6 THE PRESIDENTIAL ADDRESS 
the chemical breakdown of carbohydrate which necessarily precede 
oxidation is the intermediate combination of a sugar (a hexose) with 
phosphoric acid to form an ester. This happening is indispensable for 
the progress of the next stage, namely the production of lactic acid from 
the sugar, which is an anaerobic process. The precise happenings 
to the hexose sugar while in combination with phosphoric acid are 
from a chemical standpoint remarkable. Very briefly stated they are 
these. One half of the sugar molecule is converted into a molecule 
of glycerin and the other half into one of pyruvic acid. Now with 
loss of two hydrogen atoms glycerin yields lactic acid, and, with a 
gain of the same pyruvic acid also yields lactic acid. The actual 
happening then is that hydrogen is transferred from the glycerin 
molecule while still combined with phosphoric acid to the pyruvic 
acid molecule with the result that two molecules of lactic acid 
are formed.1_ The lactic acid is then, during a cycle of change which 
I must not stop to discuss, oxidised to yield the energy required by 
the muscle. 
But the energy from this oxidation is byno means directly available 
for the mechanical act of contraction. The oxidation occurs indeed 
after and not before or during a contraction. The energy it 
liberates secures however the endothermic resynthesis of a sub- 
stance, creatin phosphate, of which the breakdown at an earlier 
stage in the sequence of events is the more immediate source of 
energy for contraction. Even more complicated are these chemical 
relations, for it would seem that in the transference of energy from 
its source in the oxidation of carbohydrate to the system which 
synthesises creatin phosphate, yet another reaction intervenes, 
namely, the alternating breakdown and resynthesis of the substance 
adenyl pyrophosphate. The sequence of these chemical reactions 
in muscle has been followed and their relation in time to the phases 
of contraction and relaxation is established. ‘The means by which 
energy is transferred from one reacting system to another has till 
lately been obscure, but current work is throwing light upon this 
interesting question, and it is just beginning (though only beginning) 
to show how at the final stage the energy of the reactions is con- 
verted into the mechanical response. In parenthesis it may be noted 
as an illustration of the unity of life that the processes which occur in 
the living yeast cell in its dealings with sugars are closely similar to 
those which proceed in living muscle. In the earlier stages they are 
identical and we now know where they part company. You will, I 
think, be astonished at the complexity of the events which underlie 
the activity of a muscle, but you must remember that it is a highly 
specialised machine. A more direct burning of the fuel could not 
fit into its complex organisation. Iam more particularly concerned 
1 Lecture by Otto Meyerhof: in the Press (see Nature). 
