420 PROCEEDINGS OF SECTION D. 
that the remainder has mostly been concerned with the more 
simple, mechanical, and chemical processes which are associated 
with the lives of higher animals. In the case of respiration we 
know a great deal about the ways and means by which oxygen is 
brought to the cells, but of just what happens when cells take up 
oxygen and how they do it we are ignorant. We only know that 
when once they have it they keep it somehow combined, and are 
capable of exhibiting activity and of continuing to give off CO, for 
some limited period ‘after they are deprived of. oxygen. 
Our knowledge of the anatomy and mechanical principles of the 
eye is more perfect than that of any department of physiology, but 
to understand vision we would like to know next something more 
regarding the mechanism by means of which the varying intensity 
and pitch of light, focussed upon the rods and cones of the retina 
ag an inverted image of the object, originates a series of nerve- 
impulses in these sensitive cell-units. 
It is of the utmost importance, I think, that we should have 
obtained satisfactory evidence that notwithstanding all the com- 
plications of vitality, that a living animal is subservient to the 
principle of the conservation of energy just as an ordinary machine. 
We can trace the food possessed of potential energy of chemical 
composition through some slight transformations, until it reaches 
the cell, and we can trace the more or less fully oxidised products 
which have given up that energy, away out of the body; but here, 
just as in the other instances, the details of the tr ansformation 
itself are intracellular and for the present hidden. 
The acquisition of knowledge of the structural arrangement in 
the nervous system has been considerable, whereas regarding the 
nature of nerve-impulse and the behaviour of nerve-cells during 
nervous activity we know little or nothing. For the past fifty 
years physiologists have been principally concerned with the 
analysis of the function of organs as such, and have more or less 
left aside the physiology of cells. ‘In my opinion they have been 
quite wise in so doing. In this way all those physiological pheno- 
mena which can be measured according to physical standard and 
interpreted in terms of physics and chemistry have, to a large 
extent, been separated off from those that cannot. Processes in 
which cells participate collectively as membranes or organs have 
been more or less sharply defined from those in which they operate 
by means of their individuality, and in which cases the phenomena 
are intracellular. Surely it was wise to ascertain to what extent 
a physiological result was due to the physical or chemical proper- 
ties of the matter concerned, in order to know at what point the 
intervention of cellular activities is necessary. 
Mechanical and chemical analogy are of limited service in the 
interpretation of intracellular activities ; indeed it has not infre- 
quently been stated that we are wasting our time in attempting 
