170 THE ACTIVE PAKT OF A NEEVE-FIBKE, 



tte higher animals at an early period of development, 

 a precisely similar arrangement. In this delicate 

 membranous structure no separate nerve cords or 

 fibres can be detected, but delicate tracks crossing 

 one another at various angles may be discerned. 

 Runnins: in the same direction as the tracks are oval 

 masses of bioplasm. The lines, I believe, indicate the 

 paths taken by the nerve cui'rents as they traverse 

 the delicate nerve tissue in passing to and from the 

 nerve centres. 



228. Irreconcilcable opinions concerning the struc- 

 ture and action of a nerTC apparatus.— At this pre- 

 sent time there is the greatest disagreement among 

 authorities upon fundamental questions. It is not 

 even determined whether nerves terminate in ends 

 or form continuous circuits, — nor whether they in- 

 fluence tissues by reason of their being in structural 

 continuity with them or merely indirectly, in conse- 

 quence of currents passing along the nerve- fibres 

 situated at some short distance from the particles of 

 tissue to be influenced. And it is not known whether 

 the influence is produced by the passage of a con- 

 tinuous current varying in intensity, or by an inter- 

 rupted current. Nor is thei^e more accord as to the 

 origin of nerves in centres ; some holding that the 

 fibres invariably originate in cells ; others, that some 

 cells have no fibres at all connected with them. And 

 of those who admit the first proposition, some think 

 the fibre comes from the body of the cell, while others 

 profess to have traced it into the niicleus. Some 

 even assert that they have seen the fibre emanating 

 from the nucleolus. Or, again, concerning the fibres, 

 which unquestionably originate in nerve cells, it has 

 been stated that some pass into nerve-fibres, while 

 others have no special relation to nerves at all. 



229. The active part of a nerve-flbrc. — It is most 

 remarkable how very closely the nerve-fibre in the 

 lowest and most simple creatures which possess a 



