THE MAXILLARY AND MANDIBULAR BREATHING VALVES OF TELEOST FISHES

Abstract

In the light of the above observations and experiments the act of breathing in the teleost fishes may be described as follows: The respiratory stream enters the oral cavity by the mouth and leaves by the two gill openings, coming in contact with the respiratory surfaces of the gills just before it passes out. It is urged on its course by the pump-like construction and action of the oral cavity and its two sets of valves, an anterior set, which are those under consideration, and a posterior set, the branchiostegal valves. In inspiration the stream enters at the mouth, in response to a dilation of the oral cavity produced by the outward lateral movement of the opercular frames. At the same time water is prevented from entering at the gill openings by the branchiostegal valves which, although they are attached to the opercular frames, move independently of and contrary to them; so that, while this outward movement of the frames extends the gill openings, the branchiostegal valves close them automatically by the action of the water which tries to enter. In expiration the water is forced out of the gill openings by a corresponding contraction of the oral cavity. At the same time the water is prevented from regurgitating through the mouth, not by the contraction of the latter, but by the automatic operation of the maxillary and mandibular breathing valves which move as accurately and efficiently as any of the heart's valves. Caught on their posterior edges by the first movement of regurgitation, they snap together and completely prevent any water from leaving the oral cavity by the mouth which, meanwhile, is left partly open, almost as much open as during inspiration. That these valves are of value as breathing organs is evident upon casual observation; that they are of much importance is shown by the compensatory action brought about by injury; that they are not of immediate vital importance is proved by the fish's ability to get along without their services until they are repaired.