542 Analytical Notices of Books. 



' One suggestion advanced by Dr. Kidd, appears to be deserving 

 of further investigation. If the sanguineous circulation of insects 

 be carried on by the transudation of the chyle through the coats 

 of the intestines, by its subsequent general diffusion through the 

 interior of the body, (a diffusion, of which however he denies the 

 existence,) and then by its absorption into the substance of par- 

 ticular organs, as the hepatic tubes, the vesiculae seniinales, the 

 ovaries, &c. ; how, he enquires, does it happen, that the bile, for 

 instance, does not transude through the coats of the same vessels, 

 the pores of which have admitted the blood from which it has 

 been formed ? It may, he observes, be answered, that the altera- 

 tion which the blood undergoes in the several organs, changes its 

 properties to such an extent, as to render it incapable of repassing 

 through the pores which admitted it. Such may indeed be the 

 fact; but the circulation of insects, if the term may be allowed, 

 though beset with difficulties, presents an interesting field of en- 

 quiry, to the acute physiologist, whose ambition may prompt him 

 to attempt the elucidation of a subject, in which even Cuvier has 

 been foiled. Dr. Kidd conceives that the tracheae, which in their 

 minute ramifications pervade every part of the body, may possibly 

 be the instruments of the circulation in insects ; that they may 

 absorb the blood or chyle in the first instance from the internal 

 surface of the alimentary canal ; that, by the exhaustion of the air 

 from individual tracheae, the absorbed fluid may be drawn on, 

 towards the two lateral tracheal tubes, which are apparently a 

 general medium of communication between all the other tracheae 

 of the body, and that, having once reached this point, it is for- 

 warded to the most distant parts of the body, by a modification of 

 the same means by which the air itself is forwarded. That blood 

 has not been seen in the tracheae, excepting apparently in two 

 instances by Dr. Kidd, cannot be admitted in refutation of the 

 hypothesis of their employment in the circulation, since, in the 

 higher orders of animals, the arteries are found, after death, 

 equally devoid of any traces of that fluid. 



