THE CIRCULATION OP CEREBRO-SPINAL SUBSTANCE. 587 



cellular elements, but also asepticized plasma. The outside of 

 a vessel covered with a neuroglia-ridden sheath is shown in 

 Fig. 3. 



The two kinds of neuroglia-cells may be seen to take part 

 in the formation of the external net-work of fibrils in the illus- 

 tration on page 586. It is here, it seems to us, that the gov- 

 erning attribute of the "protoplasmic neuroglia-cell" shows 

 itself, as suggested by the dendritic appearance of its exten- 



f/g.3 



BLOOD-VESSEL OF THE HUMAN BRAIN, SHOWINO SEVERAL NEU- 

 ROGLIA FIBER-CELLS SURROUNDING IT AND FORMING A FELT-WORK 

 (PERIVASCULAR SYSTEM). (Andriezen.) 



a, An encircling cell. 6, Perpendicular neuroglia-flber entering the 

 sheath at right angles from a distant (extrinsic) cell. 



sions. Indeed, Eanvier has been lead to conclude that the 

 other variety of neuroglia (stellate) cell is not much more 

 than a mass of aggregated fibrils in transit, the latter passing 

 through them without forming part of the cellular structure. 

 Still, as it contains a nucleus, it may possess a regulative ac- 

 tion. The larger cell, however, imposes itself as an organ of 

 a very active kind capable probably of playing in the brain 

 the part that the vagus plays in various organs: *.., to incite 



