164 SCIENCE ‘PROGRESS 
Other proofs of the existence of intramolecular oxygen are 
mostly of the type first brought forward by Engelmann in 1868. 
Engelmann kept ciliated epithelial cells in an atmosphere of 
hydrogen till all movement ceased, and then added sufficient 
oxygen to start the movements again. On replacing this mixed 
atmosphere with pure hydrogen, it was found that cessation of 
ciliary movement ensued earlier than on the first occasion. The 
longer the cells were kept in oxygen, however, the longer time 
did the ciliary movements persist in the hydrogen, so it looked 
as if the cells had been able to store up a supply of intra- 
molecular oxygen on which they could gradually draw. Again, 
Ohrwall found that if the flow of perfusion liquid through an 
excised frog’s heart (kept in a hydrogen atmosphere or in salt 
solution) were stopped, the heart ceased beating in one to three 
hours. If the perfusion were resumed, the heart speedily began 
to beat again; but if the perfusion were now stopped it ceased 
beating in four to twenty-five minutes—z.e. in a very much 
shorter time than previously. 
Experiments of a similar nature have been made by Bernard 
upon fish, and by Frohlich and by Baglioni upon nerve; but, as 
Winterstein points out, none of these results afford convincing 
proof of the existence of intramolecular oxygen. Winterstein, 
indeed, denies the existence of this oxygen, and he supposes that 
the loss of excitability produced by deprivation of oxygen is due 
to the accumulation of toxic products of fatigue. In the presence 
of free oxygen, these products are gradually oxidised and 
rendered harmless, and so it follows that if the inexcitable 
tissue be exposed to oxygen for only a short time, but small 
amounts of the toxic products are oxidised, and so a second 
deprivation of oxygen induces a much speedier loss of excita- 
bility than the original deprivation did. This explanation might 
be held to be negatived by an interesting experiment of Verworn’s, 
in which a frog was perfused with oxygen-free salt solution. 
Strychnine was added to the saline so as to heighten the excita- 
bility of the cord, and for some minutes each stimulation of the 
skin resulted in a succession of brief reflex muscular contrac- 
tions. If during this initial period the flow of oxygenless saline 
were stopped, the cord speedily lost its reflex excitability ; but if 
the flow were resumed again, it was found that the excitability 
might return for a short time. It then disappeared perma- 
nently so long as the flow of oxygenless saline was continued, 
