THE NERVOUS SYSTEM. 155 



vertebne and cord, as well as in the sinuses and lax adipose tissue 

 of the canal (Luschka, 1. c). 



In the arachnoid itself I have never noticed any nerves, but 

 on the vessels by which it is penetrated, and in the processes 

 connecting it with the pia mater, they may perhaps be seen, 

 especially at the base of the brain — to which nerves, those seen 

 by Luschka (Serose Haute, p. 70), notwithstanding the divisions 

 observed in them, appear to me to belong. Bochdalek (1. i. c.) 

 has lately described nerves of the cerebral arachnoid, derived 

 from the n. accessorius, the portio 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 con- 

 nective 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 Bochdalek 

 equally supposes he has traced them. 



The nerves discovered by Purkinje in the pia mater of the 

 Ox, also exist in man, in Avhom the pia mater of the cord, 

 including the filum terminate, is richly supplied with plexuses of 

 fine nerves, measuring 0-0015 — O'OOS'", which throughout do 

 little more than accompany 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 O^OS'" in diameter, are dis- 

 tributed through the entire pia mater of the brain, accompany- 

 ing 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; Avhether there are any 

 or not on the vena Galeni, 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 



