PROPERTIES OF THE HEART MUSCLE. 563 



The Neurogenic and Myogenic Theories of the Heart Beat. — 



For many years there has been an active discussion among physi- 

 ologists as to whether the initial impulse starting the heart beat 

 originates in the heart muscle itself or in some of the intrinsic 

 nerve cells found in the heart. It is not profitable at present to 

 review this discussion in any detail. The adherents of the myo- 

 genic theories have shown clearly enough that heart muscle may 

 beat rhythmically when no nerve cells are present. The heart of 

 the embryo, for example, beats before nerve cells can be found 

 in it. Strips of heart muscle devoid of nerve cells can be made to 

 give rhythmic contractions, and pieces of heart muscle without 

 nerve cells grown in a tissue-medium contract rhythmically. It 

 can be said, therefore, that heart muscle in general is capable of 

 giving automatic rhythmic contractions, but this, of course, is a 

 different thing from saying that in a heart under normal condi- 

 tions the initial impulse that starts the beat actually originates in 

 heart muscle and not in the nerve cell. The advocates of the 

 neurogenic theory point to the fact that automatic rhythmicity is 

 a characteristic property of nerve cells and, moreover, that in cer- 

 tain hearts (limulus), in which the nerve cells are accessible, re- 

 moval of these cells stops the cardiac rhythm. One cannot, 

 however, without further evidence apply this result obtained from 

 a crustacean heart to explain what takes place in a mammalian 

 heart. It is obviously not applicable to the case of the heart of 

 the embryo. But the whole controversy has taken on a different 

 aspect, since it has come to be realized that the initiation of the 

 heart beat takes place in the nodal tissue, the sino-auricular node, 

 or, vmder some conditions, the auriculoventricular node. This 

 nodal tissue consists of modified heart muscle, but it is plentifully 

 supplied also with nerve cells and fibers. It is not possible at 

 present to determine whether the impulse originating in the sino- 

 auricular node is an outcome of automatic processes in the mus- 

 cular or the nervous constituent of the node. 



Automaticity of the Heart. — As was said above, the ques- 

 tion of the cause or causes of the automatic rhythmical con- 

 tractions must be sought for whether the phenomenon turns out to 

 be a property of the muscular tissue or of the nervous tissue of the 

 heart. When we say that a given tissue is automatic we mean 

 that the stimuli which excite it to activity arise within the tissue 

 itself, and are not brought to it through extrinsic nerves. In the 

 heart, therefore, we assume that a stimulus is continually being 

 produced, and we speak of it as the inner stimulus. Experiment and 

 speculation have been directed toward unraveling the nature of 

 this inner stimulus. Most of the physiologists who have expressed 

 an opinion upon the subject have sought an explanation in the 



