CHAP, ii.] RESPIRATION. 603 



minute isolated masses, bottled up as it were in the individual 

 corpuscles. The haemoglobin of each corpuscle is 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 surround- 

 ing itself. The corpuscle 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 cor- 

 puscle. 



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 it 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 dissolved in the fluid, 

 that is the lymph, which keeps the film moist. This film more- 

 over is composed of living matter, and the considerations which 

 a little while back ( 312) we urged concerning the diffusion 

 through a living membrane of solid substances in solution, hold 

 good also for the diffusion of gases in solution. 



We have now to consider 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 to bring 

 about by mere diffusion the association of the additional quantity 

 of oxygen whereby the venous is converted into arterial blood ? Or, 

 Is there evidence that the film spoken of above exerts an influence, 

 as a living film, on the entrance of oxygen from the alveolus into 

 the blood ? 



In certain fishes the partial pressure of the oxygen in the gas 

 contained in the swim-bladder, which may be regarded as a 

 modified lung, far exceeds that of the fish's blood, and if the gas be 

 drawn off it is soon replaced by gas having also a very high partial 



392 



