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axis-cylinder becomes distinctly defined. It may be said therefore that the medullary sheath is not an absolutely necessary constituent of a nerve, but is added to it only when it has arrived at a certain stage in its development.
Hence it follows that this substance, which was formerly regarded as the essential constituent of a nerve, according to present views plays a subordinate part. Those only who do not even now admit the existence of the axis-cylinder, regard the white substance of course not only as the greatly predominating constituent, but also as the really active element of the nerve-contents. Now it is very remarkable that this same substance is one which most extensively prevails in the animal body. I had, curiously enough, in the first instance in the examination of lungs come across forms which presented very similar qualities to those which we observe in the medulla of the nerves. Although this was very surprising, yet I did not really think there was an actual correspondence, until I was gradually led by a series of further observations which accumulated in the course of several years, to examine a number of tissues chemically. The result shewed, that there scarcely exists a tissue rich in cells in which this substance does not occur in large quantity; still it is only in the FlG 80 nerve-fibre that we observe the
A. n peculiarity, that the substance
^^«\ "^ 9 separates as such, whilst in all
®J?Js£ |^#*& other cellular parts it is con
®© fe^" tained in a finely divided state
in the interior of the cells, and is only set free when the contents undergo a chemical change or are subjected to the action of chemical reagents. From blood-cells, from pus-corpuscles, from the epithelial
Fig. 80. Drops of medullary matter (myeline—according to Gobley, lecithine) A. Differently shaped drops from the medullary sheath of cerebral nerves, after they have become swollen up with water. B. Drops from decomposing epithelium from the gall-bladder in their natural fluid. 300 diameters.
MYELINE. AXIS-CYLINDER (ELECTRICAL SUBSTANCE). 235
cells of the most various glandular parts, from the interior of the spleen and similar glands unprovided with excretory ducts, this substance can in every case be obtained by extraction. It is the same substance which forms the principal constituent of the yellow mass of yolk in the hen's egg, whence its taste and peculiarities, especially its peculiar tenacity and viscidity which are employed for the higher technical purposes of the kitchen, are familiar to every one. It is this substance, for which I have proposed the name of medullary matter (Markstoff), or myeline, that in extremely large quantity fills up the interval between the axis-cylinder and the sheath in primitive nerve-fibres.
If the nutrition of a nerve suffer disturbance, this substance diminishes in quantity and indeed may under certain circumstances totally disappear, so that a white nerve may be again reduced to the condition of a grey or gelatinous one. This constitutes grey atrophy, or gelatinous degeneration, in which the nerve-fibre in itself continues to exist, and only the peculiar accumulation of medullary matter has been affected. Herein you may seek for an explanation of the circumstance, that in many cases where, in accordance with the results of anatomical investigation, it was formerly thought one might expect to find a part completely incapable of fulfilling its functions, proof has been afforded by means of clinical observation, aided by electricity, that the nerve is still capable of performing its functions, although in a less degree than normal. Hence too it is manifest that the medulla cannot be the constituent in which lie vested the functions of the nerve as such. To the same conclusion physical investigations also have generally led, and at the present time therefore the axis-cylinder is pretty generally looked upon as the really essential constituent of the nerve, which is also present in pale nerves, whilst in white ones it can only be distinctly isolated by the separation of the investing medullary sheath. The axis-cylinder would therefore seem to be the real electrical substance of natural philosophers, and we may certainly admit the hypothesis which has been advanced, that the medullary sheath rather serves as an isolating mass, which confines the electricity within the nerve itself, and allows its discharge to take place only at the non-medullated extremities of the fibres.
The peculiar nature of the medullary matter most frequently displays itself in this way, that, when a nerve is torn across or cut (Fig. 78, m, m), the medulla usually protrudes from it, presenting, especially after it has been acted upon by water, a peculiar striated appearance (Fig. 80, A). It takes up water namely, which is a proof that it is not a neutral fatty substance in the ordinary sense of the term, but can at most, on account of its great power of swelling up, be compared to certain saponaceous compounds. The longer the action of the water lasts, the longer are the masses which protrude from the nerve. They have a peculiar, ribbon-like appearance, keep continually acquiring new streaks and layers, and give rise to the most singular shapes. Frequently also fragments become detached and swim about, forming peculiar, stratified bodies, which in recent times have been confounded with corpora amylacea, but are distinguished from them in the most positive manner by their chemical reactions.
With regard to the histological varieties of nerves amongst themselves, investigation shews that in different parts more or less highly developed forms greatly predominate. On the one hand, namely, the nerves are essentially distinguished by the breadth of their primitive fibres, on the other hand, by the presence of medulla. We have very broad, middle-sized and small white fibres, and in like manner broad and fine grey fibres. A very considerable size is generally speaking but seldom attained by the grey
Fig. 81. Broad and narrow nerve-fibres from the crural nerve with the medullar; substance irregularly swollen up. 300 diameters.
VARIETIES OF NERVE-FIBRES. 237
ones, because the size of a nerve depends more upon the greater or less quantity of medulla it contains, than upon the volume of the axis-cylinder, but still variations present themselves everywhere, so that some nerves are coarser and others finer.
Generally, we may say, that the primitive fibres usually become finer in the terminal portions of nerves, and that the ultimate ramifications of these latter are wont to contain comparatively the finest fibres; still this is not an absolute rule. In the optic nerve we commonly find from the very moment of its entrance into the eye only very narrow, pale fibres (Fig. 79, a), whilst the tactile nerves of the skin present quite up to their terminations comparatively broad and darkly bordered fibres (Fig. 83). It has not yet been found possible to arrive at any certain opinion with regard to the import of the different kinds of fibres from their breadth and the proportion of medulla they contain. For a time it was believed that a distinction of this sort could be established between them, namely, that the broad fibres were to be regarded as derived from the real cerebro-spinal parts, the fine ones as parts of the sympathetic; but this is not found to be borne out by facts, and all that can be said is, that the ordinary peripheral nerves certainly are abundantly provided with broad fibres, whilst the sympathetic nerves contain a comparatively larger portion of fine ones. In many places, as for example in the abdomen, grey, broad fibres predominate (Fig. 78, A), with regard to the nervous nature of which doubts are still entertained by some. For the present, therefore, no definite conclusions can be drawn as to any difference in the functions of a nerve from its mere structure, although it can scarcely be doubted that such differences must exist, and that a broad fibre must exhibit other properties, even if only quantitatively different, than a fine one, a medullated fibre others than a non-medullated one. However concerning all this nothing is at present known with certainty; and since it has been demonstrated by more delicate physical investigations that the nerves which had been previously assumed only to conduct in the one or the other direction, possess the power of conduction in both directions, I should not, I think, be justified in here advancing any hypotheses with regard to their centripetal or centrifugal conduction.
The great difference, gentlemen, which is to be remarked in regard to the functions of individual nerves, cannot as yet be referred so much to any difference of structure in them, as to the peculiarity of the structures with which the nerve is connected. Thus on the one hand the special function of the central organ from which the nerve proceeds, and on the other, the special nature of its distal termination, afford a clue to its own specific functions.
With reference to the terminations which the nerves present at their peripheral extremities, histology has, I should say, in the course of the last few years celebrated its most brilliant triumphs. Previously it was, as you well know, a matter of dispute whether the nerves ended in loops or in plexuses, or whether their terminations were free, and the one or the other opinion was held with equal exclusiveness. Now, we have examples of most of these modes of termination, but the fewest of that form which was for a time regarded as the regular one, namely the termination in loops.
The most manifest form of termination, though the one whose functions are, singularly enough, even now the least known, is that in the so-called Pacinian orVaterian1 bodies,— organs, concerning the import of which we are still unable to make any statement. They are found in man comparatively most marked in the adipose tissue of the ends of the fingers, but also in tolerably large numbers at the root of the mesentery; most distinctly and readily, however, in the mesentery of the cat, in which they extend a consider
1 Vater was professor at Wittenberg, and died in 1751.