WHITE MATTER OF THE SPINAL CORD. 



1049 



intervertebral efferents carry the blood into the vertebral, intercostal, lumbar and 

 lateral sacral veins. A part of the blood from the intrapial plexus is conducted 

 upward by the anterior and posterior median veins into the venous net-work covering 

 the pons and thence into the lower dural sinuses. 



Definite lymphatic vessels within the spinal cord are unknown. 



Development of the Spinal Cord. — A sketch of the general histogenetic processes leading 

 to the differentiation of the neurones and the neuroglia has been given (page 1009) ; it remains, 

 therefore, to consider here the changes in the neural tube by which the definite spinal cord is 

 evolved. From the time of its closure, probably about the end of the second week of fcetal 

 life, the neural tube presents three regions : — the relatively thick lateral walls and the thin ven- 

 tral and dorsal intervening bridges, "Cn^ floor- and roof-plates, that in front and behind complete 

 the boundaries of the canal in the mid-line. By the fifth week the lateral walls exhibit a distinct 

 differentiation into three zones — the inner ependymal layer, the middle nuclear layer and the 

 outer marginal layer, surrounded by the external limiting membrane. In contrast to the other 

 two, the rharginal zone is almost devoid of nuclei and, beyond affording support and perhaps 

 assisting in providing a medullary coat, plays a passive role in the production of the nervous 

 elements. 



By this time the former general oval contour of the developing cord, as seen in cross-sec- 

 tions, has become modified by the conspicuous thickening of the antero-lateral area of the nuclear 

 layer into a prominent mass on each side, whereby the reticular marginal layer is pushed out- 



FiG. 904. 

 Roof-plate 



Fig. 905. 



Roof-plate 



Dorsal zone 



Dorsal' 

 root-fibres 



Dorsal 

 root-fibres 



Ependymal 



"ayer 



\'entral 

 root-fibres 



Neuroblasts 



Floor-plate 



Developing spinal cord of about four 

 weeks. X 100. (His.) 



P'loor-plate 



Ventral root-fibres 



Developing spinal cord of about five weeks. 

 X 60. (His.) 



ward with corresponding increase in the width of the entire ventral part of the cord, which is 

 now broadest in front. Within this thickened ventro-lateral part of the nuclear layer, later the 

 anterior horn of gray matter, as early as the fourth week young neurones are seen from which 

 a.xones grow outward through the marginal zone and pierce the external limiting membrane as 

 the representatives of the anterior root-fibres of the spinal nerves. Postero-laterally the thin 

 nuclear layer is covered by a somewhat projecting thickened area within the marginal layer, 

 known as the oval bundle, whose presence is due to the ingrowth of the developing dorsal root- 

 fibres from the sensory neurones of the spinal ganglion, which process begins as early as the 

 end of the fourth week (His). 



Associated with these changes, the lumen of the cord becomes heart-shaped in consequence 

 of a conspicuous local increase in its transverse diameter, with corresponding bulging of the 

 lateral wall. In this manner a longitudinal furrow appears by which the side walls of the tube 

 are differentiated into two tracts, the dorsal and the ventral zones (the alar and basal laminae of 

 His). This subdivision is of much importance, since in the cord- segment, and ajso with less 

 certainty in the brain-segment of the neural tube, these tracts are definitely connected with the 

 root-fibres of the spinal nerves, the dorsal zone with the sensory and the ventral zone with the 

 motor roots. In advance of the floor-plate the ventrally protruding halves of the cord include a 

 broad and shallow furrow which marks the position of the anterior median fissure. During the 

 sixth week the form of the tube-lumen becomes further modified by the elongation and narrow- 



