500 CIRCULATION OF BLOOD AND LYMPH. 



observers, show that in the cold-blooded animals strips of heart 

 muscle taken from various parts of the heart will under proper 

 conditions develop rhythmical contractions. It is very improbable 

 that each of these strips, no matter how made, contains its own 

 resident nerve cells to act as a motor center. Histology does 

 not warrant such an assumption, and we must believe that these 

 results demonstrate an inherent property of rhythmicity in cardiac 

 muscle, whether or not this rhythmicity is directly responsible for 

 the normal beat. 



4. It has been shown that in the embryo chick the heart pul- 

 sates normally before the nerve cells have grown into it, and it 

 is stated that in the hearts of a number of invertebrates no nerve 

 cells can be found. Much weight can not be given, however, to 

 negative evidence of this kind, since, in the first place, better 

 technical methods may demonstrate the existence of such cells, 

 and even if absent from the heart itself it is conceivable that they 

 may be present in the surrounding tissue and send their fibers 

 to the heart. It is evident from this brief and imperfect presenta- 

 tion that it is not possible to claim that either the neurogenic or 

 the myogenic theory is demonstrated, but most physiologists per- 

 haps at present believe that the latter view is more in accord 

 with the facts. 



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 

 composition of the blood or lymph bathing the heart tissue, or in the 

 products of metabolism of the tissue itself. According to this last 

 view, each contraction results in the formation of certain products 

 which stimulate the muscle to a new contraction (Langendorff). 

 Regarding this view there is nothing of the nature of direct ex- 

 perimental evidence in its favor. No product of the metabolism of 

 the heart tissue capable of exerting this stimulating effect has been 

 isolated. In regard to the former view, that the inner stimulus 

 is connected with a definite composition of the blood or lymph, 

 there has been considerable experimental work which is of funda- 

 mental significance. While the older physiologists paid attention 



