Volume 1

Quain's elements of anatomy / edited by Edward Albert Schäfer and George Dancer Thane.

Date:
1895-1898
Catalogue details

Licence: Public Domain Mark

Credit: Quain's elements of anatomy / edited by Edward Albert Schäfer and George Dancer Thane. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    the part of the fibre thus separated from that centre {secondary degemr.ations), or they may follow from the prolonged disuse of a nervous tract/especially m young animals, as when a limb has been removed or from some other cause {tertiary degeiwrations). 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-ceFs 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 whicli immediately after birth the sciatic trunk is resected, when the animaf is fuU-grown the corresponding anterior comual 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-laUral ascending, the antero-lateral or ventro-lateral descending, and the dorso-lateral ascendinq (or direct cerelellar). 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 spmal 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 hes in the ano-le between the posterior horn and the outer surface of the cord, but is m most parts separated from both by fibres belonging to other systems. This ZaW 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. , ^ r-u A few fibres of the pyramidal tract are found scattered in other parts ot tne antero-lateral column. j. j. ^i. 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, formmg the anterior or direct portion olthe pyramidal tract (fig. 14) {column of 7%rc*), which graduaHT^diHimThes as it is traced downwards, and usuaUy ceases altogether at about the middle of the dorsal region of. the cord. It is probable that the decus- sation of these anterior pyramidal tracts goes on along their whole course, then- fibres passing through the anterior commissure and through the grey matter ot 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 ti-acts 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 anteiior column by a groove, the anterior intermediate sulcus of Rauber. In others they are quite small, or may even fail altogether. Tlf this case it may be assumed that the decussation of the pyra- mids, which is known to be subject to considerable variation, has been more complete than l^usual In other oases again the anterior pyramidal tracts may be unsymmetrical, being