10 THE PRESIDENTIAL ADDRESS 
may so modify existing equilibria as to affect profoundly the observed 
behaviour of the cell. 
Such relations, though by no means confined to them, reach their 
greatest significance in the higher organisms, in which individual 
tissues, chemically diverse, differentiated in function and separated 
in space, so react upon one another through chemical agencies trans- 
mitted through the circulation as to co-ordinate by chemical trans- 
port the activities of the body as a whole. Unification by chemical 
means must to-day be recognised as a fundamental aspect of all 
such organisms. In all of them it is true that the nervous system 
has pride of place as the highest seat of organising influence, but 
we know to-day that even this influence is often, if not always, 
exerted through properties inherent in chemical molecules. It is 
indeed most significant for my general theme to realise that when 
a nerve impulse reaches a tissue the sudden production of a definite 
chemical substance at the nerve ending may be essential to the 
response of that tissue to the impulse. It is a familiar circumstance 
that when an impulse passes to the heart by way of the vagus nerve 
fibres the beat is slowed, or, by a stronger beat, arrested. ‘That is, 
of course, part of the normal control of the heart’s action. Now 
it has been shown that whenever the heart receives vagus impulses 
the substance acetyl cholin is liberated within the organ. To this 
fact is added the further fact that, in the absence of the vagus influence, 
the artificial injection of minute graded doses of acetyl choline so 
acts upon the heart as to reproduce in every detail the effects of graded 
stimulation of the nerve. Moreover, evidence is accumulating to 
show that in the case of other nerves belonging to the same morpho- 
logical group as the vagus, but supplying other tissues, this same 
liberation of acetyl choline accompanies activity, and the chemical 
action of this substance upon such tissues again produces effects 
identical with those observed when the nerves are stimulated. 
More may be claimed. The functions of another group of nerves 
are opposed to those of the vagus group ; impulses, for instance, 
through certain fibres accelerate the heart beat. Again a chemical 
substance is liberated at the endings of such nerves, and this substance 
has itself the property of accelerating the heart. We find then that 
such organs and tissues respond only indirectly to whatever non- 
specific physical change may reach the nerve ending. ‘Their direct 
response is to the influence of particular molecules with an essential 
structure when these intrude into their chemical machinery. 
It follows that the effect of a given nerve stimulus may not be 
confined to the tissue which it first reaches. "There may be humeral 
transmissions of its effect, because the liberated substance enters 
the lymph and blood. This again may assist the co-ordination of 
events in the tissues. 
