CIRCULATION 



INSECTS. 



255 



Nevertheless, it has a tolerably regular circulation ; and the 

 organ by which this movement is chiefly effected is .a long 

 tube, termed the dorsal vessel, which seems to propel it for- 

 wards, whilst two principal sinuses, one on either side, convey 

 it backwards. The dorsal vessel, seen at a, is a membranous 



Fig. 143.--CiiicuLATioN IN INSECTS. 



tube lying along the back of the insect, and partly divided 

 into several compartments by incomplete valvular partitions, 

 which bear no inconsiderable resemblance to the valves of veins 

 ( 279). By the successive contraction of these different por- 

 tions, the blood which entered at the posterior extremity of 

 the dorsal vessel is gradually propelled forwards ; and when 

 it arrives at the front of the body, it passes out by a series of 

 canals, some of which convey it to the head, whilst others 

 pass sideways and backwards for the supply of the body, with 

 its appendages, the legs and wings. On returning from these 

 parts, it re-enters the posterior end of the dorsal vessel. But, 

 Asides ministering to this general circulation, the several 

 compartments of the dorsal vessel seem to act as independent 

 hearts, each for its own segment ; into which they send forth 

 blood ^by rainute arterial trunks, and from. which they receive 

 it again by minute apertures furnished with valves. It is to 

 be remarked that in Insects no special arrangement of vessels 

 for the aeration of blood is required ; since this aeration is 



