OF THE BLOOD-VESSELS. 189 



the sides of the lower valve are forced upwards by the in- 

 creased 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. 



" 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 segments of the body. 



" The structure of the upper valve appears to be a re- 

 flecting 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 be- 

 neath, 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. 



" The blood does not appear to be confined within any 

 specific vessels, previously to its entering the lateral open- 

 ings before-mentioned, as when they open, the particles 

 are seen converging towards them, from all the adjacent 

 parts of the cavity of the abdomen. The whole of the 

 blood received throughout the course of this vessel is con- 

 veyed to the extremity of the anterior part of the body, 

 where the vessel makes a curve inwardly, is lost to view, 

 and apparently discharges the entire volume of the blood 

 into the cavity of the body, for 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 received by the valves of the dorsal vessel, and 



