400 PHYSIOLOGY. 



the rest. Did the gills merely imbibe oxygen, smaller narrower vessels 

 would suffice. 



Secondly, if pure oxygen were found in the tracheae of insects that 

 breathe through gills, they would be able to live a longer space of time 

 even in such media as contain no oxygen, for instance, until the 

 oxygen contained within their tracheae was consumed. But this is 

 not the case. Those larvae which breathe through gills are deprived of 

 life as quickly in spirits of wine as those which respire in the ordinary 

 way. 



Thirdly, did insects with gills inspire pure oxygen, so would all 

 other insects, as the structure of their respiratory organs is the same, 

 be enabled without inconvenience to breathe pure oxygen. But this 

 is also not the case. Insects in pure oxygen breathe at first more 

 violently than irregularly, and die in the course of a few hours, before 

 near all the oxygen is consumed *. 



It hence appears necessary to adopt the conclusion, that even in 

 insects breathing through gills there is a direct transmission of 

 atmospheric air through the branchiae into the tracheae. 



235. 



If we next ask the object of all respiration, and the effect it exercises 

 upon the preservation and promotion of life, we shall find it to consist 

 especially in the alteration of the blood. Observations upon the 

 difference of the venous and arterial blood of the higher animals proves 

 that oxygen intermixed with arterial blood colours it more brightly, 

 and thus promotes its easier assimilation, although not by the mere 

 colouring, yet by the other changes it produces in it, the testimony of 

 which is its brighter colour. A similar alteration will necessarily take 

 place in the juices circulating in the bodies of insects, but in proof of 

 which we are the less enabled to give a striking instance, from, in the 

 first place, the blood of these animals being wholly colourless, and, from 

 the universal distribution of their respiratory organs, whence, conse- 

 quently, this alteration of the blood is constantly everywhere taking 

 place. In insects, therefore, arterial blood can alone be found, and the 

 motion of the juices which has been detected in insects of different 

 orders can consist merely in its general distribution, and not (as in 

 animals with perfectly distinct arteries and veins) have likewise for 



* Compare the Observations of Sorg, as above, pp. 19, 4i, 98. 



