282 INVERTEBRATE PHYSIOLOGY 



some of them obviously incorrect. The "fatigue theory," on the other hand, 

 is based on some sound experimental evidence. We therefore think that 

 the "back-pressure theory" must be definitely discarded. 



Now the question arises : What is this fatigue which causes periodical 

 arrest of a leading center ? 



A pacemaker is a region which shows rhythmic spontaneous activity, by 

 which the heart is induced to contract. If the pacemaker is neurogenic, as 

 in most anthropods, its activity consists of nerve impulses. If the pace- 

 maker is myogenic, it can be composed of modified muscle tissue, as in 

 homoiothermic vertebrates, which cannot contract itself but sends im- 

 pulses to the contractile system. Or it contracts itself, as in poikilothermic 

 vertebrates and molluscs, and thus induces contraction on neighboring 

 parts. 



Let us assume that the pacemakers of the tunicate heart have a myogenic 

 nature and contract themselves. After all, this assumption is in accordance 

 with many observations. Now what is the cause of these rhythmic con- 

 tractions of a pacemaker and why does it become exhausted after some 

 time ? Some authors state or rather imply that this fatigue is an exhaustion 

 of the contractile power, in other words a prolonged refractory period of 

 its muscular activity. As long as a pacemaker is working at normal fre- 

 quency the other one would have no chance, because it is kept refractory 

 all. the time by the oncoming waves which induce it to contract before it 

 can start spontaneous activity. However, this cannot be correct. We may 

 mention a few arguments which militate against this explanation. An 

 isolated center shows periods of activity, alternated by rest. These periods 

 of rest occur in spite of the fact that no oncoming waves from the other 

 side induce refractoriness. Further, the pause between reversals in the 

 intact heart is shorter than the resting period in the halved heart. This 

 has been shown by several workers and by experiments in our laboratory 

 (see Krijgsman, 1956). Moreover, in the halved heart the pacemaker 

 remains passive for some time after its contractile power has certainly 

 been restored. And, finally, we may mention the fact that at the end 

 of a pulsation series both pacemakers are equally exhausted as far as their 

 contractile power is concerned, one by spontaneous, the other one by in- 

 duced contractions. Thus during the rest period both have the same chance 

 to recover. Why, then, should there be a regular reversal ? One would ex- 

 pect a competition of the pacemakers at the end of the rest period. In 

 short, regular reversal would be an incomprehensible phenomenon if in- 

 terpreted in terms of contractility. The conclusion must be drawn that 

 contractile power and pacemaker activity are two different things. 



What, then, is the particular stimulus which releases the spontaneous 

 activity of a pacemaker of the tunicate heart ? In vertebrates and molluscs 



