IS15.J On the Uses of the Dorsal Vessel. 197 



formsa kind of oxygenized pommade. The fat of the gryllo-talp 

 treated with muriatic acid coagulates very quickly, but becomes less 

 yellow, the muriatic acid giving it less oxygen. 



From all that has been said, it is evident that it is exceedingly 

 difficult, not to say impossible, by direct obsfervations to know the 

 ramifications of the dorsal vessel ; but it would have been improper 

 to have admitted proofs purely negative, without having attempted 

 direct observations in the first place. The first means that presented 

 itself was injections. To them, therefore, I had recourse in the 

 first place. 



The membranes of the dorsal vessel being very thin, oppose a 

 great obstacle to the success of mercurial injections. Accordingly, 

 the dorsal vessel almost always bursts before the injection has 

 reached its extremity ; so that we cannot depend upon the results 

 of such injections. Yet I ought to observe that, in those parts of 

 the dorsal vessel in which the injection has succeeded, I have never 

 observed tlie mercury make its way into a single fibrilla, or indicate 

 the smallest ramification. This method appearing to me insuffi- 

 cient, I soon abandoned it, and tried various injections with liquids 

 coloured with cochineal or indigo, according to the colour of the 

 dorsal vessel. These injections, forced in by a very fine syringe, 

 succeeded perfectly, especially in the larvae of the great butterfly 

 (paoii), and of the sphinx of the lithymale, and likewise in the 

 mole cricket. In all of them, the injection having penetrated to the 

 extremity of the dorsal vessel, this vessel always appeared to me a 

 simple cylindrical canal without any ramification. 1 repeated these 

 injections in a great number of individuals of each class, and I 

 always had the same result. 



Though these coloured liquids did not show any ramifications of 

 the dorsal vessel, it was possible that these ramifications might 

 become sensible by making this vessel absorb these liquids. For 

 this purpose I put living insects into liquids strongly coloured, and 

 allowed the absorption to take place through the pores of the organs. 

 I had the greater hopes of success, because the conformation of the 

 tracliea; gave me hopes that these vessels would absorb very little o 

 the coloured liquids. In all the insects exposed lo this kind of proof, 

 1 saw the pulsations of the dorsal vessel cease by little and little, 

 and the humor contained in it coagulate. It was only after this 

 coagulation that the membranes of the dorsal vessel absorbed the 

 coloured liquids, and even much more rapidly than the other 

 organs. This vessel thus coloured formed along the back a bluish 

 «r rcWdish line, which had no ramification whatever ; but the white 

 fibrill.'e of the trachea^, were always (listinguishablc, which, as has 

 been said, form the first membrane, in this absorption the tracheal 

 do not sensibly change their colour. They merely assume a deeper 

 tint ; but tlioy sink down completely. The adipose tissue which 

 surrounds the dorsal vessel assumes the form of clots. The muscles 

 .vparafe in such a maimer that their fibres become perfectly dis- 

 tinct, and that their attachments may be easily recognized. The 



