368 RESPIRATION. 



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 maybe regarded as nil, while 

 carbonic acid is so abundant that the pressure of carbonic acid in them may 

 be regarded as exceeding 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 general venous blood ; but 

 this probably is due to the fact that the lymph in its passage onward is 

 largely exposed to arterial blood in the connective tissues and in the lym- 

 phatic glands, where the production of carbonic acid is slight as compared 

 to that going on in muscles. All the facts point to the conclusion that it is 

 the tissues, and not the blood, which become primarily 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 conditions that it is no 

 longer removable by diminished pressure. 



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 shown 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 abso- 

 lutely 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 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 pro- 

 duce 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 until the 

 vital clock has run down. 



303. 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 alve- 

 olus, through the thin alveolar walls, through the thin capillary sheath, 

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

 haemoglobin of the venous blood becomes wholly, or all but wholly, oxy- 

 hsemoglobin. Hurried to the tissues, the oxygen, at comparatively high pres- 

 sure in the arterial blood, passes largely into them. In the tissues the oxy- 

 gen-pressure is always kept at an exceedingly low pitch by the fact that they, 

 in some way at present unknown to us, pack away at every moment into 

 some stable combination each molecule of oxygen which they receive from 

 the blood. With its oxy-hsemoglobin largely but not wholly reduced, the 

 blood passes on as venous blood. To what extent the haemoglobin is reduced 

 will depend on the activity of the tissue itself. The quantity of haemoglobin 

 in the blood is the measure of limit of the oxidizing power of the body at 

 large ; but within that limit the amount of oxidation is determined by the 

 tissue, and by the tissue alone. 



