RESPIRATION. 253 



oxygen for every one hundred grammes of muscular tissue ; while the 

 bones absorb only a little over 17 cubic centimetres for the same weight. 

 The absorbent capacity of the tissues for oxygen is even greater 

 than that of the blood. This was shown by the experiments of Spal- 

 lanzani, and more recently by those of Bert. In Bert's experiments, 

 three equal portions of recently drawn defibrinated dog's blood were 

 placed in test-tubes, a piece of fresh muscular tissue from the same 

 animal being added to one of them, a portion of the spleen-tissue to 

 another, and the third left to itself. After a time it was found that 

 the tissues had abstracted oxygen from the blood with which they were 

 in contact, so that in these two specimens the quantity of oxygen 

 remaining was less than in the third, as follows : 



QUANTITY OF OXYGEN BY VOLUME REMAINING IN 



Blood left to itself 18 per cent. 



Blood containing spleen tissue .... 12 " 

 Blood containing muscular tissue ... 6 " 



Finally, successive analyses of the blood, as it passes from the 

 arteries to the veins, show that its loss of oxygen is mainly in the 

 capillary circulation. In general, according to Pfliiger, the quantity of 

 oxygen, by volume, in arterial blood is 15.6 per cent. ; in venous blood 

 8 per cent. ; that is, it is reduced about one-half in the capillaries of the 

 general circulation. But in the blood of the hepatic veins, which has 

 passed through a double set of capillary vessels, the loss of oxygen 

 is much greater. Bernard* found that in the same dog, blood from 

 different parts of the circulatory system, yielded the following quantities 

 of oxygen : 



QUANTITY OF OXYGEN BY VOLUME IN 



Arterial blood 18.93 per cent. 



Venous blood from right side of heart . . 9.93 " 



Venous blood from hepatic veins . . . 2.80 " 



Thus the blood-globules serve as carriers of oxygen from the lungs 

 where it is absorbed, to the tissues where it is consumed ; the first 

 object of respiration being to supply oxygen to the blood, in order that 

 the blood may supply it to the tissues. 



Exhalation of Carbonic Acid by the Blood. The venous blood, as 

 it returns to the heart, is charged with carbonic acid to such an extent 

 that a portion of this gas is exhaled through the pulmonary mem- 

 brane, and discharged with the breath. Its quantity in the blood 

 has not been determined with the same accuracy as that of oxygen. 

 Carbonic oxide, which is so efficient for the extraction of oxygen from 

 the blood, displaces only a portion of its carbonic acid ; and in the 

 experiments of Bernard, the maximum quantity of carbonic acid 

 obtained from venous blood by this means was only about 6.5 per cent. 

 A much larger proportion may be extracted by the mercurial air-pump, 

 amounting on the average, in the experiments of Ludwig, to about 28 



* Liquides de 1'Organisme. Paris, 1859, tome i., p. 394. 



