CIRCULATION IN INSECTS. 239 



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 totally 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 de- 

 tecting 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 de Serres, who 

 bestowed great pains in investigating this sub- 

 ject, 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 accumulated particularly 

 around the dorsal vessel.* A more attentive 

 examination of the structure of the vessel itself 

 brought to light a valvular apparatus, of which 

 the only conceivable purpose is that of deter- 

 mining the motion of the contained fluid in one 

 constant course ; a purpose necessarily incom- 

 patible with its supposed alternate undulation 



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

 Nat. ; torn. iv. and v. 



