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posed nerve-fibres. This epithelial layer was what Purkinje called ependyma ventriculorum.1 This assumption, it is true, was never shared in by pathologists. Pathological observation held on its course pretty unconcernedly by the side of these histological assertions. However, it appeared desirable that some understanding should be come to on the subject, since in a merely epithelial ependyma an inflammation would scarcely take place, like that which is wont to be attributed to serous membranes. The result of my investigations was, that there certainly does exist a layer beneath the epithelium of the ventricles, which in many parts has quite the character of connective tissue, but in other places possesses great softness, so that it is extremely difficult to give a description of its appearance. Every, even the slightest, traction of the part alters its appearance, and a substance now granular, now striated, now reticulated, now of any other form, is seen.
At first I thought I had succeeded in shewing that a tissue analogous to connective tissue did actually exist in this part, and that the presence of a membrane could be demonstrated. But, the more I occupied myself with the examination of it, the more did I become convinced that a real boundary between this membrane and the deeper layers of tissue did not exist, and that a membrane could only be spoken of improperly, inasmuch as the notion of a membrane involves the supposition that it is more or less different from the parts beneath it, and constitutes a separable object. Now in the present instance a separation of a rough kind may certainly not unfrequentlv be effected, but a more delicate kind of separation is altogether impossible. When the surface of any section of
1 This term has had its signification extended by the Author, who takes it to include the whole of the layer (connective tissue as well as epithelium), which rests upon the nerve-fibres and is interposed between them and the cavity of the ventricles.—From a MS. note by the Author.
EPENDYMA OF THE VENTRICLES OF THE BRAIN. 275
the ventricular wall is examined with a tolerably high power, the first thing noticed on the surface is an epithelium, sometimes in a better, sometimes in a worse state of preservation (Fig. 94, E). In the most favourable cases we find cylindrical epithelium with cilia, extending throughout the whole extent of the cavity of the spinal marrow (central canal) and of that of the brain (ventricles). Beneath this layer follows a sometimes more, sometimes less pure layer of a structure resembling connective tissue, which at first sight certainly appears to be separated by a sharp outline from the deeper parts, for even with the naked eye, and especially after the addition of acetic acid, an external grey and translucent layer is very distinctly seen, whilst the deeper layer looks white. This wrhite appearance is due to the
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Fig. 94. Ependvma ventriculorum and neuro-glia from the floor of the fourth cerebral ventricle. E. Epithelium, N, nerve-fibres. Between them the free portion of the neuro-glia with numerous connective-tissue-corpuscles and nuclei, at v a vessel. In addition, numerous corpora amylacea, which are moreover represented separately at ca. 300 diameters.
presence of medullated nerve-fibres which first occur singly, then continually become more numerous and closely aggregated, and as a rule run parallel to the surface. Thus it may certainly appear as if a particular membrane existed here, which could be separated from the uppermost nervefibres. But now,if we compare the substance which advances to the surface with that which lies between the nerve-fibres, no essential difference presents itself; on the contrary, it turns out that the superficial layer is nothing more than an extension upwards, beyond the nervous elements, of a portion of the interstitial tissue which is everywhere present between them, but in this layer alone is seen in all its purity. The connection therefore is a continuous one.
You see from this description that it was a very idle dispute, when it was discussed for years, whether the membrane which clothed the ventricles was a continuation of the arachnoid or pia mater, or was a special membrane. There is, strictly speaking, no membrane at all present, but it is the surface of the brain itself which directly meets the eye. In the case of articular cartilage also we must call it idle to dispute, what kind of membrane invests the cartilage, since the cartilage itself advances right up to the free surface of the joint. Neither is there any prolongation from the arachnoid or the pia mater to the surface of the ventricle; the last processes which these membranes send inwardly are the choroid plexuses and the tela chorioides [velum interpositum]. Beyond these there is no serous covering found investing the internal surface of the ventricles of the brain. For this reason the conditions of the cerebral cavities cannot be exactly compared with those of ordinary serous sacs. In the tela chorioides or the plexuses, a series of phenomena may certainly manifest themselves, which are parallel to the diseases of other serious parts, but this can never take place in the same manner on the ventricular surface of the brain.
NEURO-GLIA. ITS CELLULAR ELEMENTS. 277
This peculiarity of the membrane, namely, that it becomes continuous with the interstitial matter, the real cement, which binds the nervous elements together, and that in all its properties it constitutes a tissue different from the other forms of connective tissue, has induced me to give it a new name, that of neuro-glia} (nerve-cement). The view that the substance in question belongs to the class of connective tissues has recently been admitted on nearly all sides, but with regard to the extent to which any isolated structures that occur in it are to be considered as belonging to this substance, opinions are still divided. Even when I instituted my first special investigations into the structure of the ependynia of the brain and spinal cord, it turned out that certain stellate cells which are met with in the middle of the spinal marrow (in the wall of the central canal, the existence of which was afterwards more accurately demonstrated, namely, in what I called the central thread of ependynia), and which up to that time had been regarded as nerve-cells, unquestionably belonged to the neuro-glia. Afterwards, and especially by the Dorpat school with Bidder at its head, a series of investigations were published, in which a great number of cells in the spinal marrow were set down as belonging to this connective tissue. Bidder himself was ultimately led to regard all the cells which are found in the posterior half of the spinal marrow, and therefore those sympathetic and sensitive cells also which you have just seen, as connectivetissue-corpuscles. On the other hand, Jacubowitsch has utterly denied the occurrence of the cellular elements of connective tissue in any part of the brain or spinal cord, and has asserted that the interstitial tissue, which by him too, indeed, is regarded as connective tissue, is an altogether amorphous, finely granular or reticulated matter, which nowhere contains a single corpuscular element. Between
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these extremes, I think, we are perfectly justified by experience in steering a middle course. There can, according to my firm conviction, be no doubt but that the larger cells which pervade the posterior horns of the spinal marrow are nerve-cells; but, on the other hand, it must be maintained with equal positiveness, that, where neuro-glia is met with, it also contains a certain number of cellular elements. Immediately beneath the surface of the cerebral ventricles we commonly meet with spindle-shaped cells lying parallel to it, just like those which are found in other kinds of connective tissue; these become larger under certain circumstances, and, in oblique sections, often display themselves in the form of stellate cells (Fig. 94).
A substance altogether similar in structure to that, with which we have already become familiar in connective tissue —especially as far as its cells are concerned—is also found between the nerve-fibres of the cerebrum; only the cells are so soft and fragile, that generally nothing but nuclei can be perceived, scattered at certain intervals throughout the mass. On making a careful search, however, even in fresh (not artificially hardened) FlG- 95> specimens, soft, cellular bodies of a
"e * e roundish, or lenticular form can be
9 Jbs^ ® detected, which possess finely granular
^1? • contents and large granulated nuclei
with nucleoli, and lie, certainly in no very great number, between the nervous elements. At certain spots it has indeed been hitherto impossible to draw a well-defined boundary-line between the two tissues, and especially so at the surface of the cereFig. 95. Elements of the neuro-glia from the white substance of the cerebral hemispheres of a human subject, a. Free nuclei with nucleoli, b, nuclei with the granular remnants of the cellular parenchyma broken up in making the preparation, c, perfect cells. 300 diameters