8. THE REGULATION OF BREATHING
339
stimulus this work did not rule out the possibility of more appropriate
ventilation responses to moderate anoxia. However, the more recent
work of Hughes and Umezawa (1968a) showed only a small increase
in breathing rate in Scyliorhinus canicula during a gradual reduction
of oxygen tension in the inspired water. Similarly, Baumgarten and
Piiper (1969) found only small changes of ventilation volume in
Scyliorhinus stellaris under these conditions. Because simple experimental procedures have so far been ineffective in causing substantial
and reproducible modifications of ventilation in elasmobranchs very little
is known of its range and control. There is obviously not a fixed ventilation volume; Baumgarten-Schumann and Piiper ( 1968 ) reported marked
changes in resting dogfish (see also Table I ) and commented that the
animals were not nearly at their maximum ventilation levels. They attributed the ventilation changes they observed to repayment of an oxygen
debt after excitement and activity. Whether some product of metabolism
constitutes the effective stimulus to ventilation change in elasmobranchs
cannot be decided on present evidence. It is possible that the level of
activity in the animal is in some way directly coupled to breathing, but
this cannot be the whole explanation since an inactive fish shows variations in ventilation. The whole field of respiratory regulation in elasmobranch fish is poorly understood and clearly requires closer study,
particularly on freely breathing preparations.
In those fish in which a fall in oxygen concentration does result
in increased ventilation, the environmental changes must influence the
respiratory center in such a way as to cause changes in the motor output and increase the rate and/or the amplitude of breathing. The relationship between rate and amplitude has been shown to vary considerably from species to species (Table I ) . Very little is known of the
underlying neurophysiology directly responsible for these alterations
in breathing. Modifications in the oscillatory activity of cells making up
the respiratory center are obviously produced by incoming sensory information related to environmental change. The receptors for this information could be in one or more of three general regions (Fig. 19):
(1) On the outside of the animal, on the walls of the pharynx and
the surface of the gills. They would detect oxygen concentrations in
the water of the environment or ventilation stream.
( 2 ) On arterial blood vessels containing blood after oxygenation
in the gills or in tissues closely associated with arterial blood. They
would detect oxygen concentration in the bloodstream.
( 3 ) On blood vessels carrying venous blood or in tissues closely
associated with venous blood. They would detect oxygen concentration
in the blood after it has passed through the tissues of the body. This
339
stimulus this work did not rule out the possibility of more appropriate
ventilation responses to moderate anoxia. However, the more recent
work of Hughes and Umezawa (1968a) showed only a small increase
in breathing rate in Scyliorhinus canicula during a gradual reduction
of oxygen tension in the inspired water. Similarly, Baumgarten and
Piiper (1969) found only small changes of ventilation volume in
Scyliorhinus stellaris under these conditions. Because simple experimental procedures have so far been ineffective in causing substantial
and reproducible modifications of ventilation in elasmobranchs very little
is known of its range and control. There is obviously not a fixed ventilation volume; Baumgarten-Schumann and Piiper ( 1968 ) reported marked
changes in resting dogfish (see also Table I ) and commented that the
animals were not nearly at their maximum ventilation levels. They attributed the ventilation changes they observed to repayment of an oxygen
debt after excitement and activity. Whether some product of metabolism
constitutes the effective stimulus to ventilation change in elasmobranchs
cannot be decided on present evidence. It is possible that the level of
activity in the animal is in some way directly coupled to breathing, but
this cannot be the whole explanation since an inactive fish shows variations in ventilation. The whole field of respiratory regulation in elasmobranch fish is poorly understood and clearly requires closer study,
particularly on freely breathing preparations.
In those fish in which a fall in oxygen concentration does result
in increased ventilation, the environmental changes must influence the
respiratory center in such a way as to cause changes in the motor output and increase the rate and/or the amplitude of breathing. The relationship between rate and amplitude has been shown to vary considerably from species to species (Table I ) . Very little is known of the
underlying neurophysiology directly responsible for these alterations
in breathing. Modifications in the oscillatory activity of cells making up
the respiratory center are obviously produced by incoming sensory information related to environmental change. The receptors for this information could be in one or more of three general regions (Fig. 19):
(1) On the outside of the animal, on the walls of the pharynx and
the surface of the gills. They would detect oxygen concentrations in
the water of the environment or ventilation stream.
( 2 ) On arterial blood vessels containing blood after oxygenation
in the gills or in tissues closely associated with arterial blood. They
would detect oxygen concentration in the bloodstream.
( 3 ) On blood vessels carrying venous blood or in tissues closely
associated with venous blood. They would detect oxygen concentration
in the blood after it has passed through the tissues of the body. This