538 W. L. Andriezên, 



and numerous like the holes between the warp and woof of a cloth. 

 The structure however is rather more complicated by the fact that 

 some of the fibres run in an oblique direction (cf. Figs. 2, 5, and 6 a); 

 thus making it more of a true feltwork, 



Besides these above 2 varieties of cell-elements which lie in the 

 ground-substance bordering the vessels and form the perivascular felt- 

 work, the ordinary stellate neuroglia cells which are scattered throughout 

 the general ground-substance (Fig. 5 h and 6 h) also contribute a few 

 fibres which enter into the constitution of the perivascular feltwork 

 (see Figs. 5 and 6). The fibres contributed by these ordinary stellate 

 cells are often seen as fibres which run into the perivascular feltwork 

 almost at right angles (Figs. 2}^, 3jj) and on reaching near the border 

 turn off at right angles and course in a direction parallel to the vessel's 

 direction. The majority of these extrinsic fibres, extrinsic because derived 

 from distant cell-bodies — run, as just mentioned, parallel to the vessels. 

 A smaller number can be seen however running transversely over the 

 circumference and, after describing such a path — frequenti}^ a semi- 

 circumference — over the vessel, emerge from the felt-work at the 

 other side and soon terminate in the gTound-substance (Fig. 1 a). 



A final word with regard to the direction of the longitudinal 

 fibre both intrinsic and extrinsic. But few of the fibres are rigorously 

 longitudinal, for apart from their wavy course, those which exhibit a 

 slight obliquity really can often be traced along the vessels' surface 

 running in a spiral direction, which again in fortunate preparation may 

 be seen exliibiting a half or even one complete turn around the vessel, 

 after the manner of the thread of a screw. 



In the larger vessels where the felted sheath is tliickest (Fig. 6) 

 it is by no means easy to unravel and follow the various fibre ele- 

 ments troni their origin to their ending. In examinig however a large 

 inimber of specimens with strong and wide-angled illumination, the 

 eye soon gets accustomed to recognize the intimate structure of the 

 perivascular feltwork, and in oblique sections of blood vessels of small 



