DARK-BORDERED NERVE-FIBRES. 175 



tion by facts demonstrated in many of the textures 

 of man and the lower animals. The arrangement 

 is the same as regards sympathetic, and spinal 

 motor and sensitive, nerve-fibres — except that in the 

 latter the constituent fibres of the plexuses one or 

 more removes from the terminal plexus are dark 

 bordered. ("' How to "Work with the Microscope," 

 4th Ed.) 



233. Dark bordered nerve fibres of the trunks of 

 nerres. — Every peripheral nerve network is connected 

 with its nerve centre by fibres, and whenever the 

 distance between the centre and peripheral organ is 

 considerable, the nerve-fibres are protected from each 

 other, and from the tissues through which they pass, 

 by a thick layer of oleo-albuminous matter, which 

 forms an investment to each bundle of delicate fibres, 

 by which it is insulated and separated from its 

 neighbours, and from other structures, by a distance 

 equal to from five to twenty times its own diameter. 

 "When the trunks pass through narrow canals, as 

 through holes in the cranium, this insulating pro- 

 tective covering is much reduced in thickness, so that 

 a large bundle of nerve-fibres is made to pass through 

 a space not more than one-fourth of the diameter 

 which the nerve trunk possesses in other parts of its 

 course. The fibres which have this thick covering 

 are known as '■^ darJc-bordered fibres,''' from the dark 

 double-contour line they always exhibit when examined 

 in water or weak serum ; the covering itself is known 

 as the "white substance of Schwann," or the "me- 

 dullary sheath." The double- contour line is not seen 

 in specimens mounted in glycerine or syrup. "When 

 one trunk diverges from another, many of these 

 fibres divide, one of the resulting subdivisions con- 

 tinuing onwards in the trunk, while the other passes 

 off to help to form the branch trunk. I have figured 

 many of these branchings. See Plate II, figs. 3, 4, 

 p. 182. 



