August Krogh

I found it possible to observe at least the superficial capillaries of muscles both in the frog and in mammals through a binocular microscope, using strong reflected light as a source of illumination. Resting muscles observed in this way are usually quite pale, and the microscope reveals only a few capillaries at fairly regular intervals. August Krogh

Light Possible Resting

In the numerous observations made in my laboratory upon this object, we have only once seen a combination of vessels in which there might be a direct communication between a small artery and a vein, though the two observers could not come to a final conclusion on the point. August Krogh

Communication Laboratory Small

In the tissues generally, and in such a cavity as the abdomen, the pressure is everywhere and practically always very nearly atmospheric, and must be so, because the integuments give way very easily to any excess of pressure, whether positive or negative. August Krogh

Always Negative Positive

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. August Krogh

Doubt Mouth Opening

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. August Krogh

Bicycle Man Swimming

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. August Krogh

Birds Flight Flying