THE SPINAL MEDULLA. 



521 



stance, composed at first mainly of the axons 

 intercalated cells in the spinal medulla; and 

 as these funiculi increase in size they help 

 to mould the form of the gray columns. 

 This is displayed best in the case of the 

 posterior column (O.T. posterior cornu). The 

 major portion of the white substance, funiculus 

 posterior, which accumulates behind (and after- 

 wards lies on the medial side of) the posterior 

 column, does not consist of fibres springing 

 from intercalated cells, either of the spinal 

 medulla or any other part of the central 

 nervous system, but of the direct continua- 

 tions of the central processes of the cells in the 

 spinal ganglion on the posterior root (Figs. 463 

 and 464). A large proportion of the fibres of 

 the posterior root do not enter the gray 

 columns immediately after their insertion into 

 the alar lamina, but bifurcate to form two 

 vertical nerve-fibres, one passing upwards, and 

 the other downwards, in the funiculus posterior 

 before they end in the gray column, some 

 distance above or below the place where they 

 gained admission to the medulla spinalis. As 

 the spinal medulla grows, the originally blunt 

 posterior column becomes drawn backwards 

 into an increasingly attenuated process, and the 

 funiculus posterior, which was placed originally 

 upon its lateral surface (Fig. 464, A), and then 

 upon its posterior surface (Fig.464,B), gradually 

 issumes a wedge-shaped form (Figs. 464, C, and 

 166), upon the medial side of the gray matter. 

 Development of the Anterior Median 

 Fissure, Posterior Median Septum, and of 

 :he Central Canal. As the anterior columns 

 )f gray matter and the anterior fuuiculi of 

 vhite matter increase in size, the anterior 

 .urface of the spinal medulla, on each side 

 >f the median plane, bulges forwards, and the 

 issura mediana anterior (Fig. 464, A, B, and C) 

 s produced as the natural result. 



There has been considerable discussion as 

 o the mode of formation of the posterior median 

 eptum ; but there is now no doubt as to the 

 issential facts. Early in the third month the 

 vails of the posterior three - fourths (of the 

 agittal extent) of the central canal of the spinal 

 nedulla become approximated (Fig. 464), and 

 ater they fuse to obliterate that part of the 

 anal. But the part of the septum thus formed 

 3 only an insignificant portion of the whole. 

 ?or most of the septum is produced by the 

 gradual elongation of the epithelial cells lining 

 he remnant of the central canal as the fibre- 

 lasses of the posterior funiculi expand and 

 leparate the posterior surface of the spinal 

 nedulla further and further from the situation 

 f the canal (see Fig. 453, p. 511). 

 Furrows of the Spinal Medulla. When 



of cells in the root ganglia and 



Roof-plate 



ir lamina 



Early posterior 

 funiculus 



Anterior 

 nerve-root 

 'Anterior 



Mid-ventral lamina funiculus 



A 



Fasciculus gracilis 



Fasciculus cuneatus 

 Posterior median 

 septum 



Posterior 

 column 



Posterior 

 nerve-root 



Ependyma 



Anterior nerve-root 

 Anterior funiculus 



B 



Fasciculus gracilis 



Fasciculus cuneatus 



Posterior median septum 



Posterior column 

 Posterior root 



Ependyma 



Anterior 

 column 



Anterior median 

 fissure 



Anterior root 



Anterior funiculus 



FIG. 464. THREE STAGES IN THE DEVELOP- 

 MENT OF THE SPINAL MEDULLA (His). 



cross-sections of the adult spinal 



