Problems of Life and Mind. Second series, George Henry Lewes [e book reading free TXT] 📗
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Imaginary Anatomy has not been content with the cells of the anterior horn being thus united together, to admit of united action, but has gone further, and supposed that the cells of the posterior horn, besides being thus united, send off processes which unite them with the cells of the anterior horn—and thus a pathway is formed for the transmission of a sensory impression, and its conversion into a motor impulse. What will the reader say when informed that not only has no eye ever beheld such a pathway, but that the first step—the direct union of the sensory nerve-fibre with a cell in the posterior horn—is confessedly not visible?
126. The foregoing criticisms will perhaps disturb the reader who has been accustomed to theorize on the data given in text-books; but he may henceforward be more cautious in accepting such data as premises for deduction, and will look with suspicion on the many theories which have arisen on so unstable a basis. When we reflect how completely the modern views of the nervous system, and the physiological, pathological, and psychological explanations based on these views, are dominated by the current notions of the nerve-cell, it is of the last importance that we should fairly face the fact that at present our knowledge even of the structure of the nerve-cell is extremely imperfect; and our knowledge of the part it plays—its anatomical relations and its functional relations—is little more than guesswork!
THE NERVES.127. We now pass to the second order of organites; and here our exposition will be less troubled by hesitations, for although there is still much to be learned about the structure and connections of the nerve-fibres, there is also a solid foundation of accurate knowledge.
Fig. 20.—a, axis cylinder formed by the fibrils of the cell contents, and at a’ assuming the medullary sheath; b, naked axis cylinder from spinal cord.
A nerve is a bundle of fibres within a membranous envelope supplied with blood-vessels. Each fibre has also its separate sheath, having annular constrictions at various intervals. It is more correctly named by many French anatomists a nerve-tube rather than a nerve-fibre; but if we continue to use the term fibre, we must reserve it for those organites which have a membranous sheath, and thereby distinguish it from the more delicate fibril which has none.
The nerve tube or fibre is thus constituted: within the sheath lies a central band of neuroplasm identical with the neuroplasm of nerve-cells, and known as the axis cylinder; surrounding this band is an envelope of whitish substance, variously styled myeline, medullary sheath, and white substance of Schwann: it is closely similar to the chief constituent of the yolk of egg, and to its presence is due the whitish color of the fibres, which in its absence are grayish. The axis cylinder must be understood as the primary and essential element, because not only are there nerve-fibrils destitute both of sheath and myeline yet fulfilling the office of Neurility, but at their terminations, both in centres and in muscles, the nerve-fibres always lose sheath and myeline, to preserve only the neuroplasmic threads of which the axis cylinder is said to be composed. In the lowest fishes, in the invertebrates, and in the so-called sympathetic fibres of vertebrates, there is either no myeline, or it is not separated from the neuroplasm.
128. Nerve-fibres are of two kinds—1°. The dark-bordered or medullary fibres, which have both sheath and myeline, as in the peripheral system; or only myeline, without the sheath, as in the central system. 2°. The non-medullary fibres, which have the sheath, without appreciable myeline—such are the fibres of the olfactory, and the pale fibres of the sympathetic.
Nerve-fibrils are neuroplasmic threads of extreme delicacy, visible only under high magnifying powers (700–800), which abound in the centres, where they form networks. The fibrils also form the terminations of the fibres. Many fibrils are supposed to be condensed in one axis cylinder. This is represented by Max Schultze in Figs. 17 and 20.
129. As may readily be imagined, the semi-liquid nature of the neuroplasm throws almost insuperable difficulties in the way of accurately determining whether the axis cylinder in the living nerve is fibrillated or not; whether, indeed, any of the aspects it presents in our preparations are normal. Authorities are not even agreed as to whether it is a pre-existent solid band of homogeneous substance, or a bundle of primitive fibrils, or a product of coagulation.152 Rudanowsky’s observations on frozen nerves convinced him that the cylinder is a tubule with liquid contents.153 My own investigations of the nerves of insects and molluscs incline me to the view of Dr. Schmidt of New Orleans, namely, that the cylinder axis consists of minute granules arranged in rows and united by a homogeneous interfibrillar substance, thus forming a bundle of granular fibrils enclosed in a delicate sheath154—in other words, a streak of neuroplasm which has a fibrillar disposition of its granules. We ought to expect great varieties in such streaks of neuroplasm; and it is quite conceivable that in the Rays and the Torpedo there are axis cylinders which are single fibrils, and others which are bundles, with finely granulated interfibrillar substance.155
The fibres often present a varicose aspect, as represented in Fig. 21. It is, however, so rarely observed in the fresh tissue, that many writers regard it (as well as the double contour) as the product of preparation.156 It is, indeed, always visible after the application of water.
We need say no more at present respecting the structure of nerve-fibres, except to point out that we have here an organite not less complex than the cell.
Fig. 21.—Nerve-fibres from the white substance of the cerebrum. a, a, a, the medullar contents pressed out of the tube as irregular drops.
THE NEUROGLIA.130. Besides cells and fibres, there is the amorphous substance, which constitutes a great part of the central tissue, and also enters largely into the peripheral tissue. It consists of finely granular substance, and a network of excessively delicate fibrils, with nuclei interspersed. Its character is at present sub judice. Some writers hold it to be nervous, the majority hold it to be simply one of the many forms of connective tissue: hence its name neuroglia, or nerve-cement.
In the convolutions of the frozen brain Walther finds the cells and fibres imbedded in a structureless semi-fluid substance wholly free from granules; the granules only appear there when cells have been crushed. It is to this substance he attributes the fluctuation of the living brain under the touch, like that of a mature abscess; the solidity which is felt after death is due to the coagulation of this substance. Unhappily we have no means of determining whether the network visible under other modes of investigation is present, although invisible, in this substance. The neuroglia, as it appears in hardened tissues, must therefore be described with this doubt in our minds.
If we examine a bit of central gray substance where the cells and fibres are sparse, we see, under a low power, a network of fibrils in the meshes of which lie nerve-cells. Under very high powers we see outside these cells another network of excessively fine fibrils embedded in a granular ground substance, having somewhat the aspect of hoar-frost, according to Boll. It is supposed that the first network is formed by the ultimate ramifications of the nerve-cell processes, and that the second is formed by ramifications of the processes of connective cells. In this granular, gelatinous, fibrillar substance nuclei appear, together with small multipolar cells not distinguishable from nerve-cells except in being so much smaller. These nuclei are more abundant in the tissue of young animals, and more abundant in the cerebellum than in the cerebrum. The granular aspect predominates the fresher the specimen, though there is always a network of fibrils; so that some regard the granules as the result of a resolution of the fibrils, others regard the fibrils as the linear crystallization (so to speak) of the granules.157
131. Such is the aspect of the neuroglia. I dare not venture to formulate an opinion on the histological question whether this amorphous substance is neural, or partly neural and partly connective (a substance which is potentially both, according to Deiters and Henle), or wholly connective. The question is not at present to be answered decisively, because what is known as connective tissue has also the three forms of multipolar cells, fibrils, and amorphous substance; nor is there any decisive mark by which these elements in the one can be distinguished from elements in the other. The physical and chemical composition of Neuroglia and Neuroplasm are as closely allied as their morphological structure. And although in the later stages of development the two tissues are markedly distinguishable, in the early stages every effort has failed to furnish a decisive indication.158 Connective tissue is dissolved by solutions which leave nerve-tissue intact. Can we employ this as a decisive test? No, for if we soak a section of the spinal cord in one of these solutions, the pia mater and the membranous septa which ramify from it between the cells and fibres disappear, leaving all the rest unaltered. This proves that Neuroglia is at any rate chemically different from ordinary connective tissue, and more allied to the nervous. As to the staining process, so much relied on, nothing requires greater caution in its employment. Stieda found that the same parts were sometimes stained and sometimes not; and Mauthner observed that in some cells both contents and nucleolus were stained, while the nucleus remained clear, in other cells the contents remained clear; and some of the axis cylinders were stained, the others not.159 Lister found that the connective tissue between the fibres of the sciatic nerve, as well as the pia mater, were stained like the axis cylinders;160 and in one of my notes there is the record of both (supposed) connective cells and nerve-cells being stained alike, while the nerve-fibres and the (supposed) connective fibres were unstained. Whence I conclude that the supposition as to the nature of the one group being different from that of the other was untenable, if the staining test is to be held decisive.
132. The histological question is raised into undue importance because it is supposed to carry with it physiological consequences which would deprive the neuroglia of active co-operation in neural processes, reducing it to the insignificant position of a mechanical support. I cannot but regard this as due to the mistaken tendency of analytical interpretation, which somewhat arbitrarily fastens on one element in a complex of elements, and assigns that one as the sole agent. Whether we call the neuroglia connective or neural, it plays an essential part in all neural processes, probably a more important part than even the nerve-cells, which usurp exclusive attention! To overlook it, or to assign it a merely mechanical office, seems to me as unphysiological as to overlook blood-serum, and recognize the corpuscles as the only nutrient elements. The notion of the neuroglia being a mere vehicle of support
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