16 Dr. W. H. Broadbent on the Function of 



inadequate to effect the oxidation required for the evolution of 

 the force actually exerted by the muscles. 



The entire question thus turns on the assumption that oxygen 

 can leave the capillaries only by passing through the thin mem- 

 brane of which they consist, in solution in a fluid exudate. The 

 necessity for a current of fluid to convey the oxygen is supposed 

 to arise from the fact that the oxygen, being in solution in the 

 blood, carries with it its solvent in passing through the capillary 

 wall — just as in dialysis the saline matter is accompanied by the 

 water in which it is dissolved. But this view of the process 

 leaves entirely out of consideration the fact that if oxygen 

 leaves the capillaries, the products of oxidation (carbonic anhy- 

 dride &c.) must enter them ; and when two diffusible substances 

 are in solution on opposite sides of a thin membrane, the ad- 

 verse currents of the common solvent more or less neutralize each 

 other, and there is interchange of the dissolved matters with 

 comparatively little movement of fluid. 



If oxygen can leave the blood only in solution in a current of 

 fluid, how, it may be asked, does it enter the blood in the lungs ? 

 It would seem that there ought to be a stream of fluid setting in 

 from the air-cells into the pulmonary capillaries ; and this would 

 be required were it not that, as the oxygen enters the blood, car- 

 bonic anhydride leaves it. On the hypothesis that oxidation is 

 extravascular, the exchange of oxygen for carbonic anhydride is 

 effected very similarly in the pulmonic and systemic capillaries. 

 In the lungs the oxygen is dissolved in the moisture of the walls 

 of the air-sacs; there is thus outside the capillary membrane 

 fluid containing oxygen, while in its interior is moving the blood 

 charged with CO 2 ; interchange of the two gases consequently 

 takes place. In the systemic capillaries the blood is oxygenated, 

 while outside the capillaries is the interstitial fluid of the textures 

 containing the CO 2 which has resulted from oxidation. The 

 conditions under which interchange will occur are here again 

 realized; the capillary wall stands between two fluids, one charged 

 with 0, the other with CO 2 . Here, however, the is in the 

 blood, instead of CO 2 as in the lungs. It is not the affinity of 

 a distant fibre for oxygen which overcomes the weak " molecular 

 combination " of this gas with the blood-corpuscles, but the pre- 

 sence of CO 2 in the surrounding fluid ; and the affinity of and 

 CO 2 for hsematoglobin is so nearly balanced, that they mutually 

 displace each other according as one or the other predominates. 



It is thus evident that, supposing the oxidation to take place 

 outside the capillaries, the oxygen does not require a stream of 

 fluid to convey it to the tissues ; and this being the case, the 

 calculation by which it is shown that the exudate is insufficient 

 for the purpose has no bearing whatever on the question whether 



