1.ocA'rIu\ TIIE FIBRE SYSTEMS OF TILE CORD AND I /RIGLN or TIIEIR FIBRES, The location of the fibre tracts can he best understood by reference to the accompanying diagram: The direct pyramidal tram' occupies a small oval area adjacent to tile anterior fissure, It decreases in size as it passes downward and disappears entirely in the mid or lower dorsal regions. The crossed pyramidal tract occupies the postero-lateral region of the cord. In the cervical and dorsal regions it is al tract nr column of Tiirek: (1) tile crossed pyramidal tract : (e) the descending (•militia tract of the posterior column: (d) the deAcending cerebellar tract of Lnewentiml. (2) .kseending tracts: IR ) 'flue column of (loll; (11) the column of Burdaeli; (c) the column of Lissauer; (d) separated from the surface by the direct cere bellar tract. In the lumbar region the latter tract is 110 longer present and the crossed pyra midal comes to the surface. This tract extends to the lowermost part of the c•liord, decreasing in size as it descends. The fibres of the pyramidal tracts arc• alloBes of cells situated in the brain in the region of the fissure of Rolando or motor area. These !Hires descend through the internal capsule, pons. and medulla. In the medulla they form a large bundle of fibres known am the an tf:rior pyramids, lying On either side of the ante rior fissure. .1t the junction of medulla and cord most of these fibres cross mar. in what is called the pyr«midal drenssation, to the oppo site postern-lateral region, to cent nine down the coed as the crossrd mow I ract . .1 small number of fibres remain in the anterior region and pass ihmil the cord as the direct pyramidal h•act. Both tracts as they descend send collat eral, and into the gray matter of the anterior horns, where they end ill :1110E1:tat around the motor cells there situated, These inlets thus constitute the motor pathway con necting the brain and cord. earrying voluntary motor impulses from the brain to the cells of the anterior horns, which latter directly control mu, etliar movements. The descemdiny comma true, is a small tract of fibres in the dorsal region sit uated about the middle of the column of Bur Inch. By some investigators it is believed to represent descending branches from posterior root fibres, by others, descending amines of cells sit uated in the cord. The descending cerebellum tract, or tract of Loewenthal, consists, as its name intimates, of descending axone,: of neurones whose cells are situated in the cerebellum. The results of investigations as to the paths which these axones follow in passing from cerebellum olivary nucleus of the no These axones end in the gray matter of the cord. The co/ am a of (loll and tlw column of Burdaeh are 14)111 posed of the axone:: of the cells of the ganglia of the posterior roots of the spinal nerves. Entering the posterior columns, these axones divide into as cending and descending arms. The latter is short and soon ends in the gray matter. The ascending arm may be short. of medium length. or long. The short and medium length fibres end in the gray matter of the cord, after sending in collaterals at different levels. The long fibres pass up to the medulla and end. those of the column of (loll in the nucleus gracilis, those of the column of Burdach in the nucleus cuneatus. The coiama of Lissouer consists of the shortest fibres which enter the posterior roots. These fibres end in the posterior horns. These neurones, a hose central axones form the entering fibres of the posterior roots of the spinal nerves taken in connection the analogous sensory roots of the cranial nerves, constitute the peripheral sensory neurone systcm. It is by means of these neurones that all
impulses are conveyed front the periphery or sur face of the body to the cord and brain. Ify their influence over the motor cells of the cord they determine the direct reflex actions. By their in to cord are not in accord. It is probable that in the ease of many of the fibres two neurones are involved, the connection being made through the Iluence over other cells situated in the cord, me dulla, and mid-brain, sensory impressions ate transmitted through other systems of neurones to the brain, and thus enter into consciousness. By their influence upon motor cells of the cortex they complete the cycle of a sensory impression en tering consciousness and being transformed into a voluntary motion. The direct cer(lnllar tract or dorso-latcral ascending tract lies near the lat eral surface of the cord from the posterior horn to about the mid-lateral point. It begins in the upper lumbar region and extends to the cerebel lum. Its fibres arc axones of the cells of Clark's column or the nucleus dorsalis of the same side. This nucleus is a group of cells lying in the gray matter just to the median side of the base of the posterior horn. It extends as a continuous column from the mid-lumbar to the lower cervi cal region. In the medulla the direct cerebellar tract enters the restiform body and thus reaches the cerebellum by means of the inferior cerebel lar peduncle. Grorers's trart,or the rentro-lateral osrcodiny tenet. lies along the periphery of the cord from the anterior limit of the direct cere bellar to the anterior nerve roots. The tract first appears in the upper lumbar cord and in creases in size as it passes upward. Its fibres arc axones of cells scattered through the gray matter of the same and opposite sides of the cord. INIany of the fibres of this tract are probably spinal association fibres and reenter the gray matter to terminate there. tither fibres. however, pat; upward tee higher centres, their ex act terminations being as yet undeter. mined. The remainder of the white 'nat ter of the cord is constituted by the short lilu•e systems (fundamental columns or ground bundles). It will be seen that in the main these short fibres lie adjacent to the gray matter and are more extensive in the anterior part of the cord. They tile axone, of evil, situated in the gray matter of the cord. These axones enter the white 'natter. pass up or down, or, splitting. send one branch up, the other down, and finally terminate in the gray matter. .\ long their course they send collateral, into the gray 'natter.
fibre• of the fundamental columns thus verve to connect the gray matter of different level• of the cord and may be considered of the nature of longitudinal commissaral titre sys tem-. lly means of these neurones an impulse vowing into the cord by the posterior nerve root may be conveyed to many motor or sensory cells within the gray !natter. in this way are ac•com „1;.1..,.1 111.. 111rIr0 .rH,1IIeS riv1111115 scribed are those of the anterior horns con nected with the motor spinal nerve roots. These are large multipolar cells rich in chromatic sub stance. They are arranged in columns. two long L110111115 extending the entire length of the cord for the innervation of the muscles of the trunk, while in the cervical and lumbar enlargements extra columns are added for the supply of the muscles Of the extremities.