414 PROCEEDINGS OF SECTION D. 
or not, the modern school of physiologists have concerned them- 
selves almost exclusively with one or two of the higher mammals 
and the frog. 
As my first example, I will take the subject of respiration. 
This function may be divided in a higher animal into four stages. 
(1.) The mechanical arrangements for the inhalation and 
exhalation of air. 
(2.) The entrance of O, from the air into the blood and exit 
of CO, from the blood into the air. 
(3.) The way in which O, is carried to the tissues and CO, 
away. 
(4.) The taking up of O, by the cells and the excretion of 
CO,. 
On regarding these four stages, it will be seen that of all of 
them the fourth is the essentially respiratory act, an act common 
to the whole of the animal kingdom, and I may say at once that 
it is the only one we know little or nothing about. 
Of the first three stages, the last fifty years have contributed 
greatly to our exact understanding. Inhalation and exhalation 
are accomplished by alteration in the thoracic pressure, and take 
place according to the laws of «rodynamics. 
The orderly sequence of inspiration and expiration is brought 
about by a nervous mechanism, the precise arrangement of which 
has been disclosed in considerable detail. Accurate analyses of 
inspired and expired air have been made which have shown that 
out of every 100 volumes of air about five volumes of oxygen 
disappear and about four volumes of CO, appear. 
it has been shown that the operation of inhalation and exhala- 
tion by itself only succeeds in filling or emptying that portion of 
the respiratory apparatus which is nearest the surface, viz. :-—the 
trachea and large bronchi, and that the interchange of gases 
between this point and the alveoli takes place according to the 
laws of diffusion of gases. 
The amount of the gases oxygen, nitrogen, and carbonic acid 
contained in arterial and venous blood have been determined with 
considerable accuracy, and the fact established that the oxygen 
lost to the air in the lungs is gained by the blood coming from 
them, and that the carbonic acid gained by the air whilst in the 
lungs is proportional to that lost by the venous blood in this 
situation. 
Further than this, the partial pressure of the O, & CO,, in the 
alveolar air has been determined by more or less indirect methods, 
and also the partial pressures of those same gases both in venous 
and arterial blood. The results show that the exchange of gas 
