On a system of fibre-cells. 539 



or medium size the various elements which enter into the structure 

 and constitution of the perivascular felt- layer can he universally seen 

 to the hest advantage (see Fig*. 5). 



As to the normal and physiological significance of this sheath of 

 felted fibres. In the large vessels especially, this sheath is of note- 

 worthy thickness (Fig. 7): a fact which must be nowise neglected in 

 an investigation of the problems of the dynamics of the brain circu- 

 lation on the one hand, and of the physics of the sustentacular and 

 protecting elements which support the nerve -elements proper on the 

 other. For the interlacing* fibres which unite into the perivascular felt- 

 works must obviously form a structure which on the one hand would 

 oppose considerable mechanical resistence to the undue expansions of 

 the vessels in the cortex and white matter in the various brain areas, 

 and on the other hand by its very texture and porosity would allow 

 of the fi'ee passage and transsudation of lymph and products of meta- 

 bolism, to and fi'o, through its pores and meshes, and thus also allow of 

 the interchange and diffusion of fluid and osmotic currents between the 

 Brain substance and the perivascular lymph spaces. The biological 

 importance of such a structure must therefore be considerable. This 

 function is further correlated to, and derives support from the fact 

 that, the blood-vessels of the brain have not only a small amount of 

 muscular tissue, but are wanting in the tough adventitial elements 

 which vessels elsewhere possess. For the adventitia of the cortical 

 vessels is simply according to all descriptions, a tubular membrane 

 composed of a single layer of soft protoplasmic cells, while the bundles 

 of areolar tissue which are superadded to it in other organs, are absent 

 in the Brain cortex. 



The development of a felted sheath of neuroglia fibres in the 

 gi'ound-substance immediately surrounding the blood vessels of the 

 Brain seems therefore to the author of obvious significance, affording 

 by such means avoidance of undue local pressui'e on, and consequent 

 rupture, damage, or destruction of the delicate protoplasmic processes, 

 naked nerve-fibrils, and collaterals which everywhere pervade the grey 



