OF THE BLOOD A7TD CIRCTJLATI01S'. 



205 



place of veins ; these venous sinuses swell out at the base, and 

 send a branch to each bran- 

 chia. After having circulated 

 through these organs, the 

 blood is returned to the 



heart, to perform a similar cir- 

 cuit. 



[ 370. In insects (fig. 

 223) the circulation is main- 

 tained by a dorsal vessel (), 

 which acts the part of a 

 heart : it is divided into seve- 

 ral chambers bv valves, which 



it 



permit the blood to flow only 

 towards the head ; the vessel 

 here appears to cease, and the 

 blood seems to flow in the 

 interspaces of the tissues ; cur- 

 rents of globules form arches 

 in the anteunse, wings, legs, 

 and the prolongations of the 

 abdomen ; lateral currents 

 are seen at b, the direction of 

 their course being indicated 

 by the arrows. The circulation in insects can only be 



Fig. 222. 



221 '~ Circ ^ a ^ g 



Vascular system of the lobster. 



studied in transparent aquatic larva, as those of the 

 ephemera, in which it forms a beautiful spectacle for the 

 microscopist. The chyle globules enter the dorsal vessel by 



