THE MEDULLA OBLONGATA. 



1065 



FIG. 916. 



Cerebral cortex 



motor, correspond to the ventral roots of the spinal nerves with which they are 

 in series. The other groove, the postero-lateral furrow, continues upward in a 

 general way the line of the dorsal spinal root-fibres and marks the attachment of the 

 fibres of the ninth, tenth and bulbar part of the eleventh cranial nerves. Unlike the 

 posterior root-fibres of the cord, which are exclusively sensory, those attached along 

 this groove of the medulla are partly efferent and partly afferent, the fibres belong- 

 ing to the spinal accessory being entirely motor, while those of the glosso-pharyngeal 

 and the pneumogastric include both and, therefore, are mixed. 



The Anterior Area. This subdivision of the medulla, also known as the pyra- 

 mid, includes the region lying between the anterior median fissure and the antero- 

 lateral furrow. Superficially it appears as a slightly convex longitudinal tract, from 

 6-7 mm. in width, that continues upward 

 the anterior column of the cord. Each 

 pyramid constitutes a robust strand, which 

 below beginsat thedecussationand, increas- 

 ing slightly as it ascends, above disappears 

 within the substance of the pons. Just 

 before its disappearance, or, strictly speak- 

 ing, after its emergence, the pyramid 

 is slightly contracted on account of the 

 increased width of the bounding furrows. 

 Its chief components being the descending 

 motor paths formed by the cortico-spinal 

 fibres, of which approximately four-fifths 

 pass to the opposite side by way of the 

 decussation to gain the lateral pyramidal 

 tract, it is evident that only to the extent 

 of the direct pyramidal fasciculus and, for a 

 short distance, the anterior ground-bundle, 

 are its constituents represented in the 

 anterior column of the spinal cord. 



The fibres destined for the direct 

 pyramidal tract, which above the decussa- 

 tion occupy the lateral part of the pyramid, 

 gradually converge toward the mid-line 

 as the decussating fibres disappear, until, at 

 the lower limit of the crossing, they lie 

 next the median fissure, which position 

 they retain in their further descent within 

 the cord. The space thus afforded at the 

 lower end of the medulla, to the outer side 

 of the uncrossed fibres, is occupied by 

 the prolongation of the anterior ground- 

 bundle, which, however, soon suffers 

 displacement as it encounters the pyramid. 

 The ground-bundle lies at first to the outer side of the strands of decussating fibres 

 and then behind the pyramid; higher, it is pushed backward towards the mid-line 

 by the appearance of the inferior olive and the mesial fillet until, finally, it is 

 continued as the posterior longitudinal fasciculus at the side of the median raphe 

 beneath the gray matter covering the floor of the fourth ventricle. 



The proportion of the pyramidal fibres taking part in the motor decussation is 

 not always the same, from 80-90 per cent, being the usual number. Vary rarely all 

 the fibres cross, with suppression of the direct pyramidal tracts an arrangement 

 found normally in many lower animals. On the other hand, the direct pyramidal 

 tracts may appropriate an unusually large number of the fibres, even to 90 per cent, 

 of the entire pyramid, the crossed tract, however, never being entirely unrepresented. 

 Ordinarily the tracts of the two sides are approximately of equal extent, but occasion- 

 ally they may be asymmetrical, in which case the excess of the one is offset by a 

 corresponding diminution in the fasciculus of the opposite side (Flechsig). 



Pyramidal 

 decussation 



Lateral 



pyramidal tract 

 Direct 



pyramidal tract 



Spinal nerve 



Diagram showing course and decussation of cortico- 

 spinal (pyramidal) tract ; M, medulla; P, pons; CP, 

 cerebral peduncle; T, thalamus; C, L, caudate and 

 lenticular nuclei ; CC, corpus callosum. 



