INTERNAL RESPIRATION. 265 



In animals with a thin moist epidermis (frog) the exchange of gases is much 

 greater, and in them the skin so far supports the lungs in their function, and may 

 even partly replace them functionally. In mammals with thick dry cutaneous 

 appendages, the exchange of gases is, again, much less than in man, 



132. Internal Respiration. 



Where C0 2 is Formed. By the term " internal respiration" is under- 

 stood the exchange of gases between the capillaries of the systemic 

 circulation and the tissues of the various organs of the body. As the 

 organic constituents of the tissues, during their activity, undergo gradual 

 oxidation, and form, amongst other products, C0 2 : we may assume (1.) 

 That the chief focus for the absorption of and the formation of C(X 

 is to be sought for within the TISSUES themselves. That the from 

 the blood in the capillaries rapidly penetrates or diffuses into the 

 tissues is shown by the fact, that the blood in the capillaries rapidly 

 loses and gains C0 2 , while blood containing 0, and kept warm out- 

 side the body, changes very slowly and incompletely. If portions of 

 fresh tissues be placed in defibrinated blood containing 0, then the 

 rapidly disappears (Hoppe-Seyler). Frogs deprived of their blood 

 exhibit an exchange of gases almost as great as normal. This shows 

 that the exchange of gases must take place in the tissues themselves 

 (Pfliiger and Oertmann). If the chief oxidations took place in the 

 blood and not in the tissues, then, during suffocation, when is 

 excluded, the substances which use up 0, i.e., those substances which 

 act as reducing agents, ought to accumulate in the blood. But this is 

 not the case, for the blood of asphyxiated animals contains mere traces 

 of reducing materials (Pfliiger). It is difficult to say how the is 

 absorbed by the tissues, and what becomes of it immediately it comes 

 in contact with the living elements of the tissues. Perhaps it is 

 temporarily stored up, or it may form certain intermediate less oxidised 

 compounds. This may be followed by a period of rapid formation and 

 excretion of C0 2 . On this supposition, it is evident that the absorption 

 of and the excretion of C0 2 need not occur to the same extent, so 

 that the amount of C0 2 given off at any period is not necessarily an 

 index of the amount of absorbed during the same period. 



[There are two views as to where the C0 2 is formed as the blood 

 passes through the tissues. One view is that the seat of oxidation is in 

 the blood itself, and the other is that it is formed in the tissues. If 

 we knew the tension of the gases in the tissues the problem would bo 

 easily solved, but we can only arrive at a knowledge of this subject 

 indirectly, in the following ways] : 



COa in Cavities. That the C0 2 is formed in the tissues is supported by the 

 fact, that the amount of C0 2 in the fluids of the cavities of the body is greater 

 than the C0 2 in the blood of the capillaries. 



