GREY SUBSTANCE OF SPINAL CORD 



775 



roots, by the lateral funiculus; and the space between the ventral roots and the 

 anterior median fissure, by the anterior funiculus. Each of these funiculi is 

 subdivided within into its component fasciculi. 



The dorsal and ventral nerve-roots are not attached to the cord as such, but are 

 first frayed out into numerous thread-like bundles of axones which are distributed 

 along their lines of entrance and exit. These bundles are the root filaments 

 [fila radicularia] of the respective roots. The fila of the larger spinal nerves are 

 fanned out to the extent of forming almost continuous lines of attachment, while 

 in the thoracic nerves there are appreciable intervals between those of adjacent 

 roots. Throughout, the intervals are less between the fila of the ventral than 

 between those of the dorsal roots. 



Internal Structure of the Spinal Cord 



By reflected light masses of medullated axones appear white in the fresh, and 

 such masses are known as white substance. The spinaal cord consists of a 

 continuous, centrally placed column of grey substance surrounded by a variously 

 thickened tunic of white substance. The closely investing pia mater sends 



Fig. 615. 



-A, Ventral, and B, Dorsal, Views of Portion of Spinal Cord showing 

 Modes of Attachment of Dorsal and Ventral Roots. 



Antero-lateral sulcus (line of ventral roots) 

 /Anterior median fissure 



Posteriormedian sulcus 

 / Posterior in- 



numerous ingrowths into the cord, bearing blood-vessels and contributing to its 

 internal supporting tissue. The volume of white and of grey substance varies 

 both absolutely and relatively at different levels of the cord. The absolute 

 amount of grey substance increases with the enlargements. The absolute 

 amount of white substance also increases with the enlargements coincident with 

 the greater amount of grey substance in those regions. The relative amount of 

 white substance increases in passing from the conus medullaris to the medulla 

 oblongata, due to the fact that the ascending and descending axones associating 

 the cord with the encephalon are the one contributed to the cord and the other 

 gradually terminating in it at different levels along its entire descent. 



The grey substance. — In the embryo all the nerve-cells of the grey substance 

 are derived from the cells lining the neural tube, and in the adult the column of 

 grey substance, though greatly modified in shape, still retains its position about 

 the central canal. In transverse section the column appears as a grey figure of 

 two laterally developed halves, connected across the mid-line by a more attenu- 

 ated portion, the whole roughly resembling the letter H. The cross-bar of the H 

 is known as the grey commissure. Naturally, it contains the central canal, which is 

 quite small and is either rounded or laterally or ventrally oval in section, according 

 to the level of the cord in which it is examined. " The canal continues upward, and 

 in the medulla oblongata opens out into the fourth ventricle. Downward, in the 

 extremity of the conus medullaris, it widens slightly and forms the rhomboidal 

 sinus or terminal ventricle, then is suddenly constricted into an extremely small 



