PRESIDENT’S ADDRESS. 415 
between the blood plasma and the alveolar air is regulated to some 
extent according to the law of partial pressures.* 
We know that oxygen is conveyed by the blood all round the 
body, and that the amount of oxygen which a given volume of 
blood can so transport is immensely increased by the presence of 
the proteid hemoglobin in the corpuscles. The chemical properties 
of hemoglobin and its relation to oxygen, have been accurately 
ascertained. One knows precisely the conditions under which the 
two form a stable compound, and those under which they dissociate, 
and how, on account of the peculiarity of these conditions, it is so 
admirably adapted to perform its role in the economy. How it 
becomes a saturated compound where the tension of O, is rela- 
tively high (the plasma in the lungs), and breaks down where this 
tension is iow (the capillaries). By this dissociation oxygen is 
once more set free in the plasma, and diffuses thence through the 
capillary wall into the fluid which surrounds the various cells. 
From this situation it disappears, and is absorbed by the cells, 
and firmly locked up within them in some form of chemical 
combination. Beyond this point we cannot trace it, but the next 
thing known is that it reappears as CO., having undergone this 
transformation whilst within the cell. 
The history of CO, during its passage from the tissues to the 
alveolar air has also been investigated, and we are in possession of 
facts which show that this also takes place according to more or 
less simple laws of chemistry and physics. Transit is accomplished 
according to the laws of partial pressures, assisted by chemical 
combinations for the time being with the carbonates and phosphates 
of the blood plasma which operate in a manner somewhat analagous 
to that of hemoglobin with oxygen. 
As a second example of the kind of knowledge obtained by the 
methods of recent physiology, I will take vision. 
Concerning the mechanical principles of the organ of vision, the 
knowledge obtained during the last ten years—owing largely to 
the energies of Helmholtz and Donders—is so comprehensive and 
complete that it gives one the impression that little more could 
be added to it. The minute anatomy of the eye, the optical pro- 
perties of its various media, and its various defects as an optical 
instrument, have been closely investigated. The exact mechanism 
by means of which accommodation is effected, peripheral rays 
excluded when increased refraction from the periphery of the 
* The discovery by Bohr, and later confirmation by Haldane and Lorain Smith, that the 
tension of O, is frequently higher in arterial blood than in alveolar air, whereas the tension 
of CO, is often lower in the arterial blood than in such air, cannot be explained by diffusion 
across a membrane, with which one is so far acquainted in physical experiments. If, how- 
ever, one assumes a physiological membrane such as the alveolar lining to possess such 
molecular architecture as to allow O, molecules freer passage in one direction and CO, 
molecules in the other, a heaping up of O, on one side and CO, on the opposite side would 
occur, 
