THE BRAIN. 



409 



the whole central nervous system, to which it gives 

 substance and support. It is to be remembered that 

 supporting tissue of mesodermic origin does the same 

 thing, especially in the cord. The connective-tissue 

 elements usually accompany the nutrient blood- 

 vessels. 



BLOOD-VESSELS OF THE CENTRAL NERVOUS SYSTEM. 



The spinal cord receives its blood supply from a 

 plexus of blood-vessels in the pia mater. There is an 

 anterior median artery just in front of the anterior 

 fissure. Some two hundred branches from this 

 vessel pass at right angles directly into the fissure 

 and enter the gray matter, where each divides into 

 a right and left branch that enclose the central 

 canal. Each arterial branch ultimately bifurcates, 

 just in front and external to the cell column of 

 Clarke, forming minute ascending and descending 

 terminals, which become lost in an extensive capil- 

 lary system of the central gray matter. The white 

 matter receives its blood supply from a plexus of 

 vessels situated on the dorsal and lateral surfaces of 

 the cord. From this system small branches enter 

 the cord anywhere and form capillaries among the 

 nerve fibres; that is, supplies blood to the white 

 matter. The gray matter has a more liberal blood 

 supply than the white matter. 



The brain substance also receives its blood supply 

 from a plexus of blood-vessels in its pia. The capil- 

 laries are particularly numerous and closely meshed 

 wherever the nerve cells are segregated, that is, 

 in the ganglion centers. In the cerebellum the gran- 



