110 EVENINGS AT THE MICROSCOPE. 



ramifications, a cliaracter wtereby these vessels are 

 readily distinguishable when examined under the mi- 

 croscope.* 



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) in- 

 serted in the trunk, the two wires uniting without leav- 

 ing 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 performed 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 fiU. 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 join- 

 ings or interlacings occur where new lengths are intro- 

 duced, 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 point, 

 * Nat. Hist, of Anim. i. 6. 



