SECT. H7.] 



NERVOUS SYSTEM. 



24.I 



Fig. Oft. 



exist in the dura mater of the cord, although they occur upon the 

 venous sinuses of the vertebral canal. 



The pia mate)' of the cord is everywhere rich in networks of 

 fine nerves, which arc not confined to the course of the vessels. 

 At the base of the brain, numerous nervous plexuses exist upon 

 the arteries of the circle of Willis, which, following the course of 

 the different arteries, with trunks of, at most, 0-03'" in diameter, 

 spread out through the whole pia mater of the brain. Although 

 their terminations can nowhere be recognised, still I traced 

 these nerves recently upon arteries of 004'" and less in diameter 

 into the substance of the brain itself The choroid plexuses and the 

 arachnoid are destitute of nerves. All the nerves of the pia mater 

 arise from the posterior roots of the spinal nerves, from the sym- 

 pathetic, and from the roots of some of the cerebral nerves. 



B. Vessels of the Central Nervous System. In the character 

 and mode of distribution of their blood-vessels, the brain and the 

 cord almost entirely agree. After the arteries have ramified to a 

 considerable degree in the pia 

 mater, they penetrate into the 

 nervous substance, except at 

 a few places {substantia perfo- 

 rata?, pons) as fine, though still 

 distinctly arterial vessels, and 

 break up by continued ramifi- 

 cation, mostlv at acute angles, 

 into a tolerably wide mesh- 

 work of very fine capillaries, 

 from which the roots of 

 veins arise, and unite to form 

 the well-known trunks, both 

 upon the surface and in the 

 interior (fig. 99). The grey 

 substance is invariably much 

 more vascular than the white, 

 the plexuses being much closer, 

 and the vessels themselves 

 somewhat narrower, to which 

 circumstance its colour is 

 partly due. In the spinal cord, 

 the entering vessels are, in part, very regularly arranged in series. 

 Two such series exist at the bottom of the anterior fissure, enter- 

 ing the grey substance, right and left from the process of the pia 



R 



V. reels of the cerebral snl^tanceof the sheep, 

 after an injection of Gerlach's, a. of the grey, 



b. of the white substance. 



