Page:The New International Encyclopædia 1st ed. v. 14.djvu/420

* NERVOUS SYSTEM AND BRAIN. 370 NERVOUS SYSTEM AND BRAIN. usually iiioiu ur less oblileialcd. It is sur- luuiidJd by the sutislanlia yclatinosa centralis, and divides the >;iay couimissuie into an an- ttiiur gray coiiiiiiinsure and a posterior gray commissure. I'assing uut from the anterior horns to the surface of the cord are bundles of fibres which constitute the aiiteriur or »io/oc si>inal nerve roots. Just to the median side of the pos- terior horns are seen the entering fibres of the posterior or sensory nerve roots. By the pos- terior horns the white matter of the cord is di- vided into posterior columns and untero-htteral columns, the latter being again rather indelinitely subdivided by the fibres of the anterior roots into lateral and anterior ooluiiins. The poste- rior columns are also usually subdivided at this level by a connective tissue septum from the pia mater into an internal portion, the column of Goll, and an external portion, the column of liurdach. Fibre Systems or Tracts of the Cord. As al- ready noted, the cell bodies of the neurones are grouped in the gray mailer of the brain and cord in the ganglia of the cranial and of the spinal nerves and in the end organs of certain of the nerves of special sense. This grouping is for definite physiological purposes. The a.ones from many of these neurone groups pass into the white matter as distinct bundles and constitute fibre systems or fibre tracts. In the normal aihilt cord these various tracts of fibres present the same appearance and cannot Ix' ditlerenlialed from one another. Certain of the methods de- scribed above, however, especially the method of embryology and the method of pathology, have enabled us to determine the following systems: (1) Descending tracts: (a) The direct pyraniid- AnLmMt AnLlonaflu. the direct cerebellar tract; postero-lalcral ascend- ing tract or tract of l-Techsig; (e) the tract of Uowers or the antero-lateral ascending tract. (3) Short systems, fundamental colunms or ground bundles, composed of short ascending and descending fibres intermingled. LocATiox oi' THE Fibre Systems of the Cord AND Origin of Their Fibres. The location of the fibre tracts can be best understood by reference to the accompanying diagram: The direct pyramidal tract occupies a small oval area adjacent to the anterior fissure. It decreases in size as it passes dowiiward and disajjpears entirely in the mid or lower dorsal regions. The crossed pyramidal tract occupies the posterolateral region of the cord. In the cervical and dorsal regions it is ■Batmed.iiss. JbttaintJ, sstiiiitia geliitiniisa ceiitr.-ilis; Ceii. can., central caliai; Post, rom., posteriDf conimissure; i'e., vein; .-tnt. Iw., nnti;riiir lii>rn ; L*., longitudinal librea of tlic IMwterior cornu; f!vt. aiiI/'. Ito., gelatinous substance of lolanilo; .ln(. eol., anterior eolunin; L<il. cil., lateral column; Post, col., posterior column. nl trnrt or column of Tiirck: (b) the cro.ssed pyramidal tract; (c) the descending comma tract of the posterior coltimn: (d) the descending cerebellar tract of Lopwenthal. (2) .scending tracts: In) The column of OoU; (b) the column of Biirdnch; (c) the column of Lissnuer; (d) - -, Ou/t'J. ^pui ntT SPIXALi COUD. SinE VIEW. n.in of the fissures and columns. Prnsl. mttt. n'ss., iios- teiior median lissure; Post. lai. Jiss., iiosteiior lateral lis- sure; Post rt., posterior root; ertK.;., ganglion; .Spin, ni-r., spinal nerve; Aiil. nuil. fc.s., anterior median lissure; .•/"(. lal. liss., anterior literal fissure; .itit. rt., anterior root; AnI.'cot.. anterior column; Lat. col., lateral colulun; Po!>t. col., posterior column ; Post. mtd. col., posterior median column. separated from the surface by the direct cere- bellar tract. In the lumbar region the latter tract is no longer present and the crossed pyra- midal comes to the surface. This tract extends to the lowermost part of the chord, decreasing in size as it descends. The fibres of the pyramidal tracts are axones of cells situated in the lirain in the region of the fissure of Kolando or motor area. These fibres descend through the internal capsule, pons, and medulla. In the medulla they form a large bundle of fibres known as the (in- terior pyramids, lying on either side of the ante- rior fissure. At the junction of medulla and cord most of these fibres cross over, in wliat is called the pyramidal decussation, to the oppo- site posterolateral region, to continue down the cord as the crossed pyramidal tract. A small number of fibres remain in the anterior region and pass down the cord as the f/ircc( pyramidal Iraet. Hoth tracts as they dcseend send collat- erals and terminals into the gray matter of the anterior horns, where they end in arborizations around the motor cells there situated. These tracts thus constitute the motor pathway cfin- necling the brain and cord, carrying voluntiiry tnntor impulses from the brain to the cells of the anterior horns, which latter directly control mus- cular movements. The descending comma tract is a small tract of fibres in the dorsal region sit- uated about the middle of the column of Bur- .lach. Bv some investigators it is believed to