240 THE VITAL FUNCTIONS. 



in opposite directions, from one end of the 

 tube to the other. These valves are exhibited in 

 Fig. 336, in a still more magnified view of a 

 longitudinal section of the dorsal vessel, showing 

 the semicircular folds (s, s) of its inner mem- 

 brane, which perform the function of valves by 

 closing the passage against any retrograde mo- 

 tion of the fluid. This discovery of valves in 

 the dorsal vessel, again made the balance of 

 probability incline towards the opinion that it 

 is the agent of some kind of circulation. 



All doubt as to the reality of a circulation in 

 insects is now dispelled by the brilliant dis- 

 coveries of Professor Cams, who, in the year 

 1824, first observed this phenomenon in the 

 larva of the Agrion puella. In the transparent 

 parts of this insect, as well as of many others, 

 numerous streams of fluid, rendered manifest 

 by the motions of the globules they contain, 

 are seen meandering in the spaces which inter- 

 vene between the layers of the integument, 

 but without appearing to be confined within 

 any regular vessels. The streams on the sides 

 of the body all pass in a direction backwards 

 from the head, till they reach the neighbourhood 

 of the posterior end of the dorsal vessel, towards 

 which they all converge ; they are then seen to 

 enter that vessel, and to be propelled by its pul- 

 sations towards its anterior extremity, where they 

 again issue from it, and are subsequently divided 



