226 THE ESSENTIALS OF HISTOLOGY. 



in amount (L 5). The isthmus lies nearly in the centre of the cord, 

 whereas in the cervical and dorsal regions it is nearer the anterior 

 surface. 



In the part of the spinal cord from which the sacral and coccygeal 

 nerve-roots take origin the grey matter largely preponderates, the 

 crescents forming thick irregular masses, and the grey isthmus is 

 also of considerable thickness. 



Blood-vessels of the spinal cord. The blood-supply of the grey 

 matter is derived mainly from a series of arterioles, which come off 

 from the mesially-situated anterior spinal artery, pass into the anterior 

 median fissure, and at the bottom of this divide each into two branches, 

 one for the grey matter of each lateral half of the cord. In the grey 

 matter is a very close capillary plexus which is supplied not alone by 

 the vessels just mentioned, but also by small arterioles, which con- 

 verge from the small arteries of the pia mater, passing through 

 the white matter, and supplying this as they pass through it. These 

 arterioles are branches of the above-mentioned anterior spinal artery 

 and of the posterior spinal arteries (which run on each side along the 

 line of the posterior roots). The capillary plexus of the white matter 

 is far less dense than that of the grey matter. It forms longitudinal 

 meshes. 



The veins of the spinal cord accompany the arteries. Two longi- 

 tudinal venous vessels, accompanying corresponding anastomotic 

 arterioles are seen, one on either side of the central canal, in most 

 transverse sections of the cord. 



