7.

1, a cell of the cortex cerebri; 2, its axis-cylinder or nerve-process passing down in the pyramidal tract, and giving off collaterals, some of which, 3, 3, end in arborisations around cells of the anterior horn of the spinal cord, the main fibre having a similar ending at 4; call., a collateral passing in the corpus callosum to the cortex of the opposite side; str., a collateral passing into the corpus striatum; 5, axis-cylinder process of anterior cornu-cell passing to form a terminal arborisation in the endplate of a muscle-fibre, m.

m.

6, a cell of one of the spinal ganglia. Its axis-cylinder process bifurcates, and one branch, 7, passes to the periphery to end in an arborisation in the sensory surface, s. The other (central) branch bifurcates after entering the cord (at 8), and its divisions pass upwards and downwards (the latter for a short distance only); 9, ending of the descending branch in a terminal arborisation around a cell of the posterior horn, the axis-cylinder process of which, again, ends in a similar arborisation around a cell of the anterior horn; 10, a collateral passing from the ascending division directly to envelop a cell of the anterior horn; 11, one passing to envelop a cell of Clarke's column; 12, a collateral having connections like those of 9; 13, ending of the ascending division of the posterior root-fibre around one of the cells of the posterior columns of the bulb; 14, 14, axis-cylinder processes of cells of the posterior horn passing to form an arborisation around the motor cells; 15, a fibre of the ascending cerebellar tract passing up to form an arborisation around a cell of the cerebellum; 16, axis-cylinder process of this cell passing down the bulb and cord, and giving off collaterals to envelop the cells of the anterior horn; 17, axiscylinder process of one of the cells of the posterior column of the bulb passing as a fibre of the fillet to the cerebrum, and forming a terminal arborisation around one of the smaller cerebral cells; 18, axiscylinder process of this cell, forming an arborisation around the pyramidal-cell, 1.

the part of the fibre thus separated from that centre (secondary degenerations), or they may follow from the prolonged disuse of a nervous tract, especially in young animals, as when a limb has been removed or from some other cause (tertiary degenerations). The degeneration which follows a lesion of part of the nervous centre, and especially section of the spinal cord, is in some tracts above the lesion, in which case the degenerated tract is styled "ascending," in others below ("descending ") (see also p. 27). The place of the degenerated nervous substance is ultimately taken by a non-nervous fibrillar tissue, which, by its difference of behaviour to staining fluids, can easily be distinguished from the surrounding undegenerated white substance. (stage of sclerosis). In new-born and young animals, after a longer interval, groups of nerve-cells are affected by the degenerative processes, and the cells may even eventually disappear altogether (v. Gudden). When this is the case it may be assumed that they are in connection with the fibres which have been cut; these being probably the axis-cylinder processes of the cells.

Thus in a rabbit in which immediately after birth the sciatic trunk is resected, when the animal is full-grown the corresponding anterior cornual cells are found to have become atrophied, and similar results are obtained with motor nerves generally, both spinal and cranial (v. Gudden, Forel, Ganser, Mayser).

Tracts of the antero-lateral column.-Several tracts can by these methods be traced in the antero-lateral column, not only along a great part of the spinal cord, but into or from certain parts of the encephalon. The long tracts in this column are the direct and crossed pyramidal (descending), the antero-lateral or ventro-lateral ascending, the antero-lateral or ventro-lateral descending, and the dorso-lateral ascending (or direct cerebellar).

Descending tracts in the antero-lateral column.-The pyramidal tract is directly traceable down from the opposite pyramid of the medulla oblongata, and ultimately from the cerebral cortex (Rolandic region). The greater number of the fibres which compose the pyramid cross at the upper limit of the spinal cord, down which they pass in the posterior part of the lateral column as a compact bundle of fibres occupying in transverse section a somewhat triangular area, which lies in the angle between the posterior horn and the outer surface of the cord, but is in most parts separated from both by fibres belonging to other systems. This lateral or crossed part of the pyramidal tract (figs. 14 and 24), can be traced as far as the third or fourth pair of sacral nerves, becoming gradually smaller below and approaching the surface of the cord.

A few fibres of the pyramidal tract are found scattered in other parts of the antero-lateral column.

Some of the fibres of the pyramids of the medulla oblongata do not decussate at the upper limit of the cord. These pass down close to the anterior median fissure, forming the anterior or direct portion of the pyramidal tract (fig. 14) (column of Türck), which gradually diminishes as it is traced downwards, and usually ceases altogether at about the middle of the dorsal region of the cord. It is probable that the decussation of these anterior pyramidal tracts goes on along their whole course, their fibres passing through the anterior commissure and through the grey matter of the opposite side to reach the lateral pyramidal tract on the other side of the cord.

There is much variation in the development of the anterior pyramidal tracts in different individuals. In some they are so well marked as to form a visible prominence on the surface of the cord close to the anterior median fissure and separated from the rest of the anterior column by a groove, the anterior intermediate sulcus of Rauber. In others they are quite small, or may even fail altogether. In this case it may be assumed that the decussation of the pyramids, which is known to be subject to considerable variation, has been more complete than usual In other cases again the anterior pyramidal tracts may be unsymmetrical, being

UNIVERSITY

COURSE OF NERVE-FIBRES.

CALIFORNIA

25

more developed on one side than on the other, or the tract on one side may be wholly undeveloped. The direct tract is said to be wanting in 15 per cent. of cases in man. In different animals there is also much variation in the position and size of the pyramidal tracts. A well-marked direct pyramidal tract appears to be absent in most animals, even in monkeys. In some (mouse, rat, guinea-pig) the pyramidal tracts are in the posterior columns, but in most animals (rabbit, cat, dog) they run in the lateral columns as in man. The fibres of the pyramidal tract are probably connected with the anterior horn by collateral fibres, which ramify amongst the large cells that give origin to the anterior nerve-roots (figs. 16, 20).

The pyramidal tracts are undoubtedly the paths by which voluntary impulses pass from the brain to the various spinal segments. All the fibres within the area embraced by the "tract" are not, however, of the same nature, although fibres of the one function predominate : and this is probably correct of all the so-called "tracts" of conduction.

There are at least two descending tracts in the antero-lateral column, besides the direct and crossed pyramidal. One, the antero-lateral descending cerebellar tract (anterior marginal bundle of Loewenthal), consists of fibres which are connected with cells in the cerebellar cortex of the same side, and which undergo degeneration on removal of the corresponding half of the cerebellum (Marchi). These fibres form an extensive circumferential tract in the anterior three-fourths of the antero-lateral column, spreading inwards in front of the crossed pyramidal tract to reach the intermedio-lateral tract of the grey matter. The tract which is thus marked out (see fig. 26, p. 32) embraces (in the dog) the part of the anterior column which in man is occupied by the direct pyramidal tract, and also the whole region of the tract of Gowers (see below), the fibres of these two tracts being intermingled. Some of the fibres of the anterior roots also exhibit degeneration after removal of the cerebellar hemisphere, and are therefore probably directly continued from fibres of this tract.

In the monkey a few fibres in this column degenerate after lesions of the cerebral hemisphere of the opposite side. They are intermingled with those of the descending cerebellar tract and with those of the tract of Gowers, and are connected with cells in the Rolandic region of the cerebral cortex, as shown by the fact that they degenerate after lesions of that region. These fibres may perhaps be regarded as belonging to the system of the direct pyramidal tract, which in the monkey does not exist as a well-marked tract as in man: it has not yet been ascertained whether they occur in man as well as the direct pyramidal.

Ascending tracts in the antero-lateral column.-The dorso-lateral ascending cerebellar tract (direct lateral cerebellar tract of Flechsig) (fig. 14) lies between the lateral pyramidal tract and the outer surface of the cord, occupying a somewhat narrow area of the transverse section, which in the upper regions of the cord reaches to the tip of the posterior horn, but lower down becomes more limited, and is separated from the horn by the intervention of the adjoining pyramidal tract. It begins to appear at the lower dorsal region in man, and is then seen in all sections of the cord and lower part of the bulb, passing eventually by the restiform body into the cerebellum (middle lobe) (see fig. 27, p. 33).

It is found that there are a few fibres scattered through the neighbouring parts of the lateral column which, from their development simultaneously with those of the cerebellar tract, should be apparently reckoned with it. The axis-cylinder processes of the cells of Clarke's column are said to give origin to the fibres of the dorso-lateral cerebellar tract. The fibres of this tract acquire their medullary sheath somewhat earlier than those of the pyramidal tract. They are also considerably larger.

The ventro-lateral or antero-lateral ascending cerebellar tract (anterolateral ascending tract of Gowers) occupies a position in the sectional area of the lateral column which is anterior or ventral to the dorso-lateral cerebellar tract, and it has in section an arched shape curving from immediately in front of the crossed

pyramidal tract (where it is most strongly marked) round the superficial part of the lateral column in front of the dorso-lateral cerebellar tract, and tailing off gradually between the issuing anterior roots to reach the anterior column. It can be traced upwards into the medulla oblongata and pons Varolii (fig. 27), and eventually enters the cerebellum along with the superior peduncle, passing mainly to the vermis. It is uncertain from what cells the axis-cylinders of its constituent fibres spring, but it is probable that they arise from some of the cells of the posterior horn. Its fibres are intermingled with those of the descending cerebellar tract.

The remainder of the antero-lateral column which is not comprised in the above tracts encircles the anterior cornu; it is termed the antero-lateral ground-bundle. It varies in sectional area with the size of the nerve-roots and of the grey matter, and many of its fibres not improbably are of a commissural nature, serving to connect the grey matter of different segments of the cord. It receives fibres also from the grey matter of the other side through the anterior commissure (fig. 15, B, a.c.), and is in part composed of fibres of the anterior roots which course for a certain distance obliquely within it before leaving the cord. Many intersegmental fibres also occur scattered amongst the fibres of the long tracts.

Tract of Lissauer.-At the posterior part of the lateral column, close to the entering fibres of the posterior roots and directly derived from them, there is constantly to be seen a well-marked bundle of fine nerve-fibres, which was first described by Lissauer, and since by Bechterew and others. It is not always confined to the lateral column, but may extend into the postero-lateral column. ⚫

Tracts of the posterior white column.-This column is mainly composed of two long ascending tracts, viz., the tract of Goll, which about corresponds with the postero-mesial column, and the tract of Burdach, which embraces the remainder of the posterior column. In the lower part of the cord these two tracts are not marked off from one another in the adult, but from the middle of the dorsal region upwards the postero-mesial column is separated from the postero-lateral by a septum. of pia mater. Of the two parts of the posterior column the tract of Burdach is mainly composed of rather large fibres which are joined by, i.e., are continuous with, the large fibres of the entering posterior roots. After a certain course, in which they give off numerous collaterals to the grey matter, they appear to enter the grey matter and to come into intimate relationship, although not into actual continuity, with its cells, especially those of Clarke's column. The fibres of the tract of Goll, on the other hand, are for the most part of smaller diameter. They also are derived from posterior root-fibres, but instead of soon entering the grey matter of the cord many of them run up the postero-mesial column into the medulla oblongata, where they terminate amongst the cells of the nucleus gracilis.

The column of Goll in the embryo shows a distinction into two parts, one mesial, close to the posterior median fissure, and a dorso-lateral part, lying near the dorsal surface of the cord (Flechsig).

Descending fibres of the posterior column.-In the middle of the sectional area of the postero-lateral column a few fibres are constantly found, which undergo descending degeneration after lesions of the cord. These are often described as constituting a special tract (" comma" tract), but they are a good deal intermingled with fibres of the adjacent ascending tracts. Whether they originate from cells higher up in the cord or are derived from the descending branches of the posterior root-fibres is uncertain. The latter, it may be remembered, extend a certain distance down the cord from the Y-shaped division of those fibres, giving off collaterals to the grey matter, and ultimately themselves turning into the grey matter. The extent to which these descending branches of the posterior roots may pass down the cord has not as yet been determined.

DEGENERATIONS IN THE SPINAL CORD AS THE RESULT OF LESIONS.

It necessarily results from the Wallerian law of degeneration of nerve-fibres (Vol. I., p. 356) that every lesion of the nervous system in which fibres are cut or crushed must be followed by degeneration of nerve-fibres either above or below the lesion according to the position of the cells from which the fibres have originally grown, and which serve throughout life to maintain the nutrition of the fibres with which they are connected. Any such degeneration which occurs above the lesion is usually termed "ascending," and below the lesion "descending."

These terms must not be taken to imply, as is erroneously done by some authors, either that the degeneration starts from the lesion and extends upwards or downwards along the fibres, or that the nervous impulses which the fibres conduct necessarily have an ascending or a descending direction. For it is known (1) that the degeneration in the peripheral part of a cut nerve-fibre occurs simultaneously along its whole course; (2) that a nerve-fibre may undergo "descending" degeneration, although it normally conveys ascending impulses, c.g., the peripheral afferent nerves.

Since the terms ascending and descending as applied to nerve-degeneration lead to much confusion of ideas it would be better, if possible, to discard them and to adopt words which merely imply that the degenerative process occurs above or below a lesion. But unfortunately those terms have taken such deep root in the literature of the subject that this course would be very difficult, and might lead to even worse confusion than their retention.

Similarly, if the nerve-cells from which the fibres are thus derived are removed or destroyed, the fibres, with any branches which they may give off, will degenerate along their whole course. Hence any tracts of fibres in the spinal cord which are derived from cells in parts of the brain, degenerate on destruction of those parts. Further, those fibres which have grown into the spinal cord or brain from nerve-cells situated external to those organs (e.g., the fibres of posterior roots of spinal nerves and corresponding fibres of cranial nerves) will, if cut between the spinal cord or brain and the ganglion-cells from which they are derived, undergo degeneration from the point of section, not only as far as the surface of the central organ, but also along their whole course within that organ, and the degenerative process will include all their branches. Thus much having been said to explain the meaning of the results which are obtained by the study of nerve-degenerations, we may proceed to consider the degenerations which occur within the spinal cord: (1) from section of the posterior roots of the nerves; (2) from lesion or removal of parts of the brain; (3) from section or other lesion of the spinal cord itself.

1. Degenerations resulting from section of the posterior roots.-These have been investigated in the dog by Singer and Kahler, in the monkey by Mott and Tooth. It is convenient in describing the effect of sections of the posterior nerve-roots to consider them in the sacro-lumbar and in the dorso-cervical regions respectively, although the course of the degenerative process is essentially similar in both.

(a.) Section of the posterior roots of the sacro-lumbar nerves.-This has been most satisfactorily investigated by Singer in the dog and by Mott in the monkey. Section of these nerves, e.g., of the whole cauda equina, is followed by "ascending" degeneration in the posterior column of the same side along the whole length of the spinal cord and as far as the nucleus gracilis of the medulla oblongata. But the degeneration does not occupy the same position and extent in every part of the cord. At the lowest part it involves the whole of the posterior column (fig. 21, a), but as soon as a point is reached at which uncut nerve-fibres enter the cord, these now occupy the part of the column nearest the horn of grey matter, while degenerated fibres are seen in that part only which is nearest the posterior median fissure (b).