454 THE ORGANS 



of section relative to transverse section above described. Note in the white 

 matter longitudinally-running fibres from which branches pass off into the gray 

 matter (Fig. 318). The longitudinal fibres of the posterior columns are the as- 

 cending and descending branches of the central processes of the spinal ganglion 

 cells, and the branches passing into the gray matter are their collaterals and 

 terminals. If the section happens to include the entering fibres of a posterior 

 root, these can be seen branching in the posterior columns into ascending and 

 descending arms (Fig. 318). The longitudinal fibres of the lateral and anterior 

 columns are axones of column cells and of cells situated in higher centres (see 

 pages 450 and 451). These also send collaterals and terminals into the gray 

 matter. 



General Topography of the Cord, Cell Groupings, Arrangement of 



Fibres and Finer Structure 



The further description of the cord is best combined with the 

 practical study of sections of the cord, taking sections through the 

 lumbar enlargement as a type. 



PRACTICAL STUDY OF SECTIONS THROUGH LUMBAR 



ENLARGEMENT 



General Topography (Figs. 319 and 320). — The general features of the sec- 

 tions can be best seen with the naked eye or with a low-power dissecting lens. 



Note the shape and size of the cord, and that it is surrounded by a thin mem- 

 brane, the pia malcr spinalis; the deep anterior median fissure, into which the 

 pia mater extends; the posterior median septum consisting principally of neuroglia, 

 over which the pia mater passes without entering; and the postero-lateral grooves 

 or sulci at the entrance of the posterior root fibres. The gray matter is seen in 

 the central part of the section (stained more lightly in the Weigert preparation, 

 on account of the presence of numerous unstained cell bodies and dendrites) and 

 arranged somewhat in the form of the letter H. Dorsally the gray matter extends 

 almost to the surface of the cord as the dorsal gray columns {posterior horns or 

 cormia). The ventral gray columns {anterior horns) are, on the other hand, 

 short and broad, and do not approach the surface of the cord. Surrounding the 

 gray matter is the white matter (stained deep blue in the Weigert preparation). 

 This is divided by the posterior horn into two parts, one lying between the horn 

 and the posterior median septum, the posterior funiculus {dorsal white column); 

 the other comprising the remainder of the white matter, the antero-lateral funicu- 

 lus {ventro -lateral white column). This latter is again divided by the anterior 

 horn and anterior nerve roots into a lateral funiculus {lateral white column) and 

 an anterior funiculus {ventral white column). In the concavity between the an- 

 terior and posterior horns some processes of the gray matter extend out into the 

 white matter where they interlace with the longitudinally running-fibres of the 

 latter to form the reticular process (not well marked in the lumbar cord). 



For the study of further rletails the low-power objective should be used. 



