« AnteriorContinuar »
BACILLAR LAYER OF CEREBRUM AND CEREBELLUM. 269
almost entirely disappear. All nervous centres, the lowest as well as the most highly developed, are disposed upon an analogous plan; the only thing which, at least as yet, can be regarded as an especially characteristic peculiarity of the encephalon, is the circumstance, to which I called your attention in the last lecture, namely, that in the cerebrum and cerebellum processes from ganglion-cells are connected with particularly complicated apparatuses, which most resemble the granular and bacillar layers of the retina (Fig. 91) which I have brought before your notice. For here too we find branched,
Fig. 91. Diagrammatic representation of the disposition of the nerves in the cortex of the cerebellum, after Gerlach (' Mikroscopische Studien,' plate I, fig. 3). A. White matter. B, C, grey matter, B, granular layer, C, cellular layer.
almost arborescent filaments, which bear upon them minute granules, often in several rows, and attach themselves to the ganglion-cells in a manner essentially differing from, and much more delicate than that observed in the case of the proper nerve-processes. This kind of ganglion-cells may very likely stand in some close connection with the psychical functions, but at present we have no accurate information upon the subject, and it will, I expect, still be a long time before anything positive can be made out about it, seeing that parts which are much more accessible to investigation, like the retina, present the very greatest difficulties to those who seek to discover the functions of the individual segments.
The conformation which we have found to exist in the spinal marrow of man is essentially the same throughout the whole series of vertebrate animals, only that in man it is generally more complicated, and exhibits a greater abundance both of nerve-fibres and ganglionic matter. I have brought you for comparison a section from the spinal marrow of one of the lowest of the vertebrataB, namely the lamprey (petromyzon). In this animal the spinal marrow
Fig. 92. Transverse section through the spinal marrow of Petromyzon fluviatilis. F. Anterior fissure, F, posterior fissure, c, central canal with epithelium, gn, large, many-rayed ganglion-cells with processes in the direction of the anterior roots, gp, smaller, many-rayed cells with processes running to the posterior roots, gs, large, roundish cells in the neighbourhood of the posterior SPINAL MARROW OF THE LAMPRET. 271
forms a very small, flat band which has somewhat of a depression on the surface, and at first sight looks like a real ligament. On making a transverse section of it, it is found to contain the same parts that we see in man, but all only in a rudimentary form. What in ourselves we call grey matter, is also found there on both sides in the shape of a flattened oblong lobe which contains a few scattered ganglioncells, but only very few, so that perhaps only four or five are met with on each side of the transverse section. In the centre a central canal can likewise be detected, and that too lined with an epithelial layer similar to that which occurs in man. Below and in front of it generally lie a number of largish, round cavities, which correspond to unusually large, non-medullated nerve-fibres (Fig. 93, a), which were first seen by Joh. Muller. Farther outwards lie a few other thick fibres, but greatly exceeding these in number a large quantity of very fine fibres which give the transverse section a very diversified, regularly dotted appearance. Among the ganglion-cells three different kinds can here also be distinguished. Towards the outside of the grey matter lie many-rayed cells, larger anteriorly, smaller and more simple posteriorly. More internally and posteriorly on the other hand we find larger, more rounded, seemingly diclonous (bipolar) cells, comparable to the sympathetic forms. These cells communicate across the middle-line by means of real fibres, and besides we find processes which run out from the spinal marrow forwards and backwards and form the anterior and posterior roots. This is the simplest plan we have, displaying these relations; it is the general type of the anatomical structure of these parts. A circumstance worthy of particular observation here is that in the petromyzon, in the whole substance of the spinal marrow, no medullary matter exists in an isolated form, as is the case in man; we only find simple, pale fibres, which Stannius has without hesitation pronounced to be naked axis-cylinders. But without taking into account the fact that some of them have an enormous diameter, we find upon more accurate examination, as in the case of the gelatinous grey fibres in man, a membrane very clearly seen in transverse sections, especially after it has been coloured with carmine—and in the centre a finely granular matter, so that they seem rather to correspond to entire nerve-fibres. Hitherto, gentlemen, in considering the nervous system, I have only spoken of the really nervous parts of it. But if we would study the nervous system in its real relations in the body, it is extremely important to have a knowledge of that substance also which lies between the proper nervous parts, holds them together and gives the whole its form in a greater or less degree.
commissure (sympathetic cells). », ». Transverse sections of the large, pale nerve-fibres (Mullerian fibres), «' gaps out of which the large nerves have fallen, »" gaps belonging to smaller fibres. Besides, the cut ends of numerous finer and coarser fibres.
It is by no means very long ago since the existence of such interstitial masses of tissue was really only conceded in the case of peripheral nerves, and since the neurilemma was only traced back as far as the membranes of the spinal cord and brain, such an enveloping tissue being at most allowed to exist within the ganglia and in the sympathetic. In the nervous centres properly so called, and especially in the brain, this interstitial matter of ours was regarded as
Fig. 93. Pale fibres from the spinal marrow of Petromyzon fluviatilis. A. Broad, narrow, and extremely fine fibres. B. Transverse sections of broad fibres with it distinct membrane and granular centre. 300 diameters.
LINING MEMBRANE OF THE CEREBRAL VENTRICLES. 273
essentially nervous, for a substance of the kind appeared a natural desideratum, as long as a direct transference of impulses from fibre to fibre was admitted to take place, as long therefore as the necessity for a real continuity of conduction within the nerves themselves was not recognised. Thus in the brain a finely granular substance was spoken of as existing, which insinuated itself between the fibres, and though it certainly did not establish a complete connection between them, inasmuch as it occasioned a certain difficulty in the transference of impulses, yet nevertheless seemed to render a certain amount of conduction possible, so that when the impulse reached a certain degree of intensity, a direct transference from fibre to fibre could take place. This substance is however unquestionably not of a nervous nature, and if inquiry be made as to the relation which exists between it and the familiar groups of physiological tissues, it is impossible to doubt but that the substance in question is a kind of connective tissue; and therefore an equivalent of that tissue with which we became acquainted in the shape of perineurium (p. 230). But the appearance of this substance is certainly very different from that of what we call perineurium or neurilemma. These are comparatively firm, and often indeed hard and tough tissues, whilst the substance in question is extremely soft and fragile, so that it is only with very great difficulty that we can succeed in making out its structure.
I first had my attention directed to its peculiarities in investigations which I many years ago (1846) instituted into the nature of the so-called lining membrane of the cerebral ventricles (Ependyma). At that time the view was generally held, which had been put forward first by Purkinje and Valentin, and afterwards especially by Henle, that a real lining membrane did not exist in the ventricles of the brain, but only an epithelial covering, the epithelial cells directly resting upon the surface of the horizontally dis