CIRCULATION IN INSECTS. 173 



I.. Fig. 335 gives a side view of the anterior extremity of 

 the same vessel, showing the curve (A) which it describes 

 as it bends downwards in its course towards the head. 



The function performed by the dorsal vessel, which, 

 judging from the universal presence of this organ in insects, 

 must be one of great importance in their economy, was long 

 a profound mystery. Its analogy in structure and position 

 to the dorsal vessels of the Arachnida and the Annelida, 

 where it evidently communicates with channels of circu- 

 lation, and exhibits movements of pulsation resembling those 

 of insects', was a strong argument ia favour of the opinion 

 that it is the prime mover of a similar kind of circulation; 

 but then, again, this hypothesis appeared to be overturned 

 by the fact that no vessels of any kind could be seen extend- 

 ing from it in any direction; nor could any channels for the 

 transmission of a circulating fluid be detected in any part of 

 the body. Those organs, which, in animals apparently of 

 an inferior rank, are most vascular, such as the stomach, the 

 intestinal tube, the eye, and other apparatus of the senses, 

 seemed to be constructed, and to be nourished, by means to- 

 tally different from those adopted in the former animals. 

 Although extremely minute ramifications of air tubes are 

 every where visible in the interior of insects, yet, neither 

 Cuvier, nor any other anatomist, could succeed, by the closest 

 scrutiny, in detecting the least trace of blood vessels; and 

 the presumption, therefore, was, that none existed. 



But it still remained a question, if the dorsal vessel be 

 not subservient to circulation, what is its real function? 

 Marcel des Serres, who bestowed great pains in investi- 

 gating this subject, came to the conclusion that its use is to 

 secrete the fatty matter, which is generally found in great 

 abundance in the abdominal cavity, and which is accumu- 

 lated particularly around the dorsal vessel.* A more at- 

 tentive examination of the structure of the vessel itself 

 brought to light a valvular apparatus, of which the only con- 



* See his various papers in the Memoires du Museum d'Hist. Nat.; torn. 



iv. and v. 



f 



