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sider the arrangements which prevail in certain muscles, we find there is no evidence to justify us in assuming that every portion of a muscle receives special, independent nervefibres. On the contrary, a special division of nervous action in muscles only exists to a very limited extent, as we know from our experience in our own bodies. The neuro-pathological doctrines would lead us to infer that the will, or the soul, or the brain is able by means of special fibres to act upon every single part, but in reality this is by no means the case, for the nervous centres have mostly only one single path by which they can communicate with a certain number of similar elementary apparatuses.
Now with regard to nervous plexuses, we are at the present
Fig. 87. Nervous plexus from the submucous tissue of the intestinal canal of a child, from a preparation of Herr Billroth's. «, n, n. Nerves which unite to form a network, and exhibit at their points of junction ganglioniform swellings abounding in nuclei. v, v. Vessels, and in the intervals nuclei belonging to the connective-tissue. Magnified 180 diameters.
NERVE-PLEXUSES. INTESTINES. 255
time acquainted with most extensive arrangements of the kind in man, in the submucous tissue of the intestines, where the relations have recently been more closely investigated, in the first instance by Meissner and afterwards by Billroth. The submucous layer of the intestines is therefore, as Willis long ago declared it to be, a nervous tunic. On following up the afferent nerves, they are seen, after having divided, at last to break up into real networks; these in new-born infants present at certain points very large nodules, from which the nerve-fibres spread out into interlacements, so that a certain resemblance is thereby produced to a network of capillaries.
To what extent such arrangements prevail in the body generally has not yet been determined; for these facts also are almost entirely new, and have only recently attracted the attention of observers, but probably the number of these nervous membranes will eventually be augmented. In order, however, to avoid any misunderstanding, I must at once add that these plexiform expansions are by no means simple, but that the large nodides I have mentioned have the appearance of ganglions, so that we have here in some sort new nervous centres presenting themselves, and affording the possibility of a reinforcement of, or obstruction to, the original impulses. For the functions of the part this arrangement is manifestly of great importance, for we should not well be able to explain the peristaltic movements of the intestinal canal, if some contrivance did not exist by which stimuli, that in the first instance were conveyed only to one spot in the canal, could be transferred from network to network and from part to part. The modes of distribution of nerves, with which we were till recently acquainted, were not sufficient to afford anything approaching an explanation of the nature of peristaltic action, whilst these investigations of Meissner's have at once furnished us with a most suitable groundwork for an interpretation of it. So much concerning the general forms which are, as far as we know at present, assumed by the peripheral terminations of the nerves.
On the whole, these results correspond but little with the opinions which were formerly entertained, and with the hypotheses still advanced by the neuro-pathologist. The views of a neuro-pathologist of pure water amount, as is well known, to this, that a nervous centre is able, by means of nerve-fibres, to produce particular effects upon all, even the smallest particles of the territory under its sway. If a mass of cancer or pus is to spring tip in any little spot in the body, or merely a simple disturbance of nutrition to ensue, the neuro-pathologist requires a special arrangement, by means of which the nervous centre is enabled to have its influence conveyed into even the most minute districts of the periphery, and some route along which the messengers "can travel who have been appointed to bear the order to the remotest points of the organism. Actual experience teaches us nothing of the kind. At those very spots where we know such an extremely complicated arrangement of the terminal apparatuses to exist, as I have described to you in the organs of sense, the nerves have no connection with the nutrition of the parts, and especially no demonstrable influence upon the elementary structures. In nearly all other places, either whole surfaces, or parts of organs are supplied with nerves in a uniform manner, or from these surfaces and parts of organs collective impressions are conveyed to the centres. In many parts concerning which we can certainly demonstrate that nervous influence is exercised upon them, as for example in middle-sized and small vessels, we do not yet at all know to what extent their individual constituents receive special nerve-fibres. So bad are the anatomical foundations of the neuro-pathological doctrines.
There still remains for us, gentlemen, now that we have
NEBVOUS CENTRES. GANGLION-CELLS. 257
discussed the terminal arrangements of the peripheral nerves, to consider the important series of nervous centres, or in a more restricted sense of the term, ganglionic apparatuses. As I lately remarked to you, we find these predominating in those parts of the nervous centres in which there is grey matter. Still the mere grey hue of a part is not a decisive proof of its ganglionic nature; and in particular we must not suppose that the ganglion-cells are at all essentially concerned in the production of the grey colour, seeing that we find grey matter in many places where ganglion-cells do not exist. Thus, the most external layer of the cortex of the cerebrum does not contain any well-marked ganglion-cells, although it looks grey; but we find there a translucent connective substance, pervaded by a large number of delicate vessels, and assuming, in proportion as these are more or less full, at one time more a reddish grey, at another more a whitish grey hue. On the other hand it frequently happens that, where there are ganglion-cells, the substance really does not look grey, but has a positive colour varying between brownish yellow and blackish brown. Thus we find spots in the brain, which have long been known by the names of substantia nigra, fusca, &c, in which the black or brown colour, which we perceive with the naked eye, is dependent upon the ganglion-cells, which form really coloured points. This coloration appears only in the course of years. The older an individual becomes, the more conspicuously do the colours shew themselves; still under certain circumstances pathological processes also seem to accelerate their manifestation. Thus in the ganglia of the sympathetic it is a striking phenomenon, that certain morbid processes, for example, typhoid fever, appear to exercise a powerful influence in producing an early deposit of pigment. Since the pigment however constitutes a relatively foreign mass in the internal economy of the ganglion-cells, and is not, as far as we know, subservient to their proper functions, but has all the cha
racters of an inert accidental deposit, it may really be quite possible that these conditions should be regarded as a kind of premature senescence in the ganglia. In these cases we discern in the ganglion-cells (Fig. 88, a) in addition to the very distinct, large nucleus with its large, bright nucleolus, the contents properly so-called, which consist of a finely granular substance, and at a certain spot enclose the pigment which is generally deposited excentrically, but sometimes around the nucleus. Under certain circumstances this deposit increases to such an extent that a great part of the cell is filled up with it. The more abundant it is, the darker does the whole spot appear to the naked eye.
Formerly it was imagined that the majority of ganglion-cells were merely round bodies, but the conviction has been gradually gaining strength, that this form is an artificial one, and that the real state of the case is rather, that processes strike out from the cell in various directions, and ultimately become continuous with nerves or other ganglion-cells. These processes are in the first instance pale, and even where their transition into ordinary, darkly-contoured nerve-fibres can be traced, they are observed (but generally not until a certain distance from the ganglion-cell) to become thicker and gradually to provide themselves with a medullary sheath. This circumstance which^was formerly unknown explains how it was, that during so long a period so much obscurity prevailed with regard to these conditions.
Fig. 88. Elements from the Gasscrian ganglion. a. Ganglion-cell with nucleated sheath, which is prolonged around the efferent nerve-process; in the interior, the large, clear nucleus with its nucleolus, and round about it an accumulation of pigment. b. Isolated ganglion-cell with a pale process proceeding up to it. e. Delicate nerve-fibre with pale axis-cylinder. 300 diameters.