THE NERVE-NET 127 



brate heart is a muscle freely permeated by a nerve-net. 

 It has already been pointed out that there is a sharply 

 marked difference of opinion as to the nature of the 

 heart's contraction, some investigators maintaining that 

 the beat is dependent upon a rhythmic activity of the 

 nervous tissue that is transferred to the muscle ; others, 

 and these include the majority of modern workers, that 

 the rhythm originates in the muscle itself, the nerve-net 

 and other such structures being merely means of modify- 

 ing the beat. Strong evidence in favor of the latter or 

 myogenic theory of the heart-beat is found in the fact that 

 the heart of the vertebrate embryo beats long before the 

 first appearance of nervous tissue and that in tissue cul- 

 tures single rhythmically contractile muscles fibers be- 

 yond the reach of anything possibly nervous can often be 

 found. Yet, notwithstanding this evidence, it is entirely 

 possible that the cardiac nerve-net may have come to be a 

 more intimate part of the rhythmic mechanism of the 

 adult heart than is supposed by many, in which case this 

 organ in its autonomy would have a strong resemblance 

 to the actinian tentacle, for, like the tentacle, it may be 

 removed from the animal and by appropriate means made 

 to act in a strikingly normal manner for a relatively long 

 time. 



Another portion of the vertebrate body that exhibits 

 much autonomy and at the same time possesses a nerve- 

 net is the digestive tube; especially the small intestine. 

 Under normal circumstances the food in the small intes- 

 tine is carried along from the end next the stomach to the 

 outlet into the large intestine by a process of peristalsis, 

 the essential part of which consists of a wave of con- 

 striction that starts in the higher part of the tube and 

 passes downward to its lower end. This wave of con- 

 striction is preceded by a wave of relaxation which nat- 



