562 THE NEKVOUS SYSTEM 



Two sets of vessels may be said to be distributed from these 

 sources one of which is distributed through the pia mater to the 

 adjacent white matter of the spinal cord and to the grey pallium 

 of the brain ; the other penetrates the spinal cord through the 

 anterior median fissure to be distributed to the central grey matter, 

 and in the brain is represented by the branches of the middle cere- 

 bral arteries which penetrate directly to the ganglionic grey matter 

 in the interior of the cerebrum. 



In the spinal cord the vessels of the former set are mostly distrib- 

 uted to the white cortex, the larger branches, however, penetrate 

 the white matter and aid in the formation of the capillary network 

 of the grey medulla. In the brain their distribution is similar, the 

 smaller pial vessels, the cortical arteries, being distributed to the 

 cortex, which in this case is formed by the grey matter ; the larger, 

 the medullary arteries, penetrating to the white medulla in which 

 they break up into capillary vessels. 



The veins trend in the opposite direction and in the pia mater 

 collect into large vessels, which in the brain open into the sinuses 

 of the dura mater, and which in the spinal cord form the anterior 

 and posterior median veins. 



All of the larger vessels receive thin fibrous investments from 

 the pia mater, the smaller vessels and capillaries are surrounded by 

 neuroglia. 



There are frequent anastomoses between the larger veins ; the 

 arteries, however, are all terminal arteries according to Cohnheim's 

 classification, each artery possessing no anastomosis with the capil- 

 lary areas of other vessels. 



The more important facts concerning the ultimate distribution 

 of the arterial branches are purely macroscopical, and for them the 

 reader is referred to text-books on the general anatomy of the nerv- 

 ous system. 



