THE SPINAL COED 481 



The white matter forms a covering or shell around the central 

 grey mass. It increases in thickness from below upward. This 

 peculiarity is the result of the constant addition of centripetal 

 fibres, and a corresponding loss of centrifugal fibres, through the 

 spinal nerves of each successive segment. 



The posterior median septum extends inward from the shallow 

 sulcus on the dorsal surface of the spinal cord to the central grey 

 commissure, and divides the posterior mass of white matter into 

 two dorsal white columns, lying on either side of the median line, 

 and bounded laterally by the dorsal horns of grey matter and the 

 dorsal nerve roots. The anterior median sulcus in a similar man- 

 ner, splits the ventral portion of white matter into the two ante- 

 rior white columns. This sulcus, however, does not penetrate all 

 the way to the grey commissure but leaves an interval of white 

 matter containing many transverse and obliquely disposed nerve 

 fibres. The ventral or white commissure thus formed connects 

 the two anterior columns of white matter. 



The spinal cord is thus divided into two lateral and symmetrical 

 halves by a plane passing through the anterior and posterior me- 

 dian fissures and the central canal. Each lateral half includes a 

 central mass of grey matter completely surrounded, except at the 

 grey commissure, by the white matter. The latter is subdivided 

 into an anterior, lateral, and posterior column, each of which ex- 

 tends the entire length of the spinal cord and is apparently (to 

 the naked eye only) continuous above with a similar column in 

 the medulla oblongata. 



The anterior white column is included between the anterior 

 median sulcus and the ventral grey horns and nerve roots; the 

 lateral columns extend from the ventral roots in front, around the 

 lateral surface of the spinal cord, to the dorsal roots ; the dorsal 

 or posterior columns are included between the dorsal horns of grey 

 matter and dorsal nerve roots, and the posterior median septum. 



Each of these columns of white matter is again subdivided by 

 connective tissue septa of variable size and number, which extend 

 inward from the pia mater for a considerable distance. Such 

 septa may even penetrate all the way to the central grey matter. 

 One of these septa, more constant than the others, subdivides the 

 posterior column into two portions, a poster o-internal and a postero- 

 external column. 



The larger blood vessels are distributed along the fibrous septa, 

 taking their origin from the vessels of the pia mater ; the most of 



