582 RESPIRATION OF THE TISSUES. [BOOK n. 



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 unchanged 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 excreting 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. It was shewn long ago that animals might continue 

 to breathe out carbonic acid in an atmosphere of nitrogen or 

 hydrogen ; and this has more recently been illustrated by the 

 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 commencement of the ex- 

 periment. The oxygen then absorbed was stowed away from the 

 hsemoglobin 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 a simile which has been suggested, 

 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 

 will continue to live, to move, to produce carbonic acid for a while 

 without any fresh oxygen, as we know of old it will without any 

 fresh food ; it will continue to do so till the explosive compounds 

 which the oxygen built up are exhausted; it will go on till the 

 vital clock has run down. 



360. To sum up, then, the results of respiration in its 

 chemical aspects. As the blood passes through the lungs, the low 

 oxygen pressure of the venous blood permits the entrance of 

 oxygen from the air of the pulmonary alveolus, through the thin 

 alveolar wall, through the thin capillary sheath, through the thin 

 layer of blood-plasma, to the red corpuscle, and the reduced 

 hemoglobin of the venous blood becomes wholly, or all but wholly, 

 oxyhaemoglobin. Hurried to the tissues, the oxygen, at com- 

 paratively high pressure in the arterial blood, passes largely into 

 them. In the tissues, the oxygen-pressure is always kept at an 

 exceedingly low pitch, by the fact that they, in some way at 



