The method of Stilling enables us to trace a nerve-tract through a long stretch. The identity, however, is possible and certain only so long as the fibre-bundles compos ing the tract do not suffer interruption, or so long as they are not deflected from the plane of the section, or do not separate into widely diverging fibres. The accurate iden tification and tracing of the fibre-tracts, even when they branch in the most diverse directions or resolve, have necessitated the search for new methods.
One of these additional methods is the one, based on the inves tigation of secondary degenerations. Rokitansky announced in the first edition of his Pathological Anatomy (1847), that atrophy of the brain following apoplexy and inflam mation leads to atrophy of different fibre-paths, when extensive, indeed, to the disap pearance of an entire hemisphere and the related fundamental tracts. This communica tion for a time remained unnoticed. In 185o Ludwig Tiirck described more closely such secondary degenerations and inferred from his findings, that in those cases of cross-section of the spinal cord, in which the direction of physiological conductivity and that of the degeneration were identical in the secondarily degenerating cord-paths, the degeneration itself was caused by the disturbance of functions. Notwithstanding these exceedingly important results, at first only few investigators followed Tiirck along this line of investi gation. In later years, however, this method has been universally employed and to it we are indebted for the many papers by which our knowledge of the fibre-paths of the central nervous system has been materially extended. The method depends upon the principle, that every nerve-fibre in its function is dependent upon the related nerve-cell. Destruction of the latter, or separation of the nerve-fibre from its cell, results in degenera tion of the related fibre. Let us assume that a descending tract of the spinal cord has been destroyed in some part of its course. What happens ? The portions of the nerve fibres below the injury are separated from their trophic centre ; they therefore die. This destruction or secondary degeneration within the spinal cord proceeds downward. On examining a cross-section of the cord passing below the seat of injury and comparing it with a corresponding section of a normal spinal cord, the seat of the degeneration is readily located and the involved tracts may be accurately followed by means of serial sections.
This method of investigation by secondary degeneration is closely related to the physiological method or the method of vivisection. Certain nerve-centres or nerve-fibres of an animal may be directly stimulated or destroyed, and from the resulting symptoms conclusions drawn as to the relations of the nerve-centres or nerve-tracts to the peripheral parts ; thereby a functional differentiation of the nerve-fibres is also possible.
The pathological method is based on a principle similar to that of the vivisection procedures. Here also the destruction of parts of the central nervous system is con cerned, but these mutilations are not experimental but caused by the establishing of diseased processes. In this relation, the study of the pathological changes in certain affections of the spinal cord is of primary importance.
By means of the experimental method, which has been used on animals with such great success, we are able to follow and to study the course of the fibre-bundles by degenerations. This method, employed only under certain conditions, was introduced by Gudden and his pupils and is the atrophy-method, or the method of developmental arrest. Gudden's method is distinguished from other experimental procedures in that it is directed against the young animal. The chief difference consists therein, that, follow ing an experimental impression on the new-born animal, the entire process proceeds much more rapidly and completely than in the adult. The absorption of the disintegration products from the elementary parts destroyed goes on much more rapidly and com pletely in the new-born, so that scarcely a trace of the fibres and only few remains of the cells are left. In addition, the technique is relatively easy, while a further distinct advantage, as Gudden himself pointed out, is the almost incredibly rapid and admirable healing of the injury without disturbing secondary processes.
In 1852 Waller showed, that the peripheral stump of a cross-sectioned peripheral nerve undergoes degeneration. For a long time it was believed, that the peripheral segment alone degenerated, and that the central one remained unaffected by such changes.
Since the study of Ranvier on degeneration and regeneration of sectioned nerves, how ever, we know that also the central segment suffers important modifications. Ranvier showed, that in the central segment of the axis-cylinder new fibrillm were formed, which became new nerves, using the sheath of the degenerating peripheral segment as a sup port to reach the periphery. The nerve reassumes its function—it is regenerated. If, however, from any cause the developing nerve fails to secure such support, its further development is arrested and a nervous tumor, a neuroma, is formed, as seen in amputation stumps. But in these cases, especially when of long standing, a certain grade of atrophy of the nerves, as well as a diminution in the number of the corresponding nerve cells, may be observed. These changes are exceptionally rapid and marked so soon as the interference occurs in young individuals, particularly in the new-born. If in a new-born animal a motor nerve is removed, a certain region of the cerebral cortex destroyed, or the spinal cord partially cut through, not only is always a degeneration of the fibres in the separated peripheral stump to be observed, but also atrophy and indeed complete disappearance of the cells of origin. Gudden believed at first, that this differ ence from the Wallerian degeneration was attributable to the lesion being in the new born animal. Later, he recognized that it was not the age, but the position that exer cised the influence. Then, too. Forel proved, that the death of the cell after destruction of the related fibre occurred in the adult, as well as in the new-born animal. Death of the cell depends alone on the place where the fibre is sectioned. Section of a motor nerve at the periphery is followed by only a slow impairment and diminution in the size of the cells and fibres of the central stump. Section of the same nerve at its point of emergence from the brain, results in the death of the central root, as well as of all the cells of origin within the nucleus of the nerve. The method of Gudden has been rich in results. In 1872-74 Gudden proved, by extirpation of the cortical motor zone in dogs, that the pyramidal tracts run direct from the cerebral cortex to the spinal cord. Other important results are the establishing of the nuclei of origin of almost all the motor cerebral nerves, the course of the medial fillet and the termination of the optic tract. Closely connected with the method of Gudden are the pathological cases of early lesion and consequent atrophy of certain parts of the central nervous system, as well as the cases of congenital malformation involving the cerebrospinal axis.
Our knowledge concerning the fibre-tracts, moreover, has been especially advanced by the embryological method introduced by Flechsig, based on the study of the develop ment of the nerve-fibres. This method rests on the fact, that the different fibre-systems within the central nervous system acquire the medullary substance at a definite time, which, however, varies for the individual systems. On examining the infantile brain, it is found that certain fibres are already medullated, while others have not yet acquired this sheath. This difference between the medullated and non-medullated fibres is readily appreciable microscopically and, therefore, the examination of the nervous system in its various develop mental stages affords the possibility of delimiting and tracing certain fibre-systems.
An additional means, which has contributed much not only to the morphology but also especially to the accurate investigation of the fibre-tracts, is the comparative anatom ical method. Since in the different classes of animals this or that part of the brain is varyingly developed, in correspondence with different functional development, the investi gations in the domain of comparative anatomy have supplied numerous explanations con cerning the many-sided connections of individual parts of the central nervous system.
Finally, a combination of these various methods has been attempted. Edinger united the comparative anatomical method and that of Flechsig. Bechterew combined vivisection with the study of development and created the method. Admirable results were also achieved by Bechterew with his logical method, which consisted in studying secondary degenerations with simultaneous stimulation of the degenerated parts by means of the electrical current.