Flechsig believes that these are posterior root-fibres. They may easily decussate in the septum, but such decussation has not been proved. In the lower lumbar region there is a narrow tract of fibres close to the posterior median septum, that is apparently of different nature from the rest of the column. It is lenticular on transverse section, and is indicated by the clear area in L. 4, Fig. 70, and faintly in L b, Fig. 75. It is distinguished by a difference in time of development (Flechsig), and by freedom from the secondary degeneration that involves the rest of the column. It is very doubtful whether all the fibres that enter the posteromedian columns continue in these to the medulla. Some certainly do, but the upward increase in size of the columns seems to be far too small for the accommodation of all the fibres that seem to pass to them. At the same time we have, at present, no indication of the mode in which fibres leave these columns. At the medulla oblongata this column becomes filled with nervecells, the post-pyramidal nucleus, so called because the highest portion of this column has been termed the posterior pyramid of the medulla. It is also called the nucleus gracilis. The discovery that the fibres of this column are continuous with the nerve-roots invests the post-pyramidal nucleus with considerable importance, since its FIG. 78. Posterior cornu and column at the last dorsal segment. P. M. C., posteromedian column; P. E. C., postero-external column; P. M. S., posterior median septum; P. C., posterior commissure; v, commissure vein. P. V. C., posterior vesicular column; c. C., caput cornu; P. R., posterior root; a, an artery; d, d, d, a jacent to a strip of the lateral column, indicate the tracts of fibres passing from the vicinity and interior of the posterior vesicular column along the septa of the lateral column, to form the direct cerebellar tract; a, a, tracts of fibres passing from the neck of the horn, near the post. vesic. col., to the post-median column. nerve cells are the first with which these root-fibres are connected, and they arrest secondary degeneration. The upward degeneration of the median fibres implies upward conduction; their probable function will be considered in the next section. The postero-external column (" column of Burdach," containing the "posterior root-zone" of Charcot) consists chiefly of vertical fibres. Many of the posterior root-fibres pass through it, horizontally or obliquely (Fig. 64). These either curve inwards to the posterior horn (some first inclining upwards), or pass obliquely upwards and inwards to the median column and constitute the fibres of this column that are continued from the nerve-root. Across the anterior part fibres also pass, as just described, between the neck of the posterior horn and the median column. The vertical ground fibres of the external column have apparently only a short course, since they degenerate for only a few centimetres above or below a lesion. The longest descending degeneration is of a "comma-shaped tract," in the middle of the anterior third of the column in the dorsal region, which degenerates downwards for four or five centimetres (Fig. 76). These vertical fibres, of short course, may connect the grey matter of the posterior horn at different (but adjacent) levels. This column is larger in the swellings than it is in the dorsal region, chiefly in consequence of the larger number of root-fibres that pass through it. Above, it also ends in a grey nucleus, the postero-external nucleus or cuneate nucleus. Both these posterior nuclei are connected directly with the cortex of the opposite hemisphere, and chiefly with the ascending parietal convolution. Fibres also pass from them to the cerebellar hemisphere of their own side, which is connected with the same region of the opposite cerebral cortex by the superior cerebellar peduncle.* The two nuclei (of both sides) are also found (by developmental researches) to send fibres through to the inferior cerebellar peduncle, and fibres pass from the posterior median nucleus to the middle lobe of the cerebellum on the same side,‡ a fact of considerable significance. The "direct cerebellar tract" is another series of fibres that degenerate upwards. It forms a layer at the periphery of the lateral column, outside the pyramidal tract (Fig. 64), but does not extend through the whole length of the cord, ceasing below at the level of the first lumbar nerve (Fig. 70). The anterior part of the tract (as seen in section) does not extend forwards beyond the level of the lateral pyramidal tract, although it was formerly thought to do so because there are other fibres in front of it that also degenerate upwards, and these were not distinguished from those of the tract. But near its upper and lower extremities, at the level of the second cervical See Flechsig and Hosel, 'Neurol. Cent.,' 1890, p. 417. nerves, and also in the lowest part of the dorsal region, the tract lies a little anterior to its position in the rest of the cord, and hence at these places the pyramidal tract comes to the surface behind the cerebellar tract, close to the posterior nerve-roots, up to which elsewhere the cerebellar tract extends. The tract increases somewhat in size from below upwards, and hence receives fibres throughout its course, but most of its constituent fibres enter it at the level of the lowest dorsal and first lumbar nerves, i. e. at its lowest part. These fibres come from the grey substance, passing through the lateral column, along the septa that cross the pyramidal tract from the grey matter. In the latter these fibres are conspicuous objects at this level. (Fig. 78, d, d), passing transversely and obliquely from the front of the posterior vesicular column. Into this many of them can be traced; others change their direction and become vertical, perhaps passing into the vesicular column at a different level. It is probable that the fibres come from this column; the cerebellar tract is chiefly formed at that part of the cord at which the column is most developed, and the cells of this column have been found atrophied when the cerebellar tract is degenerated.* Besides the fibres that are gathered into the compact cerebellar tract, others, apparently belong. ing to it, ascend among those of the adjacent pyramidal tract. The cerebellar tract only degenerates when a lesion of the cord is at, or above, its place of origin, the junction of the dorsal and lumbar regions. If the lesion is in the lumbar enlargement, as in the case shown in Fig. 74, the tract does not degenerate. At the level of the first cervical nerve (where the pyramidal tract leaves the lateral column to cross into the anterior pyramid of the medulla) the cerebellar tract lies in front of the "grey substance of Rolando" (formed from the caput cornu posterioris), and passes up, in the restiform body, to the cerebellum. Since it degenerates upwards we must conclude that it also conducts upwards.† We have seen that the lateral pyramidal tract, although in contact, behind, with the head of the posterior cornu, does not extend quite up to the neck of the horn or to the intermediate grey substance, the two being separated by a " lateral limiting layer" of vertical fibres, in part broken up by processes from the grey matter. In the upper cervical cord, and also to a less extent in the dorsal cord, this layer extends forwards outside the anterior horn, which, in these parts, is E. g. by Minkowski, Deut. Arch. f. kl. Med.,' Bd. xxxiv, p. 433. Slight degene ration of the pyramidal tracts was the only other lesion. In the absence of evidence of contrary character, the direction in which fibres conduct must be assumed to be that in which they degenerate. We know of no exceptions to it within the central nervous system. Wherever we can observe the relations this law obtains, except in the peripheral sensory nerves, in which the conditions are conspicuously exceptional. Hence the probability is very great that the correspondence obtains where we have not yet been able to prove it, and to ignore the probability because it is not a certainty is scarcely reasonable, although not uncommon. narrow. It consists of fine fibres that seem to pass into and out of the grey matter. This course, and the fact that the tract does not degenerate through any considerable extent, suggest that its fibres have but a short course, and connect the grey matter of adjacent regions. There is a small tract of fibres, all of small size, at the junction of the tip of the posterior horn and lateral column known as Lissauer's tract. The fibres come from the posterior roots; they course upward for a short distance, forming the tract, and then enter the posterior horn. FIG. 79. Cervical section. Ascending degeneration in postmed. col. and ant.-lat. ascending tract, secondary to a lesion in the dorsal region. (From a section prepared by Dr. Mott.) The rest of the lateral column, in front of the pyramidal and cerebellar tracts, consists of fibres that vary in size, course, and date of development. Secondary degeneration, however, as I pointed out some years ago,* enables us to distinguish an important tract which occupies an irregular area in front of the pyramidal and cerebellar tracts, and degenerates upwards throughout the cord. It extends across the lateral column, as a band which fills up the angle between the pyramidal and cerebellar tracts, and it reaches the surface of the cord in front of the latter tract, nearly on a level with the anterior commissure; it then extends forward in the periphery of the anterior column, almost to the anterior median. fissure, and up to the direct pyramidal tract when this exists. I have termed it the antero-lateral ascending tract (Figs. 64, 70, 73A, 74, and 79). It has often been confounded with the cerebellar tract, and hence the impression arose that the cerebellar tract extends farther forwards than it really does. Posteriorly the tract extends across the lateral column towards the posterior commissure, from which its fibres probably come (Fig. 70, L. 4). In the cervical (ib., c. 3), especially in the upper part, where the direct cerebellar tract extends farther forward, the broadest part of the ascending tract lies just within the anterior part of the cerebellar tract, and extends, like a wedge, between the latter and the pyramidal tract. This tract has been found by Bechterew to undergo development at a different period from the rest of the lateral column. It apparently constitutes an upward path from the opposite posterior roots, since its degeneration, if unequal on the two sides, is greater on the side opposite to that on which the uncrossed post.-median column is most degenerated. Its fibres do not degenerate when the nerve-roots are divided, and hence probably spring from cells in which the rootfibres end. Fibres apparently pass to it through the whole length of the cord, and these are mingled together, so that the degeneration Diagnosis of Diseases of the Spinal Cord,' first ed., 1879. from a lesion of the lower part of the cord is not dense.* The precise form of the tract seems subject to individual variations. Above, its fibres probably end in the lateral nucleus. Some fibres of the anterior part of the lateral column are large in size, and curve upwards and downwards into the anterior horn. They must be of short course, and may be fibres of the anterior nerve-roots, which ascend and descend to nerve-cells on a different level from that of the roots by which they leave the cord. The fibres of the anterior column, excluding the anterior pyramidal tract, are termed by Flechsig the anterior ground-fibres (Fig. 6. They are not separable, structurally or by development, from those of the lateral limiting layer. The ground-fibres do not degenerate through any considerable extent of the cord, and some probably connect the anterior cornua at different levels. Moreover, some of the fibres pass to the anterior commissure, and thus, by the agency of the anterior columns, a connection may be established between the two anterior cornua at different levels. Whether any of the ground-fibres are of long course but do not degenerate at all we do not know. The Commissure of the spinal cord lies between the bottom of the anterior fissure and the posterior columns, and in it alone is there a passage of fibres from one half of the cord to the other. It consists of two parts, an anterior or white, and a posterior or grey commissure. The white commissure varies in thickness in different parts of the cord, and is largest in the lumbar region. It is composed of medullated fibres, which cross in the commissure in such a manner that the anterior fibres on each side pass out in the posterior part of the commissure on the other side. The fibres in front pass into the anterior white column, those behind into the grey substance; hence they appear to establish a connection between the anterior column of one side and the anterior cornu of the other; but many of those entering the anterior column merely pass through this to the cornu, and this is probably true also of some of the fibres that turn upward in the column. Some seem to go to ganglion-cells of the grey matter or to the fibrillary plexus, others to the anterior nerve-roots. Through this commissure must also pass the fibres of the anterior pyramidal tract, and from it may come certain fibres that have been traced across the intermediate grey substance to the lateral column, where Other illustrations of the degeneration of this tract will be found in the section on Locomotor Ataxy. For an example of its degeneration and references to previous observations, see Tooth, St. Bartholomew's Hospital Reports,' 1885, p. 137. See also his Lectures on Secondary Degeneration in the Spinal Cord,' Lond., 1890. The degeneration found by Hadden and Sherrington ( Brain,' Oct., 1888), and regarded as evidence that the tract passes to the cerebellum, is, I am convinced, the anterior extremity of the cerebellar tract. This is manifest on a comparison of fig. 5 in their plate with Fig. 79, p. 183. It is important, moreover, to note that, in many cases of transverse lesions, a superficial myelitis extending up for a short distance in the middle of each side of the cord has determined a secondary degene ration of partial, and sometimes perhaps combined character. |