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Cardiac output and pulsatile aortic flow in the teleost, Gadus morhua
Comp. Biochem. Physiol., 1962, Vol. 7, pp. 169 to 174. Pergamon Press Ltd., London. Printed in Great Britain
CARDIAC O U T P U T AND PULSATILE AORTIC FLOW IN T H E TELEOST, G A D U S M O R H U A KJELL JOHANSEN Institute for Experimental Medical Research, University of Oslo, Ullevaal Hospital, Oslo, Norway (Received 28 J u l y 1962)
Abstract.--1. Stroke volume and cardiac output have been measured in the teleost Gadus morhua using square wave electromagnetic flowmeter technique. Simultaneous values of intraventricular pressure and pressure in the bulbus arteriosus were recorded. 2. In a specimen weighing 2"9 kg the stroke volume was determined to be 0'90 cm 3. The heart rate was 30 beats/rain, giving a cardiac output of 9"3 cma/ kg/min. 3. In response to an increased venous return, the stroke volume was almost doubled within a few beats. No cardiac acceleration accompanied this response. 4. The bulbus arteriosus exerted a pronounced pressure chamber effect. The intraventricular pressure was converted to a slowly rising pressure in the bulbus extending in time far into ventricular diasto.le. Ventral aortic outflow was consequently maintained far into diastole, thereby differing markedly from conditions in mammals. N o MEASUREMENTS of stroke volume, aortic flow or cardiac output using direct methods have been reported for fish. Similarly, determinations of intracardiac and intravascular pressures recorded with adequate techniques are very scarce (Mort, 1951; Johansen, 1960; Satchell, 1960). O u r knowledge of cardiovascular function in fishes is henceforth strikingly inferior to what is known f r o m all other classes of vertebrates. T h i s is surprising, as the cardiovascular system of fishes shows m a n y unique features. T h e fact that the respiratory capillaries are in direct confluence with the systemic vessels in one single circuit poses interesting problems f r o m a haemodynamical point of view. I n the present investigation, intracardiac and bulbus arteriosus pressures have been recorded while monitoring the pulsatile ventral aortic outflow (stroke volume) simultaneously, using electromagnetic flow-meter technique. MATERIALS AND M E T H O D S T h e cod, G a d u s m o r h u a , was used as experimental animal in the present investigation. T h e specimens weighed f r o m 1.5 to 3.0 kg. T h e y were brought to the laboratory in large steel tanks filled with well-aerated sea water. Starting an experiment, the animal was tied to a rack ventral side up. N o anaesthesia was used. T h e animal was re-immersed in the salt-water tank to a water level 169
170
KJELL Jorx~qsErq
permitting exposure of the heart and adjoining great vessels in air. The fish kept on breathing spontaneously as the gills were always submerged. The ventral aorta was carefully dissected free just anterior to the bulbus arteriosus. Two ligatures were passed around the vessel allowing for easier manipulation. The vessel was subsequently fitted into the lumen of an electromagnetic flow probe. The size of the flow probe was carefully selected so that the vessel filled out the entire space between the pick-up electrodes without causing an actual compression of the vessel wall. The flow probe was made to rest in a position so that no compression was exerted on adjoining vessels and tissues. The incision was filled with Ringer solution to secure good and stable electrical contacts. Intraventricular and bulbus pressures were recorded through polyethylene catheters equipped with steel cannula at the tip. They were introduced into the proper sites by penetration of the ventricular wall and bulbus wall respectively. No obstruction to normal blood flow occurred as a result of the catheters or flow probe. Statham pressure transducers model p23BB were used and all recordings made on a Sanborn 4-channel recorder. The pressures were calibrated against water. The flowmeter was of a square-wave electromagnetic type designed and produced by the Kiger-Dennard Associates, Winston Salem, N.C., U.S.A. It was calibrated in vitro using an excised artery fitting closely on the flow probe used during the actual recording in situ. The deflexions obtained by injecting known amounts of blood or Ringer solution through the artery served as calibration. Increased venous return to the heart was obtained by slow infusion of blood or Ringer solution through an indwelling catheter placed in an intestinal vein. RESULTS Figure 1 depicts simultaneous records of pulsatile ventral aortic flow (upper tracing) and intraventricular pressure. The stroke volume in this particular 0"90 ~
'
~
cms /stroke "
'/~
~ _
Stroke
volume
CmHzO 4O
0
Sec Fzo. 1. Stroke volume and intraventricu!ar pressure in a 2.9 kg cod (Gadusmorhua).
fish was determined to be 0"90 cm a which represents a cardiac output of 27.0 cm3/min a t the prevailing heart rate of 30 beats/rain. The fish weighed 2.9 kg
171
CARDIAC O U T P U T I N TELEOSTS
giving a cardiac output of 9.3 cma/kg/min. The intraventricular pressure shows systolic values around 65 cm H20 or 47.7 mm Hg. Diastolic values are close to zero. Figure 2 demonstrates ventricular and bulbus pressure together with pulsatile aortic flow (stroke volume). Note the striking pressure chamber effect ("wind
Stroke volume B.P. V.P Sec
FIG. 2. Stroke volume and pressures in the bulbus arteriosus and ventricle of the cod, Gadus morhua. kessel" effect) of the bulbus arteriosus. The rapidly climbing intraventricular pressure is converted to an extended, smooth output in the bulbus on account of the large amount of elastic tissue in this segment. Thus the peak systolic pressure reached in the bulbus is considerably smaller than peak intraventricular systolic pressure. However, the bulbus pressure extends far into ventricular diastole. This pronounced depulsating effect has a number of possible consequences (see later Discussion). An obvious effect on ventricular ejection and outflow is apparent from Fig. 2. Whereas positive aortic flow in higher vertebrates is interrupted and reversed at the closure of the aortic valves, arterial outflow from the fish heart is maintained far into ventricular diastole. The mechanism responsible for this feature is no doubt exerted by the bulbus arteriosus. There is at no time during the cardiac cycle a backflow in the ventral aorta, and zero flow exists for a very brief period only. Considerable interest is focused on how the fish heart adjusts to an increased load, e.g. to periods of exercise. Figure 3 demonstrates the response in intraventricular and bulbus pressure as well as changes in stroke volume when an increased venous return is effected. Intraventricular and bulbus pressures increase only slightly, whereas the stroke volume almost doubles within a few beats from 0.83 cm 3 before to 1.39 cm 3 just after the increased venous return. An increased distensibility and filling of the ventricle did obviously accompany the increased venous return, likewise there was a sudden shift in end-systolic or residual volume of the ventricle. T h e m o r e complete ejection was easily observed by visual inspection of the heart. There was, however, no change in heart rate. The increased stroke volume and hence cardiac output persisted for a considerable time after the actual infusion had ceased. Thus the infused volume represented only a fraction of the net increase
KJELLJOHANSrN
172
in cardiac output. This indicates that the stimulus of an infusion brings about a general reduction in the venous reservoirs (capacity spaces) and augments venous backflow to the heart.
~ CmH20
430
0
]
Stroke volume Bul bus pressure
~
iitA f~ A f~ A~/~AAAAAAA. l/~ venfriculOrpressure 0J" ~ Increosedvenousreturn, sec v
v
v
v
v
~
~
~
~
~
~
~
FIC. 3. Response to an applied increased venous return in the cod, Gadus morhua.
DISCUSSION The few earlier reports on cardiac output in fishes were all derived from indirect methods or with crude techniques. Hart (1943) estimated the stroke volume in various species of fish by determining the mean systolic and diastolic weights of the ventricle. The weights he obtained by ligaturing the ventricle at the proper time. The technique is open to serious criticism. To get one set of values he had to use different individual fish. Furthermore, one must anticipate spontaneous changes in the end diastolic as well as the residual volume of different ventricles, a fact which cannot be accounted for by the technique used. Such spontaneous changes in the ventricular residual volume were actually demonstrated in the radiological studies of Mort (1950) and Hol & Johansen (1960). Hart concluded that stroke volume was lowest at the higher heart rates and greater in species with low blood pressure than with a higher blood pressure. He also demonstrated a direct relationship between stroke volume and weight of the fishes. Burger & Bradley (1951) working with elasmobranchs used similar techniques and claimed that ventricular distension is the main determinant of stroke volume in fishes. The approach used in these experiments was, however, crude and the results contribute little to an understanding of the normal regulation of cardiac output. Mott (1957) calculated cardiac output for some species of teleosts. She used the Fick principle and assumed a maximal oxygen content of the arterial blood calculated from values published earlier by Root (1931) for oxygen capacity. The venous blood was assumed to be totally depleted in oxygen. The values for oxygen consumption were taken from Hall (1929). The figures thus arrived at ranged from 10"1 cm3/kg/min to 15.5 cm3/kg/min. One can hardly imagine a more indirect route for evaluation of cardiac output. Besides some of the suppositions
CARDIAC O U T P U T I N T E L E O S T S
173
must be considered to be incorrect. Steen (1962) has recently shown that venous blood in fishes (teleosts) may Contain surprisingly high amounts of oxygen. The values for stroke volume and cardiac output obtained during the present investigation correspond fairly well with the computed values of Mort. The author is, however, inclined to look upon this correspondence as coincidental. One must expect large variations related to the mode of life of the fish. The demands on the cardiovascular system must be far greater in fast-swimming species than in sluggish bottom dwellers. The results obtained with increased venous return in the present study seem of significance. Recent experiments with recording of blood pressure in freeswimming teleosts (Johansen & Waage-Johannessen, 1962--unpublished) have demonstrated that cardio-acceleration never accompanies exercise. The blood pressure, however, is rapidly increased at only a slight increase in swimming rate. The prompt increase in stroke volume obtained in the present study suggests that increased volume rather than rate is the common response to an increased demand on the fish heart. This correlates with the general teaching that the innervation of the t eleost heart consists only of parasympathetic fibres in the vagus, while sympa. thetic innervation seems entirely absent. A crucial role in augmenting the necessary venous inflow resides in the emptying of venous reservoirs (capacity spaces). It can also be inferred that venous return in all likelihood is facilitated by the muscular movements during swimming. The bulbus arteriosus of the teleost heart plays a crucial role as an adjusting and regulating chamber. The main function exerted by this chamber was believed to be a protection of the delicate gill capillaries by preventing excessive pressure fluctuations in the ventral aorta (Brticke, 1852; Keith, 1924). The present investigations on the teleost heart also demonstrate an important regulatory role of the bulbus in effecting maintenance of blood flow in late systole and far into ventricular diastole. This is in striking contrast to the dynamics of ventricular ejection in the higher vertebrates, i.e. birds and mammals. In mammals positive aortic flow prevails for less than one-third of the cardiac cycle (Spencer & Greiss, 1962). In fishes positive flow in the ventral aorta exists for more than three-quarters of the cardiac cycle. The importance of the maintenance of ventral aortic flow in fishes is particularly significant when considering their low heart rate. In amphibians and reptiles where the bulbus segment also exists as a discrete segment of the heart, a similar role of the bulbus in extending aortic outflow has been proposed by March (1961) for the turtle and demonstrated for the salamander Amphiuma tridactylum by Johansen (1962). REFERENCES BRt~CKEE. VON (1852) Beitr~ige zur vergleichenden Anatomie und Physiologie des Geftisssystems. Denkschr. Akad. Wiss. Wien. 3, 335-367. BURGERJ. W. & BRADLEYS. E. (1951) The general form of the circulation in the dogfish, Squalus acanthias. J. Cell. Comp. Physiol. 37, 389-402. HALL F. G. (1929) The influence of varying oxygen tensions upon the rate of oxygen consumption in marine fishes. Amer. J. Physiol. 88, 212-218. 12
174
KJELL JOHANSEN
H~mr J. S. (1943) The cardiac output of four freshwater fish. Canad. J. Research 21, 77-84. HOL R. & JOHm'~SENK. (1960) A cineradiographic study of the central circulation of the hagfish, Myxine glutinosa L. J. Exp. Biol. 37, 469--473. JOHANS~N K. (1960) Circulation in the hagfish Myxine glutinosa L. Biol. Bull. 118, 289-295. JOH~WSEN K. (1962) Cardiovascular dynamics in the amphibian, Amphiuma tridactylum. (In preparation.) K~ITH A. (1924) Fate of the bulbus cordis in the human heart. Lancet 207, 1267-1273." MARCH H. W. (1961) The persistence of a functioning bulbus cordis homologue in the turtle heart. Amer. J. Physiol. 201, 1109-1112. MOTT J. C. (1950) Some radiological observations on the common eel, Anguilla anguilla. J. Exp. Biol. 27, 324--333. MoT'r J. C. (1951) Some factors affecting the blood circulation in the common eel (Anguilla anguilla). J. Physiol. 114, 387-398. MorT J. C. (1957) The Physiology of Fishes. The Cardiovascular System, pp. 81-108. Academic Press, New York. ROOT P. W. (1931) The respiratory function of the blood of marine fishes. Biol. Bull. 61, 427-456. SATCHELL G. H. (1960) The reflex co-ordination of the heart beat with respiration in the dogfish. J. Exp. Biol. 37, 719-731. SPSNCER M. P. & GR~ISS F. C. (1962) Dynamics of ventricular ejection. Circ. Res. 10, 274279. STUN J. B. (1962) Personal communication.
CARDIAC O U T P U T AND PULSATILE AORTIC FLOW IN T H E TELEOST, G A D U S M O R H U A KJELL JOHANSEN Institute for Experimental Medical Research, University of Oslo, Ullevaal Hospital, Oslo, Norway (Received 28 J u l y 1962)
Abstract.--1. Stroke volume and cardiac output have been measured in the teleost Gadus morhua using square wave electromagnetic flowmeter technique. Simultaneous values of intraventricular pressure and pressure in the bulbus arteriosus were recorded. 2. In a specimen weighing 2"9 kg the stroke volume was determined to be 0'90 cm 3. The heart rate was 30 beats/rain, giving a cardiac output of 9"3 cma/ kg/min. 3. In response to an increased venous return, the stroke volume was almost doubled within a few beats. No cardiac acceleration accompanied this response. 4. The bulbus arteriosus exerted a pronounced pressure chamber effect. The intraventricular pressure was converted to a slowly rising pressure in the bulbus extending in time far into ventricular diasto.le. Ventral aortic outflow was consequently maintained far into diastole, thereby differing markedly from conditions in mammals. N o MEASUREMENTS of stroke volume, aortic flow or cardiac output using direct methods have been reported for fish. Similarly, determinations of intracardiac and intravascular pressures recorded with adequate techniques are very scarce (Mort, 1951; Johansen, 1960; Satchell, 1960). O u r knowledge of cardiovascular function in fishes is henceforth strikingly inferior to what is known f r o m all other classes of vertebrates. T h i s is surprising, as the cardiovascular system of fishes shows m a n y unique features. T h e fact that the respiratory capillaries are in direct confluence with the systemic vessels in one single circuit poses interesting problems f r o m a haemodynamical point of view. I n the present investigation, intracardiac and bulbus arteriosus pressures have been recorded while monitoring the pulsatile ventral aortic outflow (stroke volume) simultaneously, using electromagnetic flow-meter technique. MATERIALS AND M E T H O D S T h e cod, G a d u s m o r h u a , was used as experimental animal in the present investigation. T h e specimens weighed f r o m 1.5 to 3.0 kg. T h e y were brought to the laboratory in large steel tanks filled with well-aerated sea water. Starting an experiment, the animal was tied to a rack ventral side up. N o anaesthesia was used. T h e animal was re-immersed in the salt-water tank to a water level 169
170
KJELL Jorx~qsErq
permitting exposure of the heart and adjoining great vessels in air. The fish kept on breathing spontaneously as the gills were always submerged. The ventral aorta was carefully dissected free just anterior to the bulbus arteriosus. Two ligatures were passed around the vessel allowing for easier manipulation. The vessel was subsequently fitted into the lumen of an electromagnetic flow probe. The size of the flow probe was carefully selected so that the vessel filled out the entire space between the pick-up electrodes without causing an actual compression of the vessel wall. The flow probe was made to rest in a position so that no compression was exerted on adjoining vessels and tissues. The incision was filled with Ringer solution to secure good and stable electrical contacts. Intraventricular and bulbus pressures were recorded through polyethylene catheters equipped with steel cannula at the tip. They were introduced into the proper sites by penetration of the ventricular wall and bulbus wall respectively. No obstruction to normal blood flow occurred as a result of the catheters or flow probe. Statham pressure transducers model p23BB were used and all recordings made on a Sanborn 4-channel recorder. The pressures were calibrated against water. The flowmeter was of a square-wave electromagnetic type designed and produced by the Kiger-Dennard Associates, Winston Salem, N.C., U.S.A. It was calibrated in vitro using an excised artery fitting closely on the flow probe used during the actual recording in situ. The deflexions obtained by injecting known amounts of blood or Ringer solution through the artery served as calibration. Increased venous return to the heart was obtained by slow infusion of blood or Ringer solution through an indwelling catheter placed in an intestinal vein. RESULTS Figure 1 depicts simultaneous records of pulsatile ventral aortic flow (upper tracing) and intraventricular pressure. The stroke volume in this particular 0"90 ~
'
~
cms /stroke "
'/~
~ _
Stroke
volume
CmHzO 4O
0
Sec Fzo. 1. Stroke volume and intraventricu!ar pressure in a 2.9 kg cod (Gadusmorhua).
fish was determined to be 0"90 cm a which represents a cardiac output of 27.0 cm3/min a t the prevailing heart rate of 30 beats/rain. The fish weighed 2.9 kg
171
CARDIAC O U T P U T I N TELEOSTS
giving a cardiac output of 9.3 cma/kg/min. The intraventricular pressure shows systolic values around 65 cm H20 or 47.7 mm Hg. Diastolic values are close to zero. Figure 2 demonstrates ventricular and bulbus pressure together with pulsatile aortic flow (stroke volume). Note the striking pressure chamber effect ("wind
Stroke volume B.P. V.P Sec
FIG. 2. Stroke volume and pressures in the bulbus arteriosus and ventricle of the cod, Gadus morhua. kessel" effect) of the bulbus arteriosus. The rapidly climbing intraventricular pressure is converted to an extended, smooth output in the bulbus on account of the large amount of elastic tissue in this segment. Thus the peak systolic pressure reached in the bulbus is considerably smaller than peak intraventricular systolic pressure. However, the bulbus pressure extends far into ventricular diastole. This pronounced depulsating effect has a number of possible consequences (see later Discussion). An obvious effect on ventricular ejection and outflow is apparent from Fig. 2. Whereas positive aortic flow in higher vertebrates is interrupted and reversed at the closure of the aortic valves, arterial outflow from the fish heart is maintained far into ventricular diastole. The mechanism responsible for this feature is no doubt exerted by the bulbus arteriosus. There is at no time during the cardiac cycle a backflow in the ventral aorta, and zero flow exists for a very brief period only. Considerable interest is focused on how the fish heart adjusts to an increased load, e.g. to periods of exercise. Figure 3 demonstrates the response in intraventricular and bulbus pressure as well as changes in stroke volume when an increased venous return is effected. Intraventricular and bulbus pressures increase only slightly, whereas the stroke volume almost doubles within a few beats from 0.83 cm 3 before to 1.39 cm 3 just after the increased venous return. An increased distensibility and filling of the ventricle did obviously accompany the increased venous return, likewise there was a sudden shift in end-systolic or residual volume of the ventricle. T h e m o r e complete ejection was easily observed by visual inspection of the heart. There was, however, no change in heart rate. The increased stroke volume and hence cardiac output persisted for a considerable time after the actual infusion had ceased. Thus the infused volume represented only a fraction of the net increase
KJELLJOHANSrN
172
in cardiac output. This indicates that the stimulus of an infusion brings about a general reduction in the venous reservoirs (capacity spaces) and augments venous backflow to the heart.
~ CmH20
430
0
]
Stroke volume Bul bus pressure
~
iitA f~ A f~ A~/~AAAAAAA. l/~ venfriculOrpressure 0J" ~ Increosedvenousreturn, sec v
v
v
v
v
~
~
~
~
~
~
~
FIC. 3. Response to an applied increased venous return in the cod, Gadus morhua.
DISCUSSION The few earlier reports on cardiac output in fishes were all derived from indirect methods or with crude techniques. Hart (1943) estimated the stroke volume in various species of fish by determining the mean systolic and diastolic weights of the ventricle. The weights he obtained by ligaturing the ventricle at the proper time. The technique is open to serious criticism. To get one set of values he had to use different individual fish. Furthermore, one must anticipate spontaneous changes in the end diastolic as well as the residual volume of different ventricles, a fact which cannot be accounted for by the technique used. Such spontaneous changes in the ventricular residual volume were actually demonstrated in the radiological studies of Mort (1950) and Hol & Johansen (1960). Hart concluded that stroke volume was lowest at the higher heart rates and greater in species with low blood pressure than with a higher blood pressure. He also demonstrated a direct relationship between stroke volume and weight of the fishes. Burger & Bradley (1951) working with elasmobranchs used similar techniques and claimed that ventricular distension is the main determinant of stroke volume in fishes. The approach used in these experiments was, however, crude and the results contribute little to an understanding of the normal regulation of cardiac output. Mott (1957) calculated cardiac output for some species of teleosts. She used the Fick principle and assumed a maximal oxygen content of the arterial blood calculated from values published earlier by Root (1931) for oxygen capacity. The venous blood was assumed to be totally depleted in oxygen. The values for oxygen consumption were taken from Hall (1929). The figures thus arrived at ranged from 10"1 cm3/kg/min to 15.5 cm3/kg/min. One can hardly imagine a more indirect route for evaluation of cardiac output. Besides some of the suppositions
CARDIAC O U T P U T I N T E L E O S T S
173
must be considered to be incorrect. Steen (1962) has recently shown that venous blood in fishes (teleosts) may Contain surprisingly high amounts of oxygen. The values for stroke volume and cardiac output obtained during the present investigation correspond fairly well with the computed values of Mort. The author is, however, inclined to look upon this correspondence as coincidental. One must expect large variations related to the mode of life of the fish. The demands on the cardiovascular system must be far greater in fast-swimming species than in sluggish bottom dwellers. The results obtained with increased venous return in the present study seem of significance. Recent experiments with recording of blood pressure in freeswimming teleosts (Johansen & Waage-Johannessen, 1962--unpublished) have demonstrated that cardio-acceleration never accompanies exercise. The blood pressure, however, is rapidly increased at only a slight increase in swimming rate. The prompt increase in stroke volume obtained in the present study suggests that increased volume rather than rate is the common response to an increased demand on the fish heart. This correlates with the general teaching that the innervation of the t eleost heart consists only of parasympathetic fibres in the vagus, while sympa. thetic innervation seems entirely absent. A crucial role in augmenting the necessary venous inflow resides in the emptying of venous reservoirs (capacity spaces). It can also be inferred that venous return in all likelihood is facilitated by the muscular movements during swimming. The bulbus arteriosus of the teleost heart plays a crucial role as an adjusting and regulating chamber. The main function exerted by this chamber was believed to be a protection of the delicate gill capillaries by preventing excessive pressure fluctuations in the ventral aorta (Brticke, 1852; Keith, 1924). The present investigations on the teleost heart also demonstrate an important regulatory role of the bulbus in effecting maintenance of blood flow in late systole and far into ventricular diastole. This is in striking contrast to the dynamics of ventricular ejection in the higher vertebrates, i.e. birds and mammals. In mammals positive aortic flow prevails for less than one-third of the cardiac cycle (Spencer & Greiss, 1962). In fishes positive flow in the ventral aorta exists for more than three-quarters of the cardiac cycle. The importance of the maintenance of ventral aortic flow in fishes is particularly significant when considering their low heart rate. In amphibians and reptiles where the bulbus segment also exists as a discrete segment of the heart, a similar role of the bulbus in extending aortic outflow has been proposed by March (1961) for the turtle and demonstrated for the salamander Amphiuma tridactylum by Johansen (1962). REFERENCES BRt~CKEE. VON (1852) Beitr~ige zur vergleichenden Anatomie und Physiologie des Geftisssystems. Denkschr. Akad. Wiss. Wien. 3, 335-367. BURGERJ. W. & BRADLEYS. E. (1951) The general form of the circulation in the dogfish, Squalus acanthias. J. Cell. Comp. Physiol. 37, 389-402. HALL F. G. (1929) The influence of varying oxygen tensions upon the rate of oxygen consumption in marine fishes. Amer. J. Physiol. 88, 212-218. 12
174
KJELL JOHANSEN
H~mr J. S. (1943) The cardiac output of four freshwater fish. Canad. J. Research 21, 77-84. HOL R. & JOHm'~SENK. (1960) A cineradiographic study of the central circulation of the hagfish, Myxine glutinosa L. J. Exp. Biol. 37, 469--473. JOHANS~N K. (1960) Circulation in the hagfish Myxine glutinosa L. Biol. Bull. 118, 289-295. JOH~WSEN K. (1962) Cardiovascular dynamics in the amphibian, Amphiuma tridactylum. (In preparation.) K~ITH A. (1924) Fate of the bulbus cordis in the human heart. Lancet 207, 1267-1273." MARCH H. W. (1961) The persistence of a functioning bulbus cordis homologue in the turtle heart. Amer. J. Physiol. 201, 1109-1112. MOTT J. C. (1950) Some radiological observations on the common eel, Anguilla anguilla. J. Exp. Biol. 27, 324--333. MoT'r J. C. (1951) Some factors affecting the blood circulation in the common eel (Anguilla anguilla). J. Physiol. 114, 387-398. MorT J. C. (1957) The Physiology of Fishes. The Cardiovascular System, pp. 81-108. Academic Press, New York. ROOT P. W. (1931) The respiratory function of the blood of marine fishes. Biol. Bull. 61, 427-456. SATCHELL G. H. (1960) The reflex co-ordination of the heart beat with respiration in the dogfish. J. Exp. Biol. 37, 719-731. SPSNCER M. P. & GR~ISS F. C. (1962) Dynamics of ventricular ejection. Circ. Res. 10, 274279. STUN J. B. (1962) Personal communication.