248 RESPIKAT1OX. 



Secondly, in the b 7 ood. There is little doubt, although the fact has 

 not been directly proved, that some of the oxygen definitely dis- 

 appears, and some of the carbonic acid is also formed, in the sub- 

 stance of the blood-globules during their circulation. Since these 

 globules are anatomical elements, and since they undoubtedly go 

 through with nutritive processes analogous to those which take 

 place in the elements of the solid tissues, there is every reason for 

 believing that they also require oxygen for their support, and that 

 they produce carbonic acid as one of the results of their interstitial 

 decomposition. While the oxygen and carbonic acid, therefore, 

 contained in the globules, are for the most part transported by 

 these bodies from the lungs to the tissues, and from the tissues back 

 again to the lungs, they probably take part, also, to a certain extent, 

 in the nutrition of the blood-globules themselves. 



Thirdly, in the tissues. This is by far the most important source 

 of the carbonic acid in the blood. From the experiments of Spal- 

 lanzani, W. Edwards, Marchand and others, the following very 

 important fact has been established, viz., that every organized tissue 

 and even every organic substance, when in a recent condition, has the 

 power of absorbing oxygen and of exhaling carbonic acid. Or. Liebig, 

 for example, 1 found that frog's muscles, recently prepared and com- 

 pletely freed from blood, continued to absorb oxygen and discharge 

 carbonic acid. Similar experiments with other tissues have led 

 to a similar result. The interchange of gases, therefore, in the 

 process of respiration, takes place mostly in the tissues themselves. 

 It is in their substance that the oxygen becomes fixed and assimi- 

 lated, and that the carbonic acid takes its origin. As the blood in 

 the lungs gives up its carbonic acid to the air, and absorbs oxygen 

 from it, so in the general circulation it gives up its oxygen to the 

 tissues, and absorbs from them carbonic acid. 



We come lastly to examine the exact _mpde by which the car- 

 bonic acid originates in the animal tissues. Investigation shows 

 that e" ven here it is not produced by a process of oxidation, or direct 

 union of oxygen with the carbon of the tissues, but in some other and more 

 indirect mode. This is proved by the fact that animals and fresh 

 animal tissues will continue to exhale carbonic acid in an atmo- 

 sphere of hydrogen or of nitrogen, or even when placed in a vacuum. 

 Marchand found 2 that frogs would live for from half an hour to an 

 hour in pure hydrogen gas ; and that during this time they exhaled 

 even more carbonic acid than in atmospheric air, owing probably 



1 In Lehmann, op. cit., vol. ii. p. 474. 2 Ibid., p. 442. 



