116 EVENINGS AT THE MICROSCOPE 



division of the trachece, even to their most minute ramifi- 

 cations, a character whereby these vessels are readily dis- 

 tinguishable when examined under the microscope. 1 



Man has imitated this exquisite contrivance in the spiral 

 wire spring which lines flexible gas-pipes; but his wire 

 does not pass between two coats of membrane. One of 

 the most interesting points of the contrivance is the way 

 in which the branches are (so to speak) inserted in the 

 trunk, the two wires uniting without leaving a blank. It 

 is difficult to describe how this is done; but by tracing 

 home one of the ramifications you may see that it is per- 

 formed most accurately the circumvolutions of the trunk- 

 wire being crowded and bent round above and below the 

 insertion (like the grain of timber around a knot), and the 

 lowest turns of the branch -wire being suitably dilated to 

 fill up the hiatus. 



You must not suppose, however, that the whole of one 

 tube is formed out of a single wire. Just as in a piece of 

 human wire-work the structure is made out of a certain 

 number of pieces of limited length, and joinings or inter- 

 lacings occur where new lengths are introduced, so, strange 

 to say, it seems to be here. It is strange, I say, that it 

 should be so, when there can be no limit to the resources, 

 either of material, or skill to use it; but so it is, as you 

 may see in this specimen, which has been dissected out 

 of the body of a silkworm. The spiral is much looser here 

 than in the air-tube of the fly, the turns of the wire being 

 wider apart; and hence its structure is much more easily 

 traced. Here you see in many places the introduction of 

 a new wire, always commencing with the most fine-drawn 



1 "Nat. Hist, of Anim.," i. 6. 



