CIRCULATION EX 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.—CincunaTion 18 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, 
besides 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 minute arterial trunks, and from which they receive 
ty 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 
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