470 EESPIEATION OF THE TISSUES. [BOOK n. 



able quantity of carbonic acid. And we may probably assert 

 with safety with regard to all the tissues that in the tissues 

 themselves, in the lymph which bathes their lymph-spaces, and 

 in the secretions which some of them pour forth free oxygen is 

 either wholly absent or so scanty that their oxygen-pressure 

 may be regarded as nil, while carbonic acid is so abundant that 

 the pressure of carbonic acid in them may be regarded as exceed- 

 ing that of venous blood. An exception seems to be presented 

 by the case of the lymph flowing along the larger lymphatic 

 vessels, for in this the amount of carbonic acid, while usually 

 higher than that of arterial blood, is lower than that of the gen- 

 eral venous blood ; but this probably is due to the fact that the 

 lymph in its passage onwards is largely exposed to arterial 

 blood in the connective tissues and in the lymphatic glands, 

 where the production of carbonic acid is slight as compared to 

 that going on in muscles. All the facts point to the conclu- 

 sion, that it is the tissues, and not the blood, which become pri- 

 marily loaded with carbonic acid, the latter simply receiving 

 the gas from the former by diffusion, except the (probably) 

 small quantity which results from the metabolism of the blood- 

 corpuscles ; and that the oxygen which passes from the blood 

 into the tissues is at once taken up and placed under such con- 

 ditions that it is no longer removable by diminished pressure 7) 



It was shewn long ago that animals might continue to 

 breathe out carbonic acid in an atmosphere of nitrogen or hydro- 

 gen ; and this is further illustrated by the 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 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 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 com- 

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

 till the vital clock has run down. 



291. 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 



