Chap, ii.] RESPIRATION. 463 



separated from its fellows by a layer, thin it may be but still 

 a distinct layer, of colourless, haemoglobinless plasma. As the 

 corpuscle makes its way through the narrow capillary paths 

 of a pulmonary alveolus, it is separated from the air of the 

 alveolus by a thin layer of plasma as well as by the film of the 

 conjoined capillary and alveolar walls ; and a like layer of 

 plasma separates it from its fellows as it journeys in company 

 with them through the wider passages of the arteries and veins. 

 Through this layer of plasma, which containing no haemoglobin 

 can hold oxygen in simple solution only, the oxygen has to 

 pass on its way to and from the corpuscle ; and every corpuscle 

 may be considered as governing, as far as oxygen is concerned, 

 a zone of plasma immediately surrounding itself. The cor- 

 puscle takes its oxygen directly from this zone and gives up 

 its oxygen directly to this zone ; and the pressure at which at 

 any moment the oxygen exists in this zone will depend on the 

 pressure of oxygen outside the zone, in the air of the pulmonary 

 alveolus for instance, and on the smaller or greater amount of 

 oxygen associated with the haemoglobin of the corpuscle. 



The evidence, however, afforded by various experiments, 

 so far as it goes, seems to shew that blood absorbs oxygen in 

 the same way as an aqueous solution of haemoglobin of the 

 same concentration ; the zone of plasma spoken of above as 

 surrounding each corpuscle seems to behave as far as regards 

 the passage of oxygen to and from the corpuscles in no essen- 

 tially different respect from the way in which the molecules of 

 water, belonging to a molecule of dissolved haemoglobin, behave 

 in regard to the absorption or the giving-off of oxygen by an 

 aqueous solution of haemoglobin. 



The film of the conjoined capillary and alveolar wall is a 

 thin membrane soaked with lymph and wet ; we cannot speak 

 of it as actually secreting a liquid secretion into the alveolus, 

 for the cavity of the alveolus is filled with air which, though 

 saturated with moisture, is air, not a liquid ; still enough passes 

 through the film to keep the film continually moist. Through 

 this film the oxygen has to make its way in order to gain access 

 to the plasma and so to the corpuscle ; it makes its way dis- 

 solved in the fluid, that is the lymph, which keeps the film 

 moist. This film moreover is composed of living matter, and 

 the considerations which a little while back (§ 253) we urged 

 concerning the diffusion through a living membrane of solid 

 substances in solution, hold good also for the diffusion of gases 

 in solution. 



If now we ask the question, Are the conditions in which 

 haemoglobin and oxygen exist in ordinary venous blood as it 

 flows to the lungs, of such a kind that the venous blood in 

 passing through the pulmonary capillaries will find the partial 

 pressure of the oxygen in the pulmonary alveoli sufficient 



