BODY FLUIDS AND CIRCULATION 103 



Systolic contraction of the ventricle in turn is accompanied by closure of 

 the A.V. valves and discharge of blood into the ventral aorta, the semilunar 

 valves in turn closing when the ventricle passes into diastole and intra- 

 ventricular pressure falls below that in the ventral aorta. 



Vertebrate heart muscle is both striped and syncytial, so that the whole 

 organ forms a single excitable unit without internal cellular boundaries. 

 Consequently, when an electrical stimulus is applied to the intact heart, 

 or to a piece of heart tissue, the excitatory wave which is evoked spreads 

 throughout the tissue. The contraction of the vertebrate heart, either dur- 

 ing rhythmic activity or following electrical stimulation, is all or nothing 

 in character, i.e. it is maximal for the condition of the heart at that time 

 and is not affected by stimulus-strength. When tested by electrical stimula- 

 tion during various phases of a normal contractile cycle the heart proves 

 to be inexcitable during most of systole (absolute refractory period), after 



isec 

 Fig. 3.7. Refractory Period and Recovery of Excitability in 

 the Heart of Torpedo o eel lata 



Effect of a strong induction shock on the ventricle at different intervals after the 

 occurrence of an auricular contraction. Exact moment of stimulation is shown by the 

 black spots on the tracings. (From Mines, 1913.) 



which excitability gradually returns (Fig. 3.7). At the end of systole and 

 throughout most of diastole an interpolated stimulus evokes a contraction 

 of submaximal height (relative refractory period); recovery becomes com- 

 plete by the end of diastole. Any extra contraction interposed between two 

 normal contractions is followed by a compensatory pause longer than a 

 normal diastolic pause. The long absolute and relative refractory periods 

 preclude tetanization of heart muscle. When quiescent strips of vertebrate 

 heart muscle are stimulated by repeated shocks at suitable intervals (longer 

 than the refractory period), they respond by single contractions of increas- 

 ing magnitude, an effect known as staircase. 



According to Starling's law of the heart the strength of contraction is 

 dependent on its degree of distension. This is an effect common to all 

 muscle by which the force of contraction increases with stretching, up to 

 some maximal value. By this means the heart adapts itself to a given load. 

 The mechanics of cardiac output in several freshwater teleosts have been 

 investigated by Hart (58). The blood pressure rises with increase in rate of 



