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completely compressed so as to arrest circulation in the lumbar part of the cord; if this arrest be maintained for two or three minutes, the posterior extremities become completely paralysed as regards sensation, voluntary motion, and reflex motion; the nerves and muscles are not paralysed, but the grey matter of the cord is rendered inexcitable; if the arrest is much prolonged (an hour or more) the cells may not recover at all, and the fibres in connection with them will then degenerate. The experiment furnishes one among other proofs that motor and sensory channels all pass to and from the brain via a station of grey matter in the cord, and that there are no direct channels of white matter in either direction between the brain and the periphery. Arrest of circulation for so brief a period, though sufficient to abolish the excitability of nerve-cells, is not sufficient to abolish the conductivity of nerve-fibres.

The same fact is demonstrable on decapitated animals. Excitation applied by means of electrodes inserted into the vertebral canal a minute or two after decapitation gives contractions of the upper extremities (by direct excitation of motor nerve-fibres), but no contraction of the lower extremities, because conduction is interrupted by inexcitable grey matter.

(2) Degenerations.-After transverse division of the cord, either experimentally upon animals or accidentally upon man―e.g. in the mid-dorsal region-degenerations occur along certain tracts above and below the seat of division; the former-called 'ascending-are found in the column of Goll and in the cerebellar tracts; the latter-called descending '-are found in the column of Türck and in the lateral column. A complete transverse division of the cord gives, of course, no information whether or no degenerated tracts cross the middle line; hemisections, or unilateral lesions, are necessary to supply such information. As regards descending degeneration, however, abundant data are furnished by cases of cerebral hemiplegia, where, in consequence of a brain lesion, a definite tract of fibres degenerates, and can be traced in the brain, in the bulb, and in the cord. In the latter, the degeneration consequent upon a lesion on one side of the brain occupies two definite situations in a transverse section of the cord; the larger tract of degeneration (crossed pyramidal tract) occupies the lateral column of the side opposite to that of the cerebral lesion, the smaller tract (direct pyramidal tract) occupies the anterior column of the same side as that of the lesion. The fibres composing this smaller tract are believed to cross the middle line along the whole length

of the cord, to be distributed to the opposite side of the body. This coincides with the results of the experiments given above, to the effect that motor channels cross in major part at the decussation of the pyramids, in minor part throughout the spinal cord. The degenerated tracts taper downwards in both cases, the crossed tract being traceable farther down the cord than the direct tract; the degeneration does not extend to the anterior roots, i.e. it is limited by the cells of the anterior cornua.

Ascending degeneration after hemisection of the cord, e.g. in the dorsal region, takes place on the same side as the injury, in the postero-median column up to the nucleus of the funiculus gracilis, and in the cerebellar tracts. It may also occur in consequence of destruction of spinal ganglia, or of division of roots. between ganglia and cord; in this case the degeneration is limited

Above

FIG. 264.-DEGENERATION OF THE
SPINAL CORD.

Secondary to a lesion of the right hemisphere; right direct pyramidal tract, left crossed pyramidal tract (Mott). (See also fig. 276.)

Below

FIG. 265.-TRACTS OF DEGENERATION
ABOVE AND BELOW A TRANSVERSE
DIVISION OF THE CORD.

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to the posterior columns and absent from the cerebellar tractsfacts which are taken to signify that ascending' fibres of the latter are connected with cells in the cord below (cells of Clarke's column), while fibres of the posterior column are in uninterrupted continuity with posterior root-fibres and ganglia. These fibres do not, however, pursue an unbroken course to the cortex; the degenerated tract ends at the spinal bulb in connection with the cells of grey matter, gracilis and cuneate nuclei-that is, the bulbar representative of the grey matter forming Clarke's column. From these bulbar nuclei fibres are traceable (1) to the opposite side of the cerebellum ria the superior pyramidal decussation and restiform body; (2) to the opposite side of the cortex cerebri via the fillet decussation and internal capsule. The posterior or dorsal cerebellar tract is traceable through the restiform body into the cerebellum; the anterior or ventral cerebellar tract

extends higher and passes to the cerebellum through its superior peduncle (Mott). Ascending degeneration of Goll's column tapers upwards, a fact that is referable to the connection of its fibres. with the cells of Clarke's column; degeneration of the posterior roots, followed up the cord, is at first postero-lateral, and higher up postero-mesial in situation, so that in a section across the cervical region, fibres derived from lower roots are situated on the mesial side of fibres derived from the upper roots.

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The above-described are the main tracts along which systematic degenerations have been followed down and up the cord; we may, however, add, without entering upon comment or discussion (1) that scattered ascending' as well as descending' fibres, in more or less abundance, have been found degenerated in other situations-in the antero-lateral and in the posterior columns; (2) that a small patch of fibres in the posterior column (referred to by pathologists under the name of comma tract') undergoes descending degeneration after section of the posterior roots; (3) that ascending and descending degenerations taken together do not include the entire area of the white columns, but leave a considerable remainder, especially in the immediate vicinity of the grey matter, the fibres of which suffer no degeneration, and are therefore considered to be commissural between cells at different levels; (4) that in cases of locomotor ataxy a well-marked area of degeneration occupies the posteroexternal columns (fig. 258) and the postero-median columns at higher levels.

It should, moreover, be expressly stated that tracts of degeneration do not constitute direct evidence that such tracts are, normally, functional channels in the direction of degeneration. They indicate lines of trophic influence derived from 'centres which have been destroyed, or from which the fibres have been separated. It is, however, considered probable that such lines of trophic influence coincide with lines of functional impulsesbut we must recognise that the inference is not unimpeachable. As a matter of fact, we know that afferent nerves of the posterior root divided beyond the ganglion, undergo descending' degeneration. The terms are also open to the objection that, as in the case of nerves, the degeneration does not gradually ascend or descend, but that it simultaneously invades the entire length of a tract separated from its trophic centre.

(3) Development.-The various white tracts of the spinal cord are not developed simultaneously, but successively; the fibres com

posing them acquire their medullary sheaths at different dates, so that in embryos at various stages various tracts may be distinguished and separately followed. As regards this order we may, without going into details, quote the main conclusions of Flechsig's investigations: the first apparent fibres are the peripheral or root-fibres connecting the spinal grey matter with the periphery-sensory, then motor; the next are the commissural fibres connecting grey matter at different levels; next, the centripetal tracts from cord to bulb and cerebellum; finally, the centrifugal tracts from cerebrum to cord. The most important point which has been thus brought out is, that the pyramidal tracts above described as degenerating in consequence of cerebral lesion are the last-comers, and that they appear simultaneously with the cortex cerebri from which they take their origin. This is corroborative evidence of the view that the pyramidal tracts are motor channels from the cortex. On man this motor or pyramidal system does not appear until birth or a few weeks later, before which it is not possible to distinguish a pyramidal tract by any of the methods used to demonstrate tracts of medullated fibres; these tracts are not well developed until the end of the first year, and even then are recognisable as the youngest of the spinal tracts, being composed of fine fibres (2 μ), whereas in the adult state they are second only to the direct cerebellar tracts as regards the diameter of the majority of their fibres (5 to 10 to 15 μ). In the development of the pyramidal tract two further points are distinctly noteworthy. Firstly, the fact that its nerve-fibres go through three recognisable stages-appearing as naked axis-cylinders (fifth month after birth); as fine medullated fibres (ninth month); finally, as coarse medullated fibres. Secondly, the probability, almost amounting to certainty, that the pyramidal tract, appearing as the last-comer in the cerebro-spinal axis, develops in a centrifugal manner, pushing and insinuating itself along the lines which it finally occupies. A third point should also be alluded to, viz., that the relative proportion between direct and crossed tracts is liable to vary; ordinarily, the direct is to the crossed as 1 is to 9, but the proportion may be occasionally 1 to 4 or 1 to 1, and, very exceptionally, the entire pyramidal mass may fail to cross, being continued as an abnormally large anterior

column.

To sum up the considerations contained in the foregoing pages - experiment, clinical observation, the study of develop

ment and of degeneration, concur to testify that in man the motor channels from one side of the brain pass to the opposite side of the body via the lateral column of the opposite side and the anterior column of the same side; the former channel, called the crossed pyramidal tract, is the larger, and crosses the middle line in the bulb; the latter channel, the direct pyramidal tract, is the smaller, and crosses the middle line in the spinal cord. As regards sensory channels, it is proved that these cross from one side of the body to the opposite side of the brain, but as to the precise locality of the crossing, evidence is conflicting. According to the older experiments, the chief crossing is in the cord; according to newer experiments, and to the collateral but not perfectly conclusive evidence of development and of ascending degeneration, the chief crossing is in the bulb, above the decussation of the pyramids.

The spinal cord as a centre of reflex action.-We have seen that the grey matter of the spinal cord constitutes a series of centres, and that the movements over which they preside are pre-eminently of a reflex character. Certain movements, however, of undoubted spinal mechanism stand out from this category, and are more commonly characterised as automatic; certain other movements, also of undoubted spinal mechanism, demonstrable only on the lower vertebrates, are manifestly appropriate to definite ends, and have the appearance of being volitional in character to such a degree that the expression psychical has even been used to characterise them. Yet these movements in nowise detract from the statement that the spinal cord is the typical centre of reflex actions in their purest form, namely, immediate, unchosen, fatal responses to peripheral excitations, without the intervention of consciousness and volition. The aimless movements of the limbs of the paraplegic patient are a glaring instance of this kind of action in its lowest form. A grade higher, and the reflex act is something more than an aimless spasm it is a defensive act with animal self-preservation as its result, a character that may very generally be detected in spinal reflex acts. A grade higher still, and the spinal reflex act is so frequent and habitual as to appear to have become independent of peripheral excitations, and now it receives the name of an automatic action. Such actions are very evidently defensive and self-preservative as regards animal life. No sharp boundary-line can be drawn between the reflex and the automatic. As we have seen in Chapter VIII., the two expressions

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