386 RESPIRATION 



tissue elements are inconsistent with this view. The experimental 

 evidence shows that we must place the diffusion pressures of oxy- 

 gen and carbonic acid in exactly the same category as the diffusion 

 pressures of water, salts, and other dissolved constituents of blood 

 plasma. This means that oxygen molecules are constantly passing 

 both outwards and inwards, although in ordinary tissues more 

 are passing inwards. It is only in oxygen-secreting tissues that we 

 find that on one side of the secreting membrane oxygen molecules 

 are passing more readily outwards, and only, so far as yet known, 

 in the green parts of plants and in the presence of light that free 

 oxygen is on all sides passing more readily outwards from living 

 tissue elements than inwards. But even in green plants, as Paul 

 Bert showed, a considerable diffusion pressure of oxygen is neces- 

 sary for life. 



We can thus compare living structures to dissociable chemical 

 molecules and particularly molecules which, like haemoglobin, 

 form molecular compounds only capable of existing in so far as 

 rate of loss is balanced by rate of gain. We must, however, assume 

 that the dissociation and association are taking place simultane- 

 ously in many different directions, corresponding to the many 

 different substances present in the blood plasma and necessary 

 for life. We have also to remember that although the individual 

 tissue elements are all in connection, direct or indirect, with the 

 blood plasma, they are also in connection with one another, and 

 that this implies additional conditions of stability in connection 

 with which molecular or ionic gains and losses are balanced 

 against one another. 



It is clear that the stability in respect of one kind of molecular 

 gain or loss determines the stability in respect of others. Thus a 

 small deficiency of oxygen molecules, or a small excess of hydro- 

 gen ions, in the blood plasma, disturbs the equilibrium of the 

 receptor elements in the respiratory center and leads to the extra 

 molecular discharges which show themselves in increased activity 

 of the center. Disturbances in other directions of the composition 

 of the blood plasma have similar results, though the receptors 

 are specially sensitive to changes in reaction or deficiency in oxy- 

 gen pressure. We can interpret similarly the mode of action of 

 various stimuli acting on living tissues, including what, for want 

 of more intimate knowledge, we call mechanical stimuli. Hence we 

 are led to the conception of a living organism as the seat of a vast 

 system of mutually dependent reversible chemical reactions. For 

 irreversible chemical reactions physiology has but little use. 



