14 THE INSECT WOELD. 



The anterior or aortic portion of the dorsal vessel shows neither 

 fan-shaped lateral expansions, nor orifices, and consists of a single 

 membraneous tube. On reaching the interior of the head it opens 

 in the lacunary inter-organic system. The whole of the blood set 

 in motion by the contractions of the cardial portion of the dorsal 

 vessel runs into the cavity of the head, and circulates after- 

 wards in irregular channels formed by the empty spaces left 

 between the different organs. It is the unoccupied portions of the 

 great visceral cavity which serve as conductors to the blood, and 

 through them run the main currents that one sees in the lateral 

 and lower parts of the body, whence these currents regain the 

 back part of the abdomen, and enter the heart after having 

 traversed externally the different organs they encountered. These 

 principal channels are in continuity with other gaps provided 

 between the muscles, or between the bundles of fibres of which 

 these muscles are composed, or else in the interior of the intes- 

 tines. 



The principal currents send into the network thus formed minor 

 branches, which, having ramified in their turn among the princi- 

 pal parts of the organism, re-enter some main current to regain 

 the dorsal vessel. 



In the transparent parts of the body the blood may be seen 

 circulating in this way in a number of inter-organic channels, 

 more or less obvious, penetrating the limbs, overspreading the 

 wings, when these appendages are not horny, and, in short, 

 diffusing itself everywhere. " If, by means of coloured injec- 

 tions," says M. Milne Edwards, "one studies the connections 

 which exist between the cavities in which sanguineous currents 

 have been found to exist, and the rest of the economy, it is 

 easy to see that the irrigatory system thus formed penetrates to 

 the full depth of every organ, and should cause the rapid renewal 

 of the nourishing fluid in all the parts where the process of vitality 

 renders the passage of this fluid necessary." 



We shall see presently, in speaking of respiration, that the 

 relations between the nourishing fluid and the atmospheric air are 

 more direct and regular than was for a long time supposed. 



In short, insects possess an active circulation, although we find 

 neither arteries nor veins : and although the blood put in motion 



