14 Journal of Comparative Neurology. 



larger pial vessels, these non-medullated, varicose nerves form a 

 primary plexus in the adventitia, the strands of which consist of 

 small bundles of non-medullated fibers, and of single fibers. The 

 small bundles of non-medullated nerves undergo repeated 

 division ; these, with the single fibers which also branch fre- 

 quently, form an interlacing network — a nerve plexus. Within 

 this plexus is formed a second, though not so well defined plexus, 

 which would seem to lie just external to the muscular coat. 

 From this, the terminal fibrils which end in the muscular tissues 

 of the vessels, are given off. In Fig. 3 is sketched a short seg- 

 ment of one of the larger branches of the middle cerebral in the 

 pia of a cat. In the figure the nerve plexus on that portion of the 

 wall of the vessel turned toward the observer is reproduced. In 

 the preparation from which this sketch was made, the sensory 

 fibers were not well stained, their endings not at all, in the region 

 of the vessel shown in the figure. It may serve to show the 

 richness of the nerve supply of the pial vessels. The plexus 

 in the adventitia being particularly well shown, the terminal 

 fibrils could not be clearly made out with the magnification used, 

 and are therefore not shown in the figure. In the smaller pial 

 vessels with a muscular coat consisting of two to four layers of 

 involuntary cells, only one plexus of non-medullated nerves is 

 made out. In Fig. 4 is reproduced such a plexus. The sketch 

 was made from a methylen blue preparation of the pia of 

 a dog as seen under the 1-12 in. oil immersion. In one 

 portion of this small artery, the non-striated muscle cells 

 were well stained — not shown in figure — and in optical 

 section it was made out that in this portion of the vessel 

 there were present two layers of involuntary muscle cells 

 in the media. I have found it much more difficult to stain 

 the perivascular nerves on the smaller vessels of the pia ; yet 

 they have been found often enough in the dog, cat and rabbit 

 in the pia-mater removed from various regions of the cere- 

 bral hemispheres so that I conclude that they are generally pres- 

 ent, and that the difficulty experienced in staining them is due 

 largely to the uncertainty of the method used. 



That the ultimate branches of the fibers forming the perivas- 



