572 On the Uses of the Dwsal Vessel. [Mat, 



several very lively individuals. I pierced'the dorsal vessel in dif- 

 ferent parts at the time when the liquid had flown back, but I 

 never perceived the least moisture issue out of it. As it is difficult 

 to determine this point in a satisfactory manner, I repeated the 

 experiments with all the precaution possible. The results were 

 constantly the same : no liquid flowed out. If this were the only 

 proof that could be given of the non-circulation of the humour 

 contained in the dorsal vessel, it would not be of much importance; 

 but, joined to those that we have already made known, it acquires a 

 certain force, and even a considerable weight. But it may be said, 

 that the reason why the humour does not exude when the dorsal vessel 

 is punctured, is because it is too thick. 1 am veiy much inclined, I 

 own, to that opinion ; for this humour lias always appeared to me 

 very little liquid, and in the voraceous larvje it has a remarkable 

 consistence. This consistence, joined to some other particularities, 

 has put us on the way to determine with some precision the func- 

 tions of the dorsal vessel in those animals that have no other circu- 

 lation than that of air. The different movements of contraction 

 and dilatation which we remark in the dorsal vessel, can never in- 

 duce us to consider that organ as a heart ; since, in the animals 

 like the Naiades [Nereis, Lin.) in which there exists only a single 

 organ, that of digestion, we observe pulsations as distinct as those 

 exhibited by the heart of other animals. Therefore the pulsations 

 of the dorsal vessel ought not to make us decide in any manner on 

 the use of that organ, nor lead us to consider it as a heart. 



In all animals which have a system of circulation and respiration, 

 the influence of the one of these on the other has been perceived.* 

 Thus often when the respiration is entire, the circulation is only 

 half so, or when the circulation is complete, the respiration docs 

 not operate in a complete manner; so that a demicirculation, mul- 

 tiplied by a complete respiration, or vice versa, gives always equal 

 products ; that is to say, a semioxygenation of the blood. But in 

 mammifcrous animals, in which the circulation and respiration are 

 complete, the oxygenation of the blood is equally so ; and in birds 

 which have a complete circulation with a double respiration, the 

 oxj'genation of the blood is doubled, in consequence of the quan- 

 tity of air that combines continually with it.f 



If the dorsal vessel of insects is an organ of circulation, it ought 

 to experience the influence of the organs of respiration like the 

 heart of animals with vertebrae. This influence ought to be so 

 much the greater, as insects, like birdsj have a double respiration. 

 The air penetrates into all the parts of their bodies, as it does in 

 birds. It continually bathes their nutritive humour, or their blood, 

 which has this particularity, that it is not confined in vessels; just 

 as in birds it acts upon the blood in the great circulation, at the 



* See Cuvicr's Anatomic Comparcc, t. iv. p. 167. 



+ Thc;c <!pcc(ilations about the oxygenation of the bluoJ are quite vas;uc and 

 uucertain. T. 



