

84 IXTltRNAL ORGANS. 



the tracheae, foffit almost forms an integral part 

 of that system. V The centre of circulation is wha,t 

 is known as the dorsal vessel, a straight tube 

 lying along the back directly beneath the skin. 

 This can often be seen through the transparent 

 skin, but on dissection it is very difficult to recog- 

 nize its true structure, so excessively delicate and 

 vague are its bounding walls ; it appears more 

 like a passage-way between other bodies than as 

 a distinct vessel ; and indeed it is often only by 

 tracing its development up to the perfect insect 

 that its actual structure can be clearly deter- 

 mined. The dorsal vessel is open posteriorly and 

 is also furnished along its sides with values open- 

 ing inward and directed forward ; the alternate 

 contraction and expansion of the vessel forces the 

 fluids of the body through these openings and 

 along the canal toward the head ; here the vessel 

 becomes very slender, with more distinct walls, 

 but at the extreme front of the head; near the 

 base of the oesophagus, it terminates, and the 

 fluids enter the general cavity of the body to flow 

 gently to the opposite extremity, and there to be 

 pumped again into the dorsal vessel. 



To understand, however, how oxygenation is 

 effected, we must glance once more at the trachea! 

 tubes. These are really involutions of the exter- 

 nal integument of the insect ; the outer chitinous 

 coat (now become the interior lining) is curiously 



