312 THE VITAL FUNCTIONS. 



on each side, and sending off radiating branches 

 from the parts wliich are opposite to the spi- 

 racles ; and these branches are further subdi- 

 vided, in the same manner as the arteries of the 

 larger animals, so that their minute ramifications 

 pervade every organ in the body. This ramified 

 distribution has frequently occasioned their 

 being mistaken for blood vessels. In the wings 

 of insects the nervures, which have the appear- 

 ance of veins, are only large air- tubes. Jurine 

 asserts that it is by forcing air into these tubes 

 that the insect is enabled suddenly to expand 

 the wings in preparing them for flight, giving 

 them by this means greater buoyancy as well as 

 tension. 



The tracheae are kept continually pervious by 

 a curious mechanism ; they are formed of three 

 coats, the external and internal of which are 

 membranous ; but the middle coat is constructed 

 of an elastic thread coiled into a helix, or cylin- 

 drical spiral (as seen in Fig. 372) ; and the 

 elasticity of this thread keeps the tube constantly 

 in a state of expansion, and therefore full of air. 

 When examined under water, the tracheae have a 

 shining silvery appearance, from the air they 

 contain. This structure has a remarkable ana- 

 logy with that of the air vessels of plants, which 

 also bear the name of tracheae ; and in both 

 similar variations are observed in the contexture 



