308
C. SHELTON
(5) Hyohyoideus, adducting the operculum and folding up the
branchiostegal apparatus
( 6 ) Levator operculi, adducting and lifting the dorsal border of
the operculum. This has the effect of rotating the operculum on
its articulation with the hyomandibula so that the operculum
base is retracted. A ligament running from this region to the
angular causes a lowering of the jaw at the end of expiration
b. Inspiration. During inspiration the whole system is increasing in
volume and, since the opercular openings are closed by their valves,
water is taken in through the mouth. The muscles involved are as follows
(see Fig. 6 ) :
(1) Levator arcus palatini, abducting the palatal complex and
hyomandibula by rotating the latter outward on its articulation
with the skull
(2) Sternohyoideus, retracting the hyoid arch thus expanding the
branchial arches and abducting the hyomandibula
( 3 ) Dilator operculi, abducting the operculum by pivoting it on
its articulation with the hyomandibula
c. Znteruction of Elements. Because the hyomandibula is articulated
with the neurocranium, the quadrate, the operculum, and the more
ventral hyoid elements, it forms a vital coupling element in the interactions between the skeletal and muscular components. The lower end
of the hyomandibula moves freely and the mechanical linkages are such
that its abduction is accompanied by retraction of the more basal elements. This expands both the buccal and opercular cavities, opens the
mouth and, because the quadrates are also moved out laterally, widens
the mouth. The branchial arches also expand. In the opposite sense,
closure of the mouth will protract the hyoid which in turn adducts the
hyomandibula and quadrate and swings in the operculum. A fuller account of this type of coupling is given by Ballintijn and Hughes (1965).
The precise timing of activity in seven of the main respiratory muscles of the trout as determined by the electromyographic studies of
Ballintijn and Hughes (1965) is shown in Fig. 7. It might seem reasonable to suppose that in a smoothly oscillating system there would be
temporal dispersion of overall activity throughout the cycle, and overlap
between different units, certainly as inspiration took over from expiration
and vice versa. This appears to be the case over most of the cycle as
Fig. 7 shows, but there is a period during the final stages of opercular
abduction when little activity is found in any muscles. It may be that
elastic elements continue the movements smoothly during this period. It
is interesting that all the muscles in Fig. 7 are active over at least two
C. SHELTON
(5) Hyohyoideus, adducting the operculum and folding up the
branchiostegal apparatus
( 6 ) Levator operculi, adducting and lifting the dorsal border of
the operculum. This has the effect of rotating the operculum on
its articulation with the hyomandibula so that the operculum
base is retracted. A ligament running from this region to the
angular causes a lowering of the jaw at the end of expiration
b. Inspiration. During inspiration the whole system is increasing in
volume and, since the opercular openings are closed by their valves,
water is taken in through the mouth. The muscles involved are as follows
(see Fig. 6 ) :
(1) Levator arcus palatini, abducting the palatal complex and
hyomandibula by rotating the latter outward on its articulation
with the skull
(2) Sternohyoideus, retracting the hyoid arch thus expanding the
branchial arches and abducting the hyomandibula
( 3 ) Dilator operculi, abducting the operculum by pivoting it on
its articulation with the hyomandibula
c. Znteruction of Elements. Because the hyomandibula is articulated
with the neurocranium, the quadrate, the operculum, and the more
ventral hyoid elements, it forms a vital coupling element in the interactions between the skeletal and muscular components. The lower end
of the hyomandibula moves freely and the mechanical linkages are such
that its abduction is accompanied by retraction of the more basal elements. This expands both the buccal and opercular cavities, opens the
mouth and, because the quadrates are also moved out laterally, widens
the mouth. The branchial arches also expand. In the opposite sense,
closure of the mouth will protract the hyoid which in turn adducts the
hyomandibula and quadrate and swings in the operculum. A fuller account of this type of coupling is given by Ballintijn and Hughes (1965).
The precise timing of activity in seven of the main respiratory muscles of the trout as determined by the electromyographic studies of
Ballintijn and Hughes (1965) is shown in Fig. 7. It might seem reasonable to suppose that in a smoothly oscillating system there would be
temporal dispersion of overall activity throughout the cycle, and overlap
between different units, certainly as inspiration took over from expiration
and vice versa. This appears to be the case over most of the cycle as
Fig. 7 shows, but there is a period during the final stages of opercular
abduction when little activity is found in any muscles. It may be that
elastic elements continue the movements smoothly during this period. It
is interesting that all the muscles in Fig. 7 are active over at least two