pac*ernaker and t~ non-linccir responsc i n the visceral one. Anderson
concludeti that h e a r t - h t t rcvcrsril ( a i m be entirely explained by the
intrinsic properties of the heart! wlls all of which have pacemaking
properties. 8hc considcrtd thatl central areas of the heart do not
normally bcvome doinintint k)ecause their electrical activity can be
dissipated in two directions along thc heart, while at the ends of the
hmrt till of the clectriccil activity is concentrated in one direction and
promotes dominance.
It is clear from the foregoiiig piiragraphs that there is still insufficient
evidence avtdable to ciirilole 11s to draw any firm conclusions concerning
the rnechanistns which control tho reversal of heart beat in ascidians.
It is possible that a f'ull explanation of thc phenomenon requires a
synthesis of the various meclianisrns involved. I n view of the behaviour
of isoltited hearts and portions of hearts it is inconceivable that back
pressure alone can explain reversal. At the same time the work of
Kriebc4 leaves us in no doubt that changes in pressure can strongly
influence thc behaviour of the heart and that it functions best under a
pressure of 5 rm watcr. However, the fact that cardiac behaviour is
influenced b-y pressure does not necessarily mcan that pressure changes
cause reversd. 'I'he discovvry by Anderson (1 968) that there is an
inherent rhythm of high and low frequencies in the visceral pacemaker
but not in tho hypobrttiichiat suggests that it is the varying responses of
this end of the heart which controls cwdiac reversal. Brocas et al.
(1966) suggcst that carbon tlioxitlr tcwsion or p H change may affect the
activity of this end ot' the iicart. Alternatively, the visceral rhythm
may bc controlled by the c*ardiaa sec*retions noted by Kalk and such
control might be exercised by r~ varying threshold of response
suggested by Krijpsmtiii (1956). One might therefore visualize the heart
being controlled in this way t).y stimulation of the virsceral centre but
being modified by changes in pressure or mechanical stress.
G. (Amclusion
The ascidian heart is IL simple tube constructed from il single layer
of' niyocndot helial cells and completely enclosed by a tough pericardium
which maintains t i pressure of betwccii 2 and 5 cm water around the
heart. Blood is forced through the heart] by peristaltic waves of contraction passing from end to end of the heart and in the absence of
structural valves a functional valvc forms a t the venous end due to
pressure from the pericardial fluid. It is powible that the pericarclial
body which floats freely in the pricurdisl fluid may also play some role
in forming func~tional vrilveh At, periodic intervals, usually of two to
three minutcu duration, I hr Iiwrt reverses the direction of contraction.
concludeti that h e a r t - h t t rcvcrsril ( a i m be entirely explained by the
intrinsic properties of the heart! wlls all of which have pacemaking
properties. 8hc considcrtd thatl central areas of the heart do not
normally bcvome doinintint k)ecause their electrical activity can be
dissipated in two directions along thc heart, while at the ends of the
hmrt till of the clectriccil activity is concentrated in one direction and
promotes dominance.
It is clear from the foregoiiig piiragraphs that there is still insufficient
evidence avtdable to ciirilole 11s to draw any firm conclusions concerning
the rnechanistns which control tho reversal of heart beat in ascidians.
It is possible that a f'ull explanation of thc phenomenon requires a
synthesis of the various meclianisrns involved. I n view of the behaviour
of isoltited hearts and portions of hearts it is inconceivable that back
pressure alone can explain reversal. At the same time the work of
Kriebc4 leaves us in no doubt that changes in pressure can strongly
influence thc behaviour of the heart and that it functions best under a
pressure of 5 rm watcr. However, the fact that cardiac behaviour is
influenced b-y pressure does not necessarily mcan that pressure changes
cause reversd. 'I'he discovvry by Anderson (1 968) that there is an
inherent rhythm of high and low frequencies in the visceral pacemaker
but not in tho hypobrttiichiat suggests that it is the varying responses of
this end of the heart which controls cwdiac reversal. Brocas et al.
(1966) suggcst that carbon tlioxitlr tcwsion or p H change may affect the
activity of this end ot' the iicart. Alternatively, the visceral rhythm
may bc controlled by the c*ardiaa sec*retions noted by Kalk and such
control might be exercised by r~ varying threshold of response
suggested by Krijpsmtiii (1956). One might therefore visualize the heart
being controlled in this way t).y stimulation of the virsceral centre but
being modified by changes in pressure or mechanical stress.
G. (Amclusion
The ascidian heart is IL simple tube constructed from il single layer
of' niyocndot helial cells and completely enclosed by a tough pericardium
which maintains t i pressure of betwccii 2 and 5 cm water around the
heart. Blood is forced through the heart] by peristaltic waves of contraction passing from end to end of the heart and in the absence of
structural valves a functional valvc forms a t the venous end due to
pressure from the pericardial fluid. It is powible that the pericarclial
body which floats freely in the pricurdisl fluid may also play some role
in forming func~tional vrilveh At, periodic intervals, usually of two to
three minutcu duration, I hr Iiwrt reverses the direction of contraction.