STRUCTURE OF THE SPINAL CORD. 223 



d). This is most distinct in the upper dorsal and lower cervical regions. 

 Another group (middle cell-group) lies in the middle of the crescent (fig. 

 252, e). The cells of the posterior cornu (g) are not collected into a 

 special group. 



Course of the nerve-roots in the spinal cord. The anterior roots 

 leave the anterior cornu in a number of bundles (fig. 250). Most 

 of their fibres are directly continued from the nerve-cells there. 

 On the other hand, these cells are surrounded by an interlacement 

 of ramified nerve-endings, which are derived from various sources, 

 especially from the collaterals of the posterior root-fibres (see below), 

 and from those of the descending fibres of the pyramidal tracts. 



The fibres of the posterior roots originate in the cells of the posterior 

 root ganglia (see diagram, fig. 253), and pass into the postero-lateral 

 column, but the smallest fibres enter the marginal bundle, and some 

 pass into the posterior horn of grey matter. On entering the spinal 

 cord the fibres bifurcate (diagram, fig. 253 and fig. 254), one branch 

 passing upwards, the other downwards. Both from the main fibre and 

 from its branches collateral fibres pass at frequent intervals into the 

 grey matter, and end in arborisations of fibrils, which envelop the 

 nerve-cells, both of the posterior and of the anterior horn (see diagram). 

 The main fibres also for the most part ultimately end in a similar manner 

 in the grey matter, some after a short course only, but others after a 

 longer course. A certain number of the last named fibres pass upwards 

 in the postero-lateral and postero-mesial columns (in the latter 

 especially those of the lower spinal nerves), until they arrive at the 

 medulla oblongata, where they have a terminal arborisation around the 

 cells in the nucleus gracilis and nucleus cuneatus. 



FIG. 253. DIAGRAM SHOWING THE PROBABLE RELATIONS OF SOME OF THE PRINCIPAL 

 CELLS OF THE CEREBRO-SPINAL SYSTEM TO ONE ANOTHER. 



1, a cell of the cortex cerebri ; 2, its axis-cylinder or nerve-process passing down in the pyramidal 

 tract, and giving off collaterals, some of which, 3, 3, end in arborisations around cells of the 

 anterior horn of the spinal cord, the main fibre having a similar ending at 4 ; call, a collateral 

 passing to the corpus callosum ; sir, another passing to the corpus striatum ; 5, axis-cylinder 

 process of anterior cornu-cell passing to form a terminal arborisation in the end-plate of a 

 muscle-fibre, m. 



<5, a cell of one of the spinal ganglia. Its axis-cylinder process bifurcates, and one branch, 7, 

 passes to the periphery to end in an arborisation in the sensory surface, s. The other 

 (central) branch bifurcates after entering the cord (at 8), and its divisions pass upwards and 

 downwards (the latter for a short distance only) ; 9, ending of the descending branch in a 

 terminal arborisation around a cell of the posterior horn, the axis-cylinder process of which, 

 again, ends in a similar arborisation around a cell of the anterior horn; 10, a collateral 

 passing from the ascending division directly to envelop a cell of the anterior horn ; 11, one 

 passing to envelop a cell of Clarke's column ; 12, a collateral having connections like these 

 of 9 ; 13, ending of the ascending division of the posterior root-fibre around one of the cells of 

 the posterior columns of the bulb ; 14, 14, axis-cylinder processes of cells of the posterior horn 

 passing to form an arborisation around the motor cells ; 15, a fibre of the ascending cere- 

 bellar tract passing up to form an arborisation around a cell of the cerebellum; 16, axis- 

 cylinder process of this cell passing down the bulb and cord, and giving off collaterals to 

 envelop the cells of the anterior horn ; 17, axis-cylinder process of one of the cells of the 

 posterior column of the bulb passing as a fibre of the fillet to the cerebrum, and forming a 

 terminal arborisation around one of the smaller cerebral cells ; 18, axis-cylinder process of 

 this cell, forming an arborisation around the pyramidal-cell, 1. 



