310
G. SHELTON
arcus palatini, the adductor arcus palatini (which also incorporates the
adductor operculi in this animal ) , and the adductor niandibulae. During
shallow breathing these muscles only are active, the sternohyoidens,
protractor hyoideus, and hyohyoideus being brought into action as the
breathing deepens. The dilator operculi only operates during hyperventilation.
B. Elasmobranchs
1. THE PATTERN OF WATER FLOW
The nature of the relationship between water flow and the gill filaments in elasmobranch fish is less well defined than it is in teleosts.
There are no direct observations of the gill configuration during breathing although it has been assumed that filaments from adjacent gill arches
are, to some extent, in contact. The fact that a measurable pressure
gradient exists from orobranchial to parabranchial cavities ( Hughes,
1960a) suggests that this is the case. Since the visceral arches themselves
are participating in the pumping process to a much greater extent than
in teleosts, there can be little doubt that changes in gill position and
hence gill resistance occur during the breathing cycle. Flow direction
over filaments and lamellae, particularly in relation to blood flow through
them, is also poorly established. Robin and Murdaugh (1967), using indirect methods of calculating oxygen uptake and the amount of oxygen
removed from the ventilation stream, came to the conclusion that countercurrent gas exchange did not occur in dogfish gills. However, other
investigations (Hanson and Martin, 1967; Piiper and Schumann, 1967;
Baumgarten-Schumann and Piiper, 1968) have shown that in many cases
the arterial oxygen tension was higher than the mean expired oxygen
tension. This can be explained by some sort of countercurrent system,
although the path of water across the secondary lamellae is ultimately
obstructed by the median scptum. The possibility that water must flow
along the length of the filaments in elasmobranchs has led Piiper and
Schumann (1967) to propose a multicapillary model which is also effective in giving a negative value for the difference between expired and
arterial oxygen tensions.
Anatomical differences between teleost and selachian are marked,
since in the latter the respiratory current passes in through the spiracle
as well as the mouth and out through a variable number of gill slits not
covered by an operculum. The flow pattern from mouth and spiracle is
curious. \Vater entering the spiracle of a dogfish will leave through the
three anterior gill slits of thc same side, whereas water entering through
G. SHELTON
arcus palatini, the adductor arcus palatini (which also incorporates the
adductor operculi in this animal ) , and the adductor niandibulae. During
shallow breathing these muscles only are active, the sternohyoidens,
protractor hyoideus, and hyohyoideus being brought into action as the
breathing deepens. The dilator operculi only operates during hyperventilation.
B. Elasmobranchs
1. THE PATTERN OF WATER FLOW
The nature of the relationship between water flow and the gill filaments in elasmobranch fish is less well defined than it is in teleosts.
There are no direct observations of the gill configuration during breathing although it has been assumed that filaments from adjacent gill arches
are, to some extent, in contact. The fact that a measurable pressure
gradient exists from orobranchial to parabranchial cavities ( Hughes,
1960a) suggests that this is the case. Since the visceral arches themselves
are participating in the pumping process to a much greater extent than
in teleosts, there can be little doubt that changes in gill position and
hence gill resistance occur during the breathing cycle. Flow direction
over filaments and lamellae, particularly in relation to blood flow through
them, is also poorly established. Robin and Murdaugh (1967), using indirect methods of calculating oxygen uptake and the amount of oxygen
removed from the ventilation stream, came to the conclusion that countercurrent gas exchange did not occur in dogfish gills. However, other
investigations (Hanson and Martin, 1967; Piiper and Schumann, 1967;
Baumgarten-Schumann and Piiper, 1968) have shown that in many cases
the arterial oxygen tension was higher than the mean expired oxygen
tension. This can be explained by some sort of countercurrent system,
although the path of water across the secondary lamellae is ultimately
obstructed by the median scptum. The possibility that water must flow
along the length of the filaments in elasmobranchs has led Piiper and
Schumann (1967) to propose a multicapillary model which is also effective in giving a negative value for the difference between expired and
arterial oxygen tensions.
Anatomical differences between teleost and selachian are marked,
since in the latter the respiratory current passes in through the spiracle
as well as the mouth and out through a variable number of gill slits not
covered by an operculum. The flow pattern from mouth and spiracle is
curious. \Vater entering the spiracle of a dogfish will leave through the
three anterior gill slits of thc same side, whereas water entering through