CHAP, ii.] RESPIRATIOX. 351 



and large respiratory surface of the animal, to supply the tissues 

 with all the oxygen they need, and to remove all the carbonic acid 

 they produce. It is difficult to believe that, in such an experiment, 

 the oxidation took place in the saline solution itself while cir- 

 culating in the blood-vessels and tissue-spaces of the animal. 



We may add, that the oxidative power which the blood itself 

 removed from the body is able to exert on substances which are 

 undoubtedly oxidized in the body is so small that it may be 

 neglected in the present considerations. If grape-sugar be added 

 to blood, or to a solution of haemoglobin, the mixture may be kept 

 for a long time at the temperature of the body, without undergoing 

 oxidation. Even "within the body a slight excess of sugar in the 

 blood over a certain percentage wholly escapes oxidation, and is 

 discharged unchanged. Many easily oxidized substances, such as 

 pyrogallic acid, pass largely through the blood of a living body 

 without being oxidized. The organic acids, such as citric, even 

 in combination with alkaline bases, are only partially oxidized; 

 when administered as acids, and not as salts, they are hardly 

 oxidized at all. It is of course quite possible that the changes 

 which the blood undergoes when shed might interfere with its 

 oxidative action, and hence the fact that shed blood has little 

 or no oxidizing power, is not a satisfactory proof that the un- 

 changed blood within the living vessels may not have such a 

 power. But did oxidation take place largely in the blood itself, 

 one would expect even highly diffusible substances to be oxidized 

 in their transit; whereas if we suppose the oxidation to take place 

 in the tissues, it becomes intelligible why such diffusible substances 

 as those which the tissues in general refuse to take up largely, 

 should readily pass unchanged from the blood through the secreting 

 organs. 



We have seen that in muscle the production of carbonic acid 

 is not directly dependent on the consumption of oxygen. The 

 muscle produces carbonic acid in an atmosphere of hydrogen. What 

 is true of muscle is true also of other tissues and of the body 

 at large. Spallanzani and W. Edwards shewed long ago that 

 animals might continue to breathe out carbonic acid in an 

 atmosphere of nitrogen or hydrogen: and more recently Pfliiger 

 has shewn, by a remarkable experiment, that a frog kept at a 

 low temperature will live for several hours, and continue to 

 produce carbonic acid, in an atmosphere absolutely free from 

 oxygen. The carbonic acid produced during this period was made 

 by help of the oxygen inspired in the hours anterior to the com- 

 mencement of the experiment. The oxygen then absorbed was 

 stowed away from the haemoglobin into the tissues, it was made 

 use of to build up the explosive compounds, whose explosions later 

 on gave rise to the carbonic acid. Or, to adopt Pfliiger's simile, the 

 oxygen helps to wind up the vital clock; but once wound up the 

 clock will go on for a period without further winding. The frog 



