THEORY. 385 



analogy, and the fact has been almost directly proved by positive 

 observations. We have purposely devoted more time to the 

 mechanism of the respiration of insects than we generally give to 

 the mechanical relations of the animal processes ; for it is pre- 

 cisely in these animals which have no true blood, but merely 

 parenchymatous juices, and no true blood-vessels, but at most 

 mere rudimentary hearts, or a very limited analogue, the so-called 

 dorsal vessel, that the atmospheric air penetrates directly through 

 the most delicate tracheal ramifications to the very elements of 

 the organs ; here the air does not come first in contact with the 

 blood, nor does it pass for any length of time with it through 

 vessels, where it may undergo metamorphoses accompanied by a 

 development of carbonic acid gas; the carbonic acid must here 

 be formed in the parenchyma of the organs themselves, and through 

 their vital activity ; for the amount rises and falls in the exhaled 

 air, as we learn from direct experiments, almost in equal pro- 

 portion to the amount of the activity of these organs. It is, 

 therefore, probable that also in the higher animals endowed with 

 true blood, the carbonic acid is almost entirely formed in the 

 functional organs, and not in the liquor sanguinis. And should 

 we then find such great differences in relation to the amount of the 

 gases, in the character of the blood flowing to and from the organs 

 (the arterial and venous blood), if all the carbonic acid of the blood 

 flowing to the right side of the heart were formed gradually and 

 alone throughout the whole extent of the blood-column passing 

 from the left to the right side of the heart through the capillaries ? 

 No further probabilities need be adduced to prove that the paren- 

 chyma of the organs is the seat of the formation of carbonic acid, 

 as we obtain the most convincing proof of the correctness of this 

 view from the admirable investigations of G. Liebig.* Although 

 many points may be susceptible of improvement in the method of 

 experimenting adopted by G. Liebig, the main results must con- 

 tinue unaffected. We have already (p. 95) noticed the most 

 essential facts which have been brought to light by these inquiries. 

 We would here only repeat, that the carefully prepared frogs' 

 muscles absorb oxygen and exhale carbonic acid so long as their 

 irritability or contractility lasts, that the latter is lost in irrespirable 

 gases, and finally, that a muscle completely deprived of blood con- 

 tinues to maintain this interchange of gases so long as it retains its 

 contractility. We have here, therefore, not the mere representa- 

 tion, but the perfect expression of a respiration of the organ of a 



* Ber, d. Akad. d. Wiss. zu Berlin, 1850, S, 339-347. 

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