose voluntary control over, and sensation in, the limb, and these powers are only gradually restored as that nervous continuity returns.

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Having arrived at this notion of an impulse travelling along a nerve, we readily pass to the conception of a sensory nerve as a nerve which, when active, brings an impulse to the central organ, or is afferent; and of a motor nerve, as a nerve which carries away an impulse from the organ, or is efferent. It is very convenient to use these terms to denote the two great classes of nerves; for, as we shall find (§ 12), there are afferent nerves which are not sensory in the sense of giving rise to a change of consciousness, or sensation, while there are efferent nerves which are not motor, in the sense of inducing muscular contraction. Such, for example, are the nerves by which the electrical fishes give rise to discharges of electricity from peculiar organs to which those nerves are distributed. The pneumogastric when it stops the beat of the heart cannot be called a motor, and yet is then acting as an efferent nerve.

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Their "nervous continuity"-because their physical continuity is not interrupted as a whole, but only that of the substance which acts as a conductor of the nervous influence; or, it may be that only the conducting power of a part of that substance is interfered with. Imagine a telegraph cable, made of delicate caoutchouc tubes, filled with mercury-a squeeze would interrupt the "electrical continuity" of the cable, without destroying its physical continuity. This analogy may not be exact, but it helps to make the nervous phenomena intelligible.

It will, of course, be understood, as pointed out above, that the use of these words does not imply that when a nerve is irritated in the middle of its length, the impulses set up by that irritation travel only away from the central organ if the nerve be efferent, and towards if it be afferent. On the contrary, we have evidence that in both cases the impulses travel both ways. All that is meant is this, that the afferent nerve from the disposition of its two ends, in the skin, &c. and in the central organ, is of use only when impulses are travelling along it towards the central organ, and similarly the efferent nerve is of use only when impulses are travelling along it, away from the central organ.

10. There is no difference in structure, in chemical or in physical character, between afferent and efferent nerves. The impulse which travels along them requires a certain time for its propagation, and is vastly slower than many other forces-even slower than sound.

11. Up to this point our experiments have been confined to the nerves. We may now test the properties of the spinal cord in a similar way. If the cord be cut across (say in the middle of the back), the legs and all the parts supplied by nerves which come off below the section, will be insensible, and no effort of the will can make them move; while all the parts above the section will retain their ordinary powers.

When a man hurts his back by an accident, the cord is not unfrequently so damaged as to be virtually cut in two, and then paralysis and insensibility of the lower part of the body ensue.

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If, when the cord is cut across in an animal, the cut end of the portion below the division, or away from the brain, be irritated, violent movements of all the muscles supplied by nerves given off from the lower part of the cord take place, but there is no sensation. On the other hand, if that part of the cord, which is still connected with the brain, or better, if any afferent nerve connected with that part of the cord be irritated, great pain ensues, as is shown by the movements of the animal, but in these movements the muscles supplied by nerves coming from the spinal cord below the cut take no part; they remain perfectly quiet.

12. Thus, it may be said that, in relation to the brain

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