AQUATIC RESPIRATION OF FISHES 19 



(fig. 4c). The operation of the two pumps ensures that the flow 

 across the gills is almost continuous despite its intermittent 

 entry and exit from the system. Evidence for this view has been 

 obtained by measuring the pressure changes in front of and 

 behind the gills. The pressure curves (fig. 5) show that a gradient 

 from the buccal to the opercular cavities is maintained through- 

 out the respiratory cycle, with the exception of a very brief 

 period when some reversal of flow may occur. This is unlikely, 

 however, because of its extreme brevity and the inertia of the 

 water. The buccal pressure pump forces water across the gills 

 whereas the opercular suction pumps draw the water through 

 them and the alternation and interaction of these two pumps 

 maintains a continuous flow. An essential part of the mechanism 

 is the presence of valves guarding the mouth and the opercular 

 openings. Both sets of valves are passive and move according 

 to the pressure gradient across them. Thus when the mouth 

 opens, thin flaps of skin projecting down from behind the upper 

 and lower jaws are displaced inwards by the reduced pressure. 

 As the buccal cavity begins to decrease in volume the increase 

 in pressure causes them to flap outwards, thereby closing the 

 mouth and preventing too great a reflux of water through it. The 

 rims of the opercula are also provided with a thin sheet of tissue 

 which comes into close contact with the posterior border of the 

 opercular cavities as they expand and prevents the entry of water 

 by this route. 



The operation of a double-pumping mechanism and the con- 

 sequent continuous flow across the gills make the terms inspira- 

 tion and expiration not entirely suitable. These are applicable 

 to tidal ventilation but for fish a subdivision of each respiratory 

 cycle into four parts is preferable (fig. 4c). The two main phases 

 are when first the pressure pump and then the suction pumps 

 are responsible for maintaining the flow. These two are separated 

 by transition phases in which the diff'erential pressure is reduced 

 and may be reversed following the pressure pump. 



(v) Action of the skeletal and muscular systems during ventilation 

 The skeleton of the teleost head is complex and varies from 



