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An elastic longitudinal ligament stretched under considerable tension lies within the dorsal aorta of Rainbow trout.

Experiments with mechanically simulated swimming movements in an artificial model demonstrate that such a ligament can act as a pump mechanism. The output of the pump increases with tail beat frequency up to a certain point where the mechanism tends to break down. Investigations on the trout dorsal aorta show that pump breakdown probably takes place at relatively low output pressures.

Fundamental blood flow considerations indicate that a fully effective dorsal aorta pump over the full range of swimming speeds is undesirable. It is concluded that the dorsal aorta ligament can act as a pump which automatically increases circulation during swimming particularly to the body musculature. The output pressure is small but may be of significance in causing reflex increase in cardiac output.