INTERNAL STRUCTURE. 



9 



as groups of nerve-cells, occurring in different parts of the grey matter, as well as 

 to the several portions of the white matter immediately to be described. 



Central canal. Extending through the whole length of the spinal cord, in the 

 substance of the grey commissure, there is a minute canal (fig. 8) which, in sections 

 of the cord, is barely visible as a speck, with the naked eye. It is continued above 

 into the medulla oblongata, where it gradually approaches the posterior surface and 

 eventually opens out at the calamus scriptorius of the fourth 

 ventricle. At the lower end of the cord, near the extremity 

 of the conus medullaris, it becomes enlarged, and shaped like 

 the letter ~|~, and is stated by some observers to open on the 

 dorsal or posterior surface of the cord ; but this is denied by 

 others. This central canal, though minute, is an object of 

 considerable interest as a typical part of the structure of the 

 cord, since it is the permanent remains of the epiblastic canal 

 from which the spinal cord is developed. It is more distinct 

 in lower vertebrata than in mammals. 



White matter. The white substance of each half of the cord completely 

 encloses the grey matter except opposite the posterior horn. This last therefore 

 serves to separate off a smaller posterior white column, which is somewhat wedge- 

 shaped in section and is bounded internally by the posterior median fissure, from the 



Fig. 9. SECTION OF 



THE SPINAL CORD 

 NEAR THE EXTRE- 

 MITY OF THE CONUS 



MEDULLARIS. Mag- 

 nified about six dia- 

 meters. 



Pig. 10. DIAGRAM SHOWING BOTH THE ABSOLUTE AND RELATIVE EXTENT OF THE GREY MATTER AND 



OF THE WHITE COLUMNS IN SUCCESSIVE SECTIONAL AREAS OF THE SPINAL CORD, AS WELL AS THE 



SECTIONAL AREAS OF THE SEVERAL ENTERING NERVE-ROOTS (adapted from Ludwig and Woroschiloff). 



(E. A. S.) 



The sectional areas of the several entering nerve-roots (n.r) as well as the extent of the grey matter 

 {gr), and of the lateral, posterior, and anterior columns of white matter (I, c., p. c., and a. c.}, are 

 represented in superposed curves, the common abscissa of which (abs.) is intersected at equal intervals 

 by as many ordinates as there are pairs of spinal nerves. In the ordinates each millimeter above the 

 abscissa represents about one square millimeter of sectional area. 



rest of the white substance which forms a large antero-lateral while column (figs. 7 

 to 9). The antero-lateral column is sometimes arbitrarily divided into anterior and 

 lateral white columns, the place of passage of the bundles of the anterior nerve-roots 

 being taken as the limit between the two ; but since these are scattered over a 

 considerable part of the transverse section it is clear that the limit cannot be 

 distinctly fixed. 



The white substance is traversed by imperfect septa of connective tissue 

 prolonged inwards from the pia mater. Most of these are irregular and somewhat 

 variable in position, with the exception of one in the cervical region extending 

 inwards towards the grey commissure from the sulcus before described as bounding 

 the postero-mesial column. This, the posterior intermediate septum before men- 

 tioned (p. 7) (fig. 8, s) cuts off a small portion of the posterior column next to the 

 posterior median fissure, corresponding to the projection of the postero-mesial column 

 on the surface. 



