SPINAL COED AND NEKVES 



281 



the height of the 5th and 6th cervical vertebrae and ends at the level 

 of the 2nd and 3rd thoracic vertebrae. The lumbo-sacral enlarge- 

 ment, which comprises the segments that send roots to the lumbo- 

 sacral plexus, begins at the level of the 10th dorsal vertebra and is 

 largest at the level of the 12th. Next comes the conus medullaris, 

 which terminates at the level of the 1st. or 2nd lumbar vertebra 

 in the filum terininale, by which the cord is attached to the 

 coccyx. 



The cord as a whole is enclosed in a sheath (theca) formed of 

 a dense fibrous membrane, the dura mater, which is attached to 



FIG. 169. Diagrammatic transverse section of spinal cord. (Brb.) , fissura longitudinalis ventralis ; 

 b, f. 1. dorsal ; c, ventral column ; d, lateral column ; e, dorsal column ; }, funiculus gracilis ; 

 g, fnniculus cuneatus ; h, ventral ; i, dorsal root ; k, central canal ; I, sulcus intermedium 

 dorsalis ; m, ventral horn ; n, dorsal horn ; o, tractus intermedio - lateralis ; d, processus 

 reticularis ; g, white or ventral commissure ; r, grey or dorsal commissure ; s, Clarke's 

 column or colnmna vesicularis. 



the periosteum that lines the interior of the vertebral canal. 

 Enclosed in the dura niater, the cord is protected against external 

 pressure, and readily gives, without undue strain, to the twisting 

 and displacement caused by the movements of the vertebral 

 column. In fact there is a space between the dura mater and the 

 cord, filled with a lymphatic fluid known as the ccrebrospinal fluid, 

 which is continually formed as fast as it diffuses through the 

 lymphatic spaces in the spinal roots. 



Inspection of a transverse section of the spinal cord (Fig. 169) 

 shows the arrangement of the central grey matter and the 

 peripheral white matter, but comparison of a series of transverse 

 sections made at different levels (Fig. 170) shows that different 

 regions present special characteristics and variations in form, 



