THE NERVOUS SYSTEM. 399 



derived from the n. accessorius, iheportio minor trigemini, and the facial 

 nerve, but fails to show that they terminate in the membrane. When 

 the same author also finds extremely numerous nerves in the arachnoid 

 covering the cauda equina, he falls into the same error, as Rainey had 

 previously encountered in regarding connective tissue, disposed in the 

 more rare reticular form, as nerves. In the cauda equina, I am 

 acquainted with nerves only on the filum terminate, accompanying the 

 vessels, and nowhere else ; not even in the dura mater, into which Boch- 

 dalek equally supposes he has traced them. 



The nerves discovered by Purkinje in the pia mater of the Ox, also 

 exist in man, in whom the pia mater of the cord, including the filum 

 terminate, is richly supplied with plexuses of fine nerves, measuring 

 0*0015-0-003 of a line, which throughout do little more than accom- 

 pany the vessels. At the base of the brain, many similar plexuses 

 occur on the arteries of the circle of Willis, which, in twigs, at most 

 0-03 of a line in diameter, are distributed through the entire pia mater 

 of the brain, accompanying and always following the course of the 

 various vessels, with the exception of those of the cerebellum ; their 

 terminations, however, can nowhere be perceived. It is certain that 

 they do not accompany the arteries into the cerebral substance, and 

 that no nerves exist in the vascular plexuses ; whether there are any 

 or not on the vena Cf-aleni, I have not yet inquired. The origin of these 

 nerves has been ascertained by Remak to be in the posterior roots, each 

 of which, as I have satisfied myself, in many situations, and as it appears 

 to me more frequently in the cervical portion of the cord, from the 

 fibres in closest contiguity to each other, sends out fine fibrils across the 

 subarachnoid space into the pia mater. As in this case, so also in the 

 cerebrum, besides the sympathetic nerves (plexus caroticus internus, 

 plexus vertebralis), the cerebral nerves may participate in the supplying 

 of the pia mater, since Bochdalek has noticed numerous fine twigs, 

 given off from the roots of many of the cerebral nerves, of the same 

 structure as the roots themselves, joining the nervous plexuses of the 

 arteries at the base of the brain and of the pia mater of that region, 

 and of the cerebellum, as well as in the plexus chorioideus ventric. 

 quart. (?). He also found that isolated fine filaments entered the pia 

 mater, directly from the medulla ollongata, the pans Varolii, and crura 

 cerebri, which were not previously conjoined with the neighboring ner- 

 vous trunks. 



B. Vessels of the central nervous system. With respect to the dis- 

 tribution and condition of the bloodvessels the brain and spinal cord 

 agree almost entirely. After ramifying to a considerable extent in the 

 pia mater, the arteries enter the nervous substance, except in a few 

 situations (substantice perforator, pons), as fine, though still distinct, 



