CONCLUSIONS 103 
most interesting problems of general physiology has been 
to determine what is the nature of the irritable response 
which living matter shows. It is this, the problem of 
problems, which we wish to have solved. Is that 
process physical or chemical? Is it simply an altera- 
tion of permeability of membranes, as: some have 
supposed, or is it in reality in the nature of an explosion ? 
Is the living thing essentially a bag of jelly with a 
wonderful membrane about it, that membrane being so 
wonderful that all the phenomena of life are to be 
ascribed to its changes in state? For this is the view 
which some maintain. They lead us to the holy of 
holies of cells and tell us to behold a membrane! _ Is life 
nothing more than a membrane? What kind of a subter- 
fuge is this which we encounter? All the riddles of life 
are but the peculiar properties of a membrane! Upon 
this membrane, as upon a magic carpet of Arabia, we 
are invited to mount and travel over that unexplored 
country whose mountain peaks shine in the distance. 
Are we, then, beings of but two dimensions, nothing 
but membranes, of which the magic proportions mock 
us derisively, since we can never hope to seize that 
which has but two dimensions? That such a view 
resembles the membrane it has conjured up, in that it is 
surface without depth, is self-evident. 
In no such simple and naive a manner can the un- 
knowns in the equation of life be determined. For we 
have found that everywhere, paralleling the irritability 
changes in a perfect degree, as far as we have been able 
to determine, go the chemical changes. Carbon dioxide, 
that very simple substance, the last term in the katabo- 
lism of living matter, rises and falls with irritability. 
