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these are developed the encephalic structures. The continuity of the walls and cavities of these vesicles is never obliterated throughout the subsequent changes. It is also traceable throughout the medulla spinalis. And microscopic investigation reveals that underneath all the morphological changes the walls of the whole cerebro-spinal axis are composed of similar elements on a similar plan.86

16. Two conclusions directly follow from this exposition:—first, that since the structure of the great axis is everywhere similar, the properties must be similar; secondly, that since there is structural continuity, no one part can be called into activity without at the same time more or less exciting that of all the rest.

THE PERIPHERAL SYSTEM.

17. Following the analytical division, we now come to the Peripheral System of nerves and ganglia. The separation, I must often repeat, is purely artificial; but the artifice has conveniences. We separate in the same way the heart from veins and arteries, and the capillary circulation from the arterial.

Each nerve has its direct connection with a particular centre, and indirectly with the whole system. It has its circumscribed territory, and individual office. Except in a few cases of anastomosis, the action of one nerve does not involve that of another: only one muscle or one group of muscles is moved, without exciting motion in a neighbor. It is through the centres that these individual territories are united; and a wave of excitation always passes throughout the central substance. Thus the centres are not simply organs of association, consequently of regulation, but are the nexus whereby the diversity of the actions is integrated into the unity of consensus.

18. Nothing further need at present be stated respecting the nerves; but it is needful to give precision to the ideas of

GANGLIA AND CENTRES,

usually spoken of as if they were convertible terms. That this is inexact may be readily shown, and that it is misleading appears in its causing physiologists to credit every ganglion, wherever found, with central functions; and, by an almost inevitable extension of the error, has led to the assignment of central functions to a single ganglionic cell! This is but part of that “superstition of the cell” against which I shall have to protest. I will not here raise the doubt which presses from various sides respecting the central functions of the ganglia in the heart and intestines, because the reader perhaps shares the general opinion on that point; but let me simply ask what central function can possibly be assigned to the ganglia on each of the spinal sensory nerves? above all to those grouped and scattered ganglionic cells which are found at the peripheral termination of some nerves, and in the very trunks of others? There may, indeed, be imagined a central function for the ganglia in the mesentery, and even in the choroid coat of the retina, on the hypothesis (quite gratuitous, I think) of their regulating the circulation; but even this explanation cannot be adopted with respect to the ganglionic cells which appear in the course of the nerve.87

The meaning of a physiological centre is, that it is a point to which stimulations proceed, and from which they are reflected. The meaning of a ganglion is, that it is a group of nerve cells dispersed among, or in continuation with, nerve fibres: it may be a centre of reflection, or it may not; and in the latter case its physiological office is at present undetermined. A ganglion is no more a centre in virtue of its cell-group than a muscle is a limb. All function depends on connection, and central function demands a connection of afferent and efferent parts.

19. The ganglia found in the ventral cord of the Invertebrate (see Fig. 1) are centres, each of which has considerable independence, each regulating a single segment of the body, or a group of similar segments. As the scale of animal complexity ascends, these separated centres tend more and more to coalesce, and with this coalescence comes an increasing combination of movements.88 Observe the caterpillar slowly crawling over a leaf; each segment of its body moves in succession; but when this caterpillar becomes a butterfly the body moves rapidly, and all at once. Open the caterpillar, and you find its nervous centres are thirteen separate ganglia, each presiding over a distinct part of the body, and each capable of independent action. Open the butterfly, and you find the thirteen ganglia greatly changed: the second and third are fused into one; the fourth, fifth, and sixth into another; the eleventh and twelfth into another; the only trace of the original separation is in a slight constriction of the surface. The movements of the caterpillar were few, simple, slow, and those of the butterfly are many, varied, and rapid.

20. In the Vertebrates the coalescence of ganglia is such that the spinal axis is one great centre. We do indeed anatomically and physiologically subdivide it into several centres, because several portions directly innervate separate organs; but its importance lies in the intimate blending of all parts, so that fluctuating combinations of its elements may arise, and varied movements result. Each centre combines various muscles; the axis is a combination of centres. The brainless frog, for instance, has still the spinal cord, and therefore the power not only of moving either of his limbs, but also of combining their separate movements: if grasped, he struggles and escapes; if pricked, he hops away. But these actions, although complex, are much less complex and varied than the actions of the normal frog.

There is not only a coalescence of ganglia, but a greater and greater concentration of the substance in the upper portions of the axis. In the inferior vertebrates, and in the mammalian embryo, the spinal cord occupies the whole length of the vertebral canal from the head to the tip of the tail; and here the centres of reflexion correspond with the several segments. But as the cranial mass develops there is a withdrawal of neural substance from the lower parts, and the centres of reflexion are then some way removed from the segments they innervate. In the animal development there is even a greater and greater predominance of the upper portions, so that the brain and medulla oblongata are of infinitely more importance than the spinal cord.

21. Besides the central group of elements which belong to fixed and definite actions, we must conceive these elements capable of variable combinations, like the pieces of colored glass in a kaleidoscope, which fall into new groups, each group having its definite though temporary form. The elements constitute really a continuous network of variable forms. It is to such combinations, and not to fixed circumscribed ganglia, that we must refer the subordinate centres of the axis. We speak of a centre for Respiration, a centre for Laughing, a centre for Crying, a centre for Coughing, and so on, with as much propriety as we speak of a centre for Swallowing or for Walking. Not that in these cases there is a circumscribed mass of central substance set apart for the innervation of the several muscles employed in these actions, and for no other purpose. Each action demands a definite group of neural elements, as each geometric form in the kaleidoscope demands a definite group of pieces of glass; but these same pieces of glass will readily enter into other combinations; and in like manner the muscles active in Respiration are also active in Laughing, Coughing, etc., though differently innervated and co-ordinated.

22. The physiological rank of a centre is therefore the expression of its power of fluctuating combination. The medulla oblongata is higher than the medulla spinalis, because of its more varied combinations; the cerebrum is higher than all, because it has no fixed and limited combinations. It is the centre of centres, and as such the supreme organ.

CHAPTER II.
THE FUNCTIONAL RELATIONS OF THE NERVOUS SYSTEM.

23. The distinguishable parts of this system are the central axis, the cranial nerves, and the spinal nerves, with the chain of ganglia and nerves composing the Sympathetic. Let us briefly set down what is known of their special offices.

Men very early discovered that the nerves were in some way ministrant to Sensation and Movement; a divided nerve always being accompanied by insensibility and immobility in the limb. Galen, observing that paralysis of movement sometimes occurred without insensibility, suggested that there were two kinds of nerve; but no one was able to furnish satisfactory evidence in support of this suggestion until early in the present century, when the experiments of Charles Bell, perfected by those of Majendie and Müller, placed the suggestion beyond dispute.

Fig. 12.—Transverse sections of spinal cord (dorsal region).

24. Fig. 12 is a diagram (not a drawing of the actual aspect, which would be hardly intelligible to readers unversed in such matters) representing two transverse sections of the spinal cord just where the nerve-roots issue. The gray substance is somewhat in the form of a rude H, in the dorsal region, and of the expanded wings of a butterfly in the lumbar enlargements (Figs. 4–6); the extremities of this gray substance are the anterior and posterior horns. We have already said that from the anterior horns of each half issue the roots of the motor nerves, which pass to the muscles. From the posterior horns issue the sensory nerves, which, soon after leaving the cord, enter the ganglia before joining the motor nerves, and then pass to the skin, in the same sheath with their companions, separating again as they reach the muscles and surfaces where they are to be distributed. When this mixed nerve is cut through, or tied, all sensation and movement disappear from the parts innervated. But if only one of the roots be cut through, above the ganglion, there will then be only a loss of movement or a loss of sensation. Thus suppose the section be made at a, b, A: we have then divided a sensory nerve, and no pinching or pricking of the part innervated by that nerve will be felt; but movement will take place if the under nerve be irritated, or if a sensation elsewhere be excited. Now reverse the experiment, as at B, c, d. Then, pricking of the skin will be felt, but no movement will respond. The nerve which enters the cord at the upper (posterior) part is therefore a sensory nerve; that which enters at the under (anterior) part is motor. The direction is in each case indicated by the arrow. The central end b, if irritated, will produce sensation; whereas the peripheral end a produces neither sensation nor movement. The central end d produces neither sensation nor movement; the peripheral end c produces movement.

25. Two facts are proved by these experiments. First, that the co-operation of the centre is necessary for Sensation, but not for Movement. Although normally all the muscles of the trunk are moved only when their centre has been excited, yet any irritation applied directly to the muscle nerve, even when separated from its centre, produces a movement. And to this we may add that a slighter stimulus will move the muscle by direct irritation of the nerve, than by indirect irritation through the centre; a slighter stimulus also will suffice when applied to the nerve than when applied to the muscle itself.

26. The second fact proved is known as Bell’s Law, that the sensory and motor channels are respectively the posterior and anterior nerves. The fact is indisputable, but its theoretic interpretation can no longer be accepted in its original form. Bell supposed the two nerves to be different in kind, endowed with different specific energies, the one sensitive, the other motor. The majority of writers

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