THE TUNICATE HEART 281 



(5) If a back-pressure should be built up, one would expect a gradually 

 increasing expansion of the larger vessels and/or a change in the rate of 

 flow in the course of a pulsation series. This has never been observed. On 

 the contrary, careful workers such as v. Skramlik (1941), said that the 

 flow of blood is a regular one-way movement as long as a pulsation period 

 lasts. 



(6) The "back-pressure theory" tries to give a causal explanation, if 

 not of the start of activity of the resting center, then at any rate of the 

 arrest of the leading center. However, there can be no certainty at all 

 that increasing pressure causes arrest of the leading center. On the con- 

 trary, it has been found in the hearts of molluscs, arthropods, and verte- 

 brates that the pacemaker is facilitated by increased tension of the heart 

 muscles. Therefore we cannot accept, without experimental evidence, the 

 postulate that a pacemaker of the tunicate heart slows or stops beating as 

 a result of increased pressure. On the contrary, one could expect that the 

 leading center would go on laboring, showing more or less isometric con- 

 tractions. However, the back-pressure would then be reduced very quickly 

 because of considerable leakage, and thus there would be no reason what- 

 ever for the cessation of the activity of the center. 



(7) If an increasing back-pressure were the cause of arrest, the slow- 

 ing.or laboring of the leading center ought to be of a gradual nature, against 

 the gradual increase in pressure. This has never been shown ; several 

 workers have observed that only the last or a few final beats are delayed, 

 but prior to this there is a fairly constant rhythm (see Krijgsman, 1956). 



(8) All authors who advocated back-pressure as a cause of arrest have 

 assumed that this pressure actually builds up gradually, i.e., increases 

 and increases until the heart can no longer counteract a final "critical" 

 pressure. However, it seems most improbable that during the course of a 

 pulsation series there is a continual increase in the back-pressure. On the 

 contrary, in such a system a new steady state will be reached very soon. 

 v. Skramlik (1930a), and Ebara (1951), observed that a pulsation series 

 in the intact animal lasts a considerable time in certain species, sometimes 

 even more than 3 hours. It would be contradictory to physical principles 

 to assume that a back-pressure continues to increase during such long 

 periods. Apart from this it has been observed that a heart ligatured in the 

 middle continues to beat ( Schultze, 1901 ; Nicolai, 1908 ; v. Skramlik, 

 1926a ; Ebara, 1954) in spite of the fact that almost immediately a constant 

 back-pressure is built up. Apparently the leading center can beat for a 

 considerable time against a constant back-pressure. Thus a "critical" 

 value of that pressure does not exist. 



■ We feel that the "back-pressure theory" is based on mere assumptions, 



