362 RESPIRATION. 



gen simply absorbed and that entering into combination with the haemo- 

 globin, it must not be understood that the latter is wholly independent of 

 pressure. On the contrary, all chemical compounds are in various degrees 

 subject to dissociation at certain pressures and temperatures ; and the existence 

 of the somewhat loose compound of oxygen and haemoglobin is dependent on 

 the partial pressure of oxygen in the atmosphere to which the haemoglobin 

 is exposed. Not only will a solution of haemoglobin or a quantity of blood 

 either absorb oxygen, and thus undergo association or undergo dissociation 

 and give off oxygen according as the partial pressure of oxygen in the 

 atmosphere to which it is exposed is high or low, but also the amount taken 

 up or given off will depend on the degree of the partial pressure ; the haemo- 

 globin as we have seen may be partially as well as wholly reduced. The 

 law, however, according to which absorption or escape thus takes place is 

 quite different from that observed in the simple absorption of oxygen by 

 liquids. The association or dissociation is further especially dependent on 

 temperature, a high temperature favoring dissociation, so that at a high 

 temperature less oxygen is taken up than would be taken up (or, as the 

 case may be, more given off than would be given off) at a lower tempera- 

 ture, the partial pressure of the oxygen in the atmosphere remaining the 

 same. 



Moreover, in the blood we have to deal not with haemoglobin in simple 

 solution, in which the molecules are dispersed uniformly through the solvent, 

 but with the haemoglobin segregated into 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 colorless, 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 cor- 

 puscle may be considered as governing, as far as oxygen is concerned, a zone 

 of plasma immediately surrounding 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 pul- 

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

 oxygen associated with the haemoglobin of the corpuscle. 



The evidence, so far as it goes, seems to show that blood absorbs oxygen 

 in the same way as an aqueous solution of haemoglobin of the same concen- 

 tration ; the zone of plasma spoken of above as surrounding each corpuscle 

 behaves as far as regards the passage of oxygen to and from the corpuscles 

 in no essentially different respect from the way 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 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 fiuid, that is the lymph, 



