6 Journal of Comparative Neurology. 



successfully stained methylen blue preparations, perivascular 

 nerves may be clearly seen on the vessels constituting the circle 

 of Willis, and on the larger arterial branches that proceed from 

 it, and may from here be traced on to the arteries of the pia- 

 mater of the hemispheres. Even under low magnification, how- 

 ever, it becomes obvious that such perivascular nerves do not 

 all present the same characteristics. On the one hand, and this 

 more particularly on the larger vessels, well formed bundles of 

 medullated nerve fibers are observed ; on the other, plexuses 

 of non-medullated nerve fibers. To anticipate somewhat, I may 

 state, for considerations which will follow, that I have come to 

 regard the medullated nerve fibers as sensory nerve fibers, the 

 perivascular plexuses of non-medullated fibers as vaso-motor 

 fibers. I shall therefore consider the nerves associated with the 

 arteries of the cranial pia-mater under these two heads. 



Sensoy nerves. In all well stained methylen blue prepara- 

 tions of the pia-mater, whether of the dog, cat or rabbit, I have 

 observed relatively large medullated nerve fibers accompanying 

 the blood vessels, not only the larger vessels found on the base 

 of the brain and medulla, but following the vessels over the con- 

 vexity of the cerebrum to the great longitudinal fissure. These 

 medullated nerve fibers reach the pial vessels, so far as I have 

 been able to determine, from two main sources. A relatively 

 large bundle of medullated nerves — sometimes several small 

 ones — ^joins the middle cerebral just after it gives off the pos- 

 terior communicating branch. This, as a rule, divides into two 

 branches, one joining the middle cerebral, the other the pos- 

 terior communicating artery. The other source of medullated 

 nerve fibers is from one or several bundles of medullated nerve 

 fibers, which proceeds upwards on the basilar artery until it 

 divides into the posterior cerebrals; the nerve-bundles on the 

 basilar undergo a similar division and follow the branches 

 of this artery. The bundles of medullated nerves on the 

 basilar artery are formed from such found on the vertebral 

 arteries. Fig. i may serve to illustrate these statements. In 

 the figure the circle of Willis with a portion of the basilar artery, 



