172 THE CIRCULATION OF THE BLOOD 



solution under pressure and starting the beating by application of elec- 

 trical stimuli. Isolated strips of ventricular muscle, in which also no 

 nervous element can be demonstrated, may under favorable conditions 

 be caused to beat quite regularly if supplied with proper nutrient fluid. 

 The rebuttal of this evidence is twofold: In the first place, skeletal mus- 

 cle itself under certain conditions, such as exposure to solutions con- 

 taining an excess of phosphate (Biedermann's), may exhibit rhythmic 

 contractility, especially on cooling, which indicates that exhibition of rhyth- 

 mic power in isolated portions of cardiac muscle need not mean that under 

 ordinary conditions such power is responsible for the normal heartbeat. 

 In the second place, it is pointed out that although we can not reveal 

 their presence by present-day histological methods, this is not conclusive 

 evidence that the heart-muscle fiber may not possess some nervous struc- 

 tures capable of functioning as nerve cells. 



The heart even of mammals can be made to continue beating for sev- 

 eral days after excision from the body. The nerve cells, as we know them 

 in the central nervous system at least, can not, on the other hand, be 

 made to functionate for more than a few hours after death. Therefore, 

 it is argued, the heartbeat in surviving mammalian hearts can not de- 

 pend on the nervous structures. The argument is however easily refuted: 

 on the one hand, we do not know that the nerve structures situated 

 peripherally in the heart muscles are of the same viable nature as those 

 composing the central nervous system; and, on the other, the survival 

 of the heart may in itself be sufficient to maintain around the nerve cells 

 embedded in it a nutrient environment which is much more physiological 

 than that which we can supply in artificial perfusions of surviving 

 nervous tissues. 



4. Circumstantial but nevertheless strong evidence is furnished by 

 the fact that many other varieties of involuntary muscle are endowed 

 with rhythmic contractility; thus, the muscle of the intestines, of the 

 ureters, of the bladder, of the uterus, of the blood vessels of certain 

 animals, and of the lymph vessels in the so-called lymph hearts, main- 

 tain rhythmic contractility after isolation from the animal body. The 

 rhythmic power seems in certain of these cases to be independent of 

 nervous control. 



Neurogenic Hypothesis 



In favor of this hypothesis the following evidence is offered: 

 1. The heart of certain animals of Limulus, the king-crab, for exam- 

 ple is definitely dependent for its rhythmic contractility upon neigh- 

 boring nervous structures. The heart of this animal is a tubular sac- 

 culated organ, and along its dorsal surface there runs longitudinally a 



