8. THE REGULATION OF BREATHING
301
flow. This resistance must be known before differential pressure records
can be used to assess 00w patterns and rates through the gills. Although
analysis would be much easier if gill resistance remained constant,
it seemed from the outset that this would be unlikely. The geometry of
the gills changes constantly during a single breathing cycle since they
are bridging a gap of changing dimensions and are attached ventrally
to the basihyal which also moves rhythmically. Parts of the gill sieve
could thus be alternately exposed and protected or, more simply, there
could be variations of the dimensions in the pores through which the
water 00ws.
Interdigitation of secondary lamellae could, as Hughes (1966) points
out, have an enormous effect on resistance. In addition, it is uncertain
that contact can be maintained under all conditions between the tips
of gill filaments from adjacent gill arches. A number of workers have
suggested that the gill filaments do separate during some stage in the
opercular cycle. Saunders ( 1961) reported separation as the operculum
was maximally abducted, Hughes ( 1961a) during opercular adduction,
and Pasztor and Kleerekoper (1962) during all phases but principally
during adduction. The latter workers were satisfied that the movements
were not a result of the force of the ventilating currents but were produced by rhythmic contraction of gill adductor muscles. They also
suggested that the filament tips parted to protect the gills against excess
water flow, although it is by no means clear that an animal could produce
a gill damaging current. In tuna and some other oceanic fishes there is
considerable fusion of gill filaments and secondary lamellae to give a
more solidly constructed hemibranch ( Muir and Kendal, 1968). Even
in these forms, which usually rely on "ram" ventilation associated with
fast swimming, water can still bypass the gills if the filament tips part.
Such determination of gill resistance as have been made support
the conclusion that it cannot remain constant. Hughes and Shelton
(1958) reported experiments on tench in which the ventilation volume
was changed by adding carbon dioxide to the water. As the volume
increased so the ratio (mean pressure) / (minute volume) decreased, and
this was taken as strong evidence that the gill resistance was decreasing. It also seemed reasonable to conclude that if resistance changed
with overall flow rate it could quite easily vary within a single cycle
since the pressure gradient was not constant. Later experiments in which
the gills of deeply anesthetized fish were artificially perfused (Hughes
and Shelton, 1962) gave results in which the resistance varied in an
unpredictable way, sometimes rising, sometimes falling, and sometimes
remaining constant with increased flow (Fig. 4a). It was quite common
for the resistance to remain constant over certain parts of the pressure-
301
flow. This resistance must be known before differential pressure records
can be used to assess 00w patterns and rates through the gills. Although
analysis would be much easier if gill resistance remained constant,
it seemed from the outset that this would be unlikely. The geometry of
the gills changes constantly during a single breathing cycle since they
are bridging a gap of changing dimensions and are attached ventrally
to the basihyal which also moves rhythmically. Parts of the gill sieve
could thus be alternately exposed and protected or, more simply, there
could be variations of the dimensions in the pores through which the
water 00ws.
Interdigitation of secondary lamellae could, as Hughes (1966) points
out, have an enormous effect on resistance. In addition, it is uncertain
that contact can be maintained under all conditions between the tips
of gill filaments from adjacent gill arches. A number of workers have
suggested that the gill filaments do separate during some stage in the
opercular cycle. Saunders ( 1961) reported separation as the operculum
was maximally abducted, Hughes ( 1961a) during opercular adduction,
and Pasztor and Kleerekoper (1962) during all phases but principally
during adduction. The latter workers were satisfied that the movements
were not a result of the force of the ventilating currents but were produced by rhythmic contraction of gill adductor muscles. They also
suggested that the filament tips parted to protect the gills against excess
water flow, although it is by no means clear that an animal could produce
a gill damaging current. In tuna and some other oceanic fishes there is
considerable fusion of gill filaments and secondary lamellae to give a
more solidly constructed hemibranch ( Muir and Kendal, 1968). Even
in these forms, which usually rely on "ram" ventilation associated with
fast swimming, water can still bypass the gills if the filament tips part.
Such determination of gill resistance as have been made support
the conclusion that it cannot remain constant. Hughes and Shelton
(1958) reported experiments on tench in which the ventilation volume
was changed by adding carbon dioxide to the water. As the volume
increased so the ratio (mean pressure) / (minute volume) decreased, and
this was taken as strong evidence that the gill resistance was decreasing. It also seemed reasonable to conclude that if resistance changed
with overall flow rate it could quite easily vary within a single cycle
since the pressure gradient was not constant. Later experiments in which
the gills of deeply anesthetized fish were artificially perfused (Hughes
and Shelton, 1962) gave results in which the resistance varied in an
unpredictable way, sometimes rising, sometimes falling, and sometimes
remaining constant with increased flow (Fig. 4a). It was quite common
for the resistance to remain constant over certain parts of the pressure-