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APRIL 3, 1858.
SPINAL CORD AND BRAIN.
The spinal cord.—White and grey matter.—Central canal.—Groups of ganglioncells.—White columns and commissures.
The medulla oblongata and the brain.—Its granular and bacillary layer.
The spinal cord of the petromyzon and its non-medullated fibres.
The intermediate substance (interstitial tissue).—Ependyma ventricnlorum.— Neuro-glia.—Corpora amylacea.
The last time, gentlemen, I laid before you the results of the most recent observations concerning the nature and distribution of the ganglion-cells in the great nervous centres; allow me now to dwell a moment upon that organ which serves as a type in the development of the vertebratae, and is at the same time the one whose structure we can best take in at one view, namely the spinal cord.
The spinal cord presents, as is well known, and can with ease be seen by the naked eye in any transverse section, in different parts of its course, a different amount of white matter, though nearly everywhere the white matter predominates over the grey. This appears in transverse sections in the form of the well-known horns, which are distinctly marked off from the pure white of the rest of the mass by their sometimes pale grey, sometimes reddish grey colour. Wherever then the sub
SPINAL CORD. GREY MATTER. CENTRAL CANAL. 265
stance appears white to the naked eye, it is essentially composed of real, medullated nerve-fibres, in which only here and there a few ganglion-cells are imbedded; and indeed a large proportion of these fibres are of considerable breadth, so that the quantity of medullary matter is at certain points extremely large.
The grey matter of the horns is the real seat of the ganglion-cells, but here too the grey colour is by no means to be entirely ascribed to the accumulation of ganglion-cells; on the contrary, they never, as you will afterwards see, form more than a small portion of this matter, and the grey hue is chiefly due to there generally being in these parts no separation of that opaque, strongly refractive substance (myeline, medullary matter) which fills the white nerves.
It is in the centre of the grey substance that, as Stilling, especially, has shewn, the central canal (canalis spinalis) actually exists, which had previously been so commonly supposed to be present, and had also frequently been described as of constant occurrence, but of which nevertheless no one had ever previously been able to furnish a regular demonstration. In the case of the old observers, as for example Portal, their investigations were in every instance confined to a few pathological specimens, from which they derived all the information they possessed upon the subject, and from which they inferred in a somewhat arbitrary manner that the presence of a canal was the rule.
This central canal is so minute that extremely successful sections are required in order that it may clearly be perceived by the naked eye. Usually nothing more than a rounded grey spot can be detected, which is distinguished from the surrounding parts by its somewhat greater density. It is by microscopical examination alone that we can detect in this spot the transverse section of the canal in the shape of a minute hole (Fig. 90, c, c), which, like nearly all the free surfaces of the body, is invested with a layer of epithelium. It has now taken up its stand as a really regular, constant and persistent canal. It is continued throughout the whole extent of the spinal marrow from the filum terminale, where it cannot at all times be very distinctly demonstrated, up to the fourth ventricle, where the orifice by which it opens into the so-called sinus rhomboidalis1 is situated in the gelatinous substance of the calamus scriptorius. Here it may in the first instance be traced as a direct continuation from the floor of the fourth ventricle into a minute funnel-shaped fissure or line.
As for the ganglion-cells, they are generally found in the largest number in the anterior and lateral parts of the
Fig. 90. The half of a transverse section from the cervical part of the spinal marrow, fat, Anterior fissure; fp, posterior fissure, c c. Central canal with the central thread of ependyma. ca. Anterior commissure with ncrve-fibrcs crossing one another; ep, posterior commissure, ra. Anterior roots; rp, posterior ones. gm. Accumulation of motor cells in the anterior
'A name given to the floor of the fourth ventricle.
WHITE MATTER OF THE SPINAL CORD. 267
anterior horns. It is at this spot that we chiefly meet with the large many-rayed corpuscles which we considered the last time—corpuscles, which have in great part been traced into efferent nerves of the anterior root, and therefore give origin to motor nerves.
An analogous, but less distinctly grouped accumulation is found in the direction of the posterior horns, but there the cells are rather the small, many-rayed ones, such as those I lately described to you; they are connected with the fibres which run into the posterior root, and are therefore probably subservient to the functions of sensation. Besides, there is generally a third, sometimes more closely aggregated, at others more scattered, group of cells to be seen, which in their whole conformation remind us of the familiar cell-forms we meet with in the ganglia (Figs. 89, C; 90, gs). Their special position in the spinal marrow is certainly not so clearly defined as that of the other parts; perhaps they should be regarded as the origin of the sympathetic roots which run from the spinal marrow to the main trunk of the sympathetic, but this is as yet by no means clearly made out.
In the white substance of the anterior, lateral and posterior columns are found the medullated nerve-fibres, which in general follow an ascending or descending course, so that in transverse sections of the spinal marrow we scarcely gain sight of anything else than transverse sections of the nerve-fibres. Under the microscope therefore we generally see dark points, every one of which corresponds to a nerve-fibre. The whole mass of fibres constituting the columns of the spinal cord is, from within outwardly, split up into a series of groups or segments chiefly following a radiating arrangement, or in some sort into wedgehorns ; gi, sensitive cells of the posterior horns; gt, sympathetic cells. The black, dotted mass represents a transverse section of the white substance of the cord (the nerve-fibres belonging to the anterior, lateral and posterior columns) and its lobular divisions. 12 diameters.
shaped lobules, in consequence of a sometimes smaller, sometimes largerquantityof connective tissue with vessels pushing its way in between the separate divisions, which are of a fascicular nature like those of the peripheral nerves. This connective tissue is directly connected with the more abundant mass of it present in the grey matter. Now with regard to the nerve-fibres themselves, it is probable that a certain number of them proceed throughout the whole length of the spinal marrow, but it ought certainly not to be assumed that they are all derived from the brain; a probably considerable portion no doubt have their origin in the ganglion-cells of the spinal marrow itself, and then bend round into the anterior or posterior columns. Besides, the conviction has more and more gained ground, that, both between the two halves of the spinal marrow and between the separate groups of ganglion-cells, direct communications, commissures, exist—fibres passing across from one cell to another and from one side to the other, some so as to cross with those of the opposite side (anterior commissure), and some so as to run in a straight and parallel direction (posterior commissure).
With the help of these anatomical observations a notion, though indeed as yet a very unsatisfactory one, can be formed of the routes along which the different processes are carried on within the nervous centres. Every special function possesses its special elementary, cellular organs; every mode of conduction finds paths distinctly traced out for it. In general too, well-defined peculiarities in the structure of the individual nervous centres correspond to the differences of function, and particularly the posterior horns become gradually more and more strongly developed as we ascend; and in proportion as this development proceeds, we see the medulla oblongata, the cerebellum and cerebrum, coming into view, whilst the motor parts withdraw more and more into the background and ultimately