WHITE MATTER OF THE SPINAL CORD. 



1043 



likely that these areas represent the principal aggregations of the downward coursing limbs of 

 the axones, derived from the posterior horn-cells of the same and opposite sides. In the 

 cervical region the descend- 



ing limbs of the posterior 

 root fibres appear as the 

 comma tract; in the lower 

 thoracic cord these are re- 

 placed by, without being di- 

 rectly continuous with, those 

 forming the* oval field, and 

 these in turn by the axones of 

 the triangular bundle. No one 

 of these fields is exclusively 

 devoted to the descending 

 limbs of endogenous fibres, 

 since in all the presence of 

 exogenous posterior root- 

 fibres has been demonstrated. 



The Fibre -Tracts 

 of the Lateral Column. 

 — These include : ( 1 ) the 

 lateral pyramidal, (2) the 

 direct cerebellar, (3) the 

 ascending antero-lateral, 

 and (4) the lateral ground- 



FiG. 897. 



Section of spinal cord at level of seventh cervical segment ; anterior 

 cornua are less robust ; root-zone is seen just behind Lissauer's tract. X 6- 

 Preparation by Professor Spiller. 



bundle. 



The lateral or crossed pyramidal tract (fasciculus cerebrospinalis lateralis) 

 forms the chief path by which motor impulses originating in the cerebral cortex are 

 conveyed to the spinal cord. It stands in close relation with the direct pyramidal 

 tract of the anterior column. Both are continuations of the conspicuous pyramidal 

 paths of the medulla oblongata and may be followed upward through the ventral 

 part of the medulla, the pons and the cerebral peduncles into the white matter of the 

 cerebral hemispheres and on to the cortical gray matter where, in the motor areas 

 bordering chiefly the Rolandic fissure, lie the nerve-cells from which the pyramidal 

 fibres arise. These fibres, therefore, are the axones of cortical motor neurones and 

 extend without interruption from the superficial gray matter of the cerebral hemi- 

 spheres to various levels in the cord, constituting long descending (corticifugal) 

 motor tracts. On reaching the lower part of the medulla, from 80-90 per cent, of 

 the component fibres of each pyramid cross to the opposite side by way of the 

 decussation of the pyramids (page 1065) and, entering the cord, descend as the 

 lateral pyramidal tract; the remaining fibres (on an average, about 15 per cent.) 

 pass downward into the ventral column of the cord as the direct pyramidal tract. 



After decussating, the crossed pyramidal tract passes outward to enter the lateral 

 column of the cord, thereby exchanging its former median and superficial position 

 for a deeper and more lateral one. Since its fibres are continually entering the gray 

 matter to end about the radicular cells from which the anterior root-fibres of the 

 spinal nerves arise, the tract progressively loses in size as it descends, until, at about 

 the level of the fourth sacral nerve, it ceases to exist as a distinct strand, although 

 continued by small scattered bundles of fibres as far as the origin of the coccygeal, 

 nerve. This diminution is not regular, since in the sacral and lumbar enlargements 

 the loss is more marked than elsewhere, on account of the relations of the tract-fibres 

 to the large motor limb- nerves. 



The relations, as well as size, of the lateral pyramidal tract vary at different levels. As 

 seen in cross-sections of the upper thoracic region of the cord, the tract occupies an area of 

 considerable size, that mesially lies against the posterior horn and laterally is in contact with 

 the direct cerebellar tract, by which it is excluded from the periphery. In front, where its limits 

 are less definite, the tract extends ventrally for a variable distance into the lateral column, but 

 seldom overreaches the plane of the gray commissure. W^ith the diminution and disappear- 

 ance of the direct cerebellar tract within the lower portions of the cord, the pyramidal field 

 approaches and finally reaches the surface, which relation it retains as it grows smaller, the 



