4 Journal of Comparative Neurology. 



once be examined microscopically. Relatively large pieces of 

 pia-mater may in this way be obtained, practically free from 

 brain tissue. I have for instance a number of times been able 

 to obtain, in one piece, the pia-mater from the base of the 

 brain of a cat, with that covering the lower surface of the med- 

 ulla and upper cervical cord, which, although slightly lacerated, 

 would contain a complete circle of Willis with its main branches, 

 the basilar and a portion of the vertebrals. When it was found 

 that the perivascular nerves were well stained, the pieces of 

 pia-mater were fixed either in an ammonium picrate solution, 

 (Dogiel) or in the ammonium molybdate solution (Bethe). 

 Some of the ammonium molybdate fixed tissue was hardened 

 in alcohol, sectioned and counterstained in alum-carmine. I 

 have given the method used by me somewhat in detail, since 

 Gulland (i) mentions having used the methylen blue method in 

 his research, without success however. If I read his account 

 correctly, the methylen blue method used by Gulland, was the 

 modification of Ehrlich's method suggested by Dogiel, accord- 

 ing to which a dilute solution of methylen blue, made with nor- 

 mal salt solution, (i-iooo used by Gulland) is applied to the tis- 

 sues. This method was used by me, with negative results. 



The existence of a perivascular plexus in the arteries carry- 

 ing blood to the brain, seems beyond question. A number of 

 anatomies consulted with reference to this point mention them. 

 We read in Quain that the ascending carotid branches of the 

 superior cervical ganglion, as they ascend into the skull with 

 the internal carotid artery, divide into two parts, known respect- 

 ively as the internal and external division. The terminal parts 

 of these two divisions extend to the cerebral and ophthalmic 

 arteries, around which they form secondary plexuses, those 

 on the cerebral arteries ascending to the pia mater. 



And again : * ' From the lowest cervical and first dorsal 

 ganglion, slender grey branches ascend along the vertebral 

 artery in its canal, forming a plexus (plexus vertebralis) around 

 the vessels by their intercommunications, and supplying it with 

 offsets. This plexus is connected with the cervical spinal nerves 

 as they cross the vertebral artery, and its ultimate ramifications 



