284 On the Structure and Functions of the Organs of Respiration. 



ration by quitting the water entirely, causes fishes to take in air from the 

 surface — a fact which may be readily observed during the summer. 



It is by no means an easy matter to arrive at the precise truth, in expe- 

 rimenting on the chemical changes produced by the respiration of animals ; 

 for the function must, in all cases, be more or less disturbed, and the results 

 will be doubtful in the same proportion. We have not yet sufficient evi- 

 dence to enable us to assert that all animals produce similar alterations in 

 the air j but the numerous experiments made on vertebrated animals, and 

 on some of the invertebrated classes, leave us little reason to doubt the 

 fact. 



The principal changes, which we recognise in air which has been re- 

 spired, are : 1. The disappearance of oxygen which is absorbed; 2. The 

 presence of carbonic acid which has been exhaled ; 3. The absorption of 

 nitrogen ; and 4. The exhalation of nitrogen. 



The oxygen which disappears is usually more than is contained in the 

 carbonic acid produced, so that it must be actually absorbed into the 

 system ; and this we find especially the case in the lower classes of verte- 

 brata, and in all young animals. The quantity varies in such proportion, 

 that it sometimes exceeds the third part of the carbonic acid formed, and 

 is sometimes so small that it may be disregarded ; the difference depending 

 not only on the constitution of the species, but on the comparative degree 

 of development, and on individual differences among adults. This fact, 

 which was first ascertained by Dr. Edwards, explains the results obtained 

 by Messrs. Allen and Pepys, who found the quantity of oxygen lost, and 

 carbonic acid produced, to be the same : they took the greatest care to 

 obtain accurate results ; but their experiments were made with few indi- 

 viduals, and only with two species, man and the guinea pig. 



With regard to the production of carbonic acid, there is now sufficient 

 evidence to show that it is not generated by the contact of oxygen and 

 carbon in the lungs, as was formerly supposed ; but that it is formed in 

 the course of the circulation, and exists in venous blood when it arrives 

 at the lungs. Dr. Edwards has performed some striking experiments in 

 support of this view. He has shown that some animals may be made to 

 breathe hydrogen for a considerable time ; and that the production of car- 

 bonic acid goes on even more rapidly, up to a certain point, than in atmos- 

 pheric air. From many recent experiments on venous blood, the fact of 

 its containing free carbonic acid has been put beyond a doubt : and it is a 

 curious effect of the power of mutual diffusion which has been already ex- 

 plained, that venous blood will give off carbonic acid when exposed to an 

 atmosphere of hydrogen, even after it has been submitted to the exhausting 

 power of a vacuum, A slight degree of absorption of oxygen and exha- 

 lation of carbon takes place by the skin of most of the mammalia, but to a 

 much less extent than in amphibia. 



The question now arises, as to the source of the carbonic acid in venous 



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