There can be little doubt that fishes swimming rapidly do not make respiratory movements at all, but obtain the necessary ventilation of the gills simply by opening the mouth.

It is no doubt technically possible to study metabolism and respiration of fishes during swimming at a constant rate, and of certain insects and birds during flight, and to obtain information similar to that obtained on man during work on a bicycle ergometer or a treadmill.

The respiratory mechanisms of birds are definitely adapted to the function of flight, as evidenced by the fact that birds which do not fly (Apteryx, Penguins) show these adaptations in a greatly reduced form.

In man, the mechanical breathing is essential to life, and it is one of the old tests for death to see whether these movements have ceased completely.

Muscles do not use oxygen at a constant rate.

The most direct evidence of the wonderful plasticity and elasticity of red corpuscles is obtained when they are watched in a current, where they can be caught against a projecting edge and bent by the pressure of the current flowing past them.