« AnteriorContinuar »
NERVOUS SYSTEM. 229
cised by the nervous system in the different processes of life, anatomically, a single glance suffices to shew, that the point of view which neuro-pathologists have been accustomed to set out with, is a very erroneous one. For they fancied they saw in the nervous system an unusually simple whole, from the unity of which resulted the unity of the body in general, of the whole organism. But even though one has nothing but very rough anatomical ideas concerning the nerves, still one ought not to shut one's eyes to the fact that this unity is in a very sorry plight, and that even the scalpel demonstrates the nervous system to be an apparatus composed of an extremely large number of parts of relatively equal value without any single discoverable central point. The more accurately we make our histological investigations, the more do the elements multiply, and the ultimate composition of the nervous system proves to be disposed upon a plan analogous to that which has been followed in all the other parts of the body. An infinite quantity of cellular elements manifest themselves side by side, more or less autonomous, and in a great measure independent of one another.
If in the first instance we exclude the ganglionic substance and confine ourselves simply to the fibrous matter, we have on the one hand the real (peripheral) nerves in the narrower sense of the word, and on the other the large accumulations of white medullary substance, of which the greater part of the cerebrum, cerebellum, and the columns of the spinal marrow is composed. The fibres of these different parts are indeed on the whole similarly constructed, but disclose in their intimate structure such numerous, and in part, such considerable differences, that there are spots, with regard to which even at this very moment we cannot say with certainty whether the elements we have before us are really nerves, or belong to an altogether different kind of fibres. The greatest certainty has been acquired with regard to the structure of the ordinary peripheral nerves; in them the following can generally be distinguished with tolerable facility. All the nerves which can be followed with the naked eye contain a certain number of subdivisions, or fasciculi, which afterwards separate in the form of branches or twigs. On tracing out these individual twigs which keep continually dividing, we find that the nerve under nearly all circumstances retains a fascicular arrangement until nearly its ultimate divisions, so that every fasciculus in its turn comprises a greater or less number of so-called primitive fibres. The term, primitive fibre, which is here employed, was originally selected, because a nerve-fasciculus was regarded as analogous to the primitive fasciculus of a muscle. This notion afterwards became almost obsolete, and Robin was the first who in more recent times again directed attention to the substance which holds the fasciculus together and which he called perineurium. It consists of
very dense connective tissue, which, upon the addition of acetic acid, is seen to contain small nuclei, and is different from the looser connective tissue which in its turn holds the fasciculi together and constitutes the so-called neurilemma.
When we use the term nerve-fibre alone in its histological sense, we always mean the primitive fibres, and not the fasciculi which to the naked eve look like fibres. These ultimate fibres in their
Fig. 77. Transverse section through one of the trunks of the brachial plexus. /, /. Neurilemma, from which one thicker partition t and finer prolongations, indicated by light-coloured lines, run through the nerve and divide it into small fasciculi. These exhibit the dark, punctated, transverse sections of the primitive fibres, and between them is seen the perineurium. 80 diameters.
NERVE-FIBRES. AXIS-CYLINDER. MEDULLARY SIIEATII.231
turn possess, one and all, a special external membrane, which, when it has been entirely freed from its contents— a matter certainly very difficult to accomplish, but sometimes occurring spontaneously in pathological conditions, as for example in certain states of atrophy—displays nuclei upon its walls (Fig. 5, c). Within these membranous tubes lie the proper nerve-contents, which in ordinary nerves may again be divided into constituents of two descriptions. These can scarcely be distinguished apart in a nerve which is quite fresh; but in a short time after it has perished or been cut out, or after the action of any medium upon it, they at once separate very distinctly from one another, one of the constituents undergoing a rapid change which has generally been termed coagulation, and by means of which it is marked off from the other constituent (Fig. 78). When this has taken place there is distinctly seen in the interior of the nerve-fibre, the so-called axis-cylinder (the primitive band of Remak), a very fine, delicate, pale structure; and round about it a tolerably firm, dark mass, here and there running together, the nerve-medulla or medullary sheath [white substance of Schwann]; this fills up the space between the axis-cylinder and the external membrane. But the nerve-tube is gener
Fig. 78. Grey and white nerve-fibres. A. A grey, gelatinous nerve-fasciculus from the root of the mesentery, after the addition of acetic acid. B. A broad white primitive fibre from the crural nerve: a the axis-cylinder laid bare, c, v a varicose state of the fibre with its medullary sheath; at the end at m, m the medullary matter (myeline) protruding in convoluted forms. C. A fine, white primitive fibre from the brain, with its axis-cylinder protruding. 300 diameters.
ally so tightly filled with its contents that, when viewed in the ordinary way, scarcely anything is seen of the separate constituents, the axis-cylinder being always with difficulty visible within the medullary substance. Hence the fact may be accounted for, that its very existence was disputed for years and the view proclaimed by many, that it was also an appearance due to coagulation, produced by a separation of the originally homogeneous contents into an internal and external mass. This view is however unquestionably incorrect; every mode of examination at last discloses this primitive band; even in transverse sections of nerves the axis-cylinder is very distinctly seen in the interior, with the medulla round about it.
It is the so-called nerve-medulla which gives the nervefibres in general their white appearance; wherever the nerves contain this constituent, they look white; wherever it is wanting, they appear translucent and grey. There are therefore nerves which are akin in colour to ganglionic matter, are comparatively transparent and possess a more clear and gelatinous appearance than the others; and they have thence been called grey or gelatinous nerves (Fig. 78, J). Between the grey and white nerve-substance therefore there does not exist the difference that the one is ganglionic and the other fibrous, but only this, that the one contains medulla and the other does not. In general the absence of medulla in a nerve stamps it as one of a lower and more imperfect kind, whilst the presence of this substance announces a more abundant nutrition and a higher development in the part.
Not long ago I made an observation in which a direct illustration of the practical importance of these two conditions was displayed in a very unexpected manner, the usually translucent grey nerve-substance having been transformed into an opaque and white matter, namely in the retina. I found, namely, entirely by accident one day, in the eyes of a man in whom I was looking for changes of quite
MEDULLATED AND NON-MEDULLATED NERVES. 233
another kind—round about the papilla of the optic nerve, where the uniformly translucent retina is ordinarily seen— a number of whitish, radiating striae like those which one sometimes meets with upon a small scale in dogs, and pretty constantly in rabbits in different directions. The microscopical examination shewed that, like as in these animals, medullated fibres had developed themselves in the retina, and that its fibrous layer had become thicker and opaque in consequence of the assumption of medullary substance. On examining the individual fibres I found, on tracing them from the fore and middle parts of the retina backwards towards the papilla, that they gradually increased in breadth, and at the same time displayed, at first in an almost imperceptible, but afterwards in a very striking manner, an investing layer of medulla. This is a kind of transformation, therefore, which essentially impairs the functions of the retina, for this delicate membrane becomes thereby more and more impervious to light, inasmuch as the white substance does not suffer the rays of light to pass through.
The same change occurs in nerves during their development. A young nerve is a delicate, tubular structure, provided with nuclei at certain intervals and containing a pale grey substance. The medulla does not appear until afterwards, and then the nerve becomes broader and the
Fig. 79. Medullary hypertrophy of the optic nerve within the eye (Cf. 'Archiv f. pathologische Anatomie und Physiologic,' vol. s, p. 190). A. The posterior half of the globe of the eye, seen from before; from the papilla of the optic nerve proceed in four directions radiating striae of white fibres. B. Fibres from this optic nerve in the retina, magnified 300 times: a, a pale, ordinary, slightly varicose fibre, b, one with a gradually thickening medullary sheath, c, a similar one with its axis-cylinder protruding.