CIUCULATION. 241 



upwards by the increased flow of the blood from the section 

 below the valve, the lateral openings are closed, and the main 

 current of the blood is projected through the two valves, as 

 shewn in Fig. 4, a. 



It is not easy to see this beautiful structure of the valves of 

 the great dorsal vessel, for it is only when the insect is in a 

 state of great exhaustion, or has been just so much compressed 

 as to destroy voluntary motion without entirely depriving it of 

 life, that it is possible to subject it to a power sufficiently high 

 to discern these extremely delicate and transparent tissues ; 

 and even then, to see them to the greatest advantage, recourse 

 should be had only to such as are in the last three or four 

 sections of the body. 



The structure of the upper valve appears to be a reflecting 

 inwards and upwards of the inner coat or coats of the artery ; 

 and of the under one, to be a contraction and projection of the 

 like parts of a portion of the artery beneath, so as to come 

 within the grasp of the lower part of the valve above it. The 

 exterior portion of the artery may be seen as an exceedingly 

 fine line, connecting the parts above and below the valves, as 

 represented at c, Figs. 2, S, and 4. 



The blood does not appear to be confined within any 

 specific vessels, previously to its entering the lateral openings 

 before-mentioned, as, when they open, the particles are seen 

 converging towards them, as shewn by the curved and straight 

 arrows. Fig. 2. 



The whole of the blood received throughout the course of 

 this vessel is conveyed to the extremity of the antei'ior part of 

 the body, where the vessel makes a curve inwardly, and is lost 

 to view at b, Fig. 1 . To all appearance, the main current of 

 the blood is now discharged into the cavity of the body, as it 

 is seen pursuing its course downwards, in a wide spreading 

 stream, on each side and beneath the great dorsal vessel. As 

 it descends, portions are again absorbed by the valves of the 

 dorsal vessel, and, at the same time, vessels passing down each 

 side of the body convey another portion of the blood to its 

 lower extremity. These are decidedly vessels, not portions of 

 the great abdominal cavity, their boundaries being clearly 

 definable. They communicate at each junction of the sections 

 of the body with the great abdominal cavity, as a part of 

 the blood they convey is discharged at these points, to supply 



NO. III. VOL. I. I I 



