566 CIRCULATION OF BLOOD AND LYMPH. 



The Neurogenic Theory of the Heart Beat. The literature 

 upon this topic is very large.* The neurogenic theory has suffered 

 some changes in its details since first proposed by Volkmann, 

 particularly in the specific functions assigned to the different ganglia 

 that exist in the heart. In general, however, the theory assumes 

 that the excitation to each beat arises within the nerve cells, and 

 since the cardiac cycle begins with a contraction at what may be 

 called the venous end of the heart, that is, at the junction of the 

 veins with the auricles, it is assumed that the excitation or inner 

 stimulus arises in the nerve cells situated in this region. These cells 

 constitute, therefore, what may be called the automatic motor 

 center of the heart. The stimuli generated within it are transmitted 

 through its axons first to the musculature of the venous end of 

 the heart. The subsequent orderly march of this contraction, to 

 auricles and then to ventricles, is also upon this theory usually 

 attributed to the intrinsic nerve cells and fibers. Through a definite 

 mechanism the impulses generated in the motor center are trans- 

 mitted to subordinate nerve centers through which the auricles are 

 excited, and then to other nerve cells lying in or near the auriculo- 

 ventricular groove or to the nervous tissue in the auriculoven- 

 tricular bundle through which the ventricles are excited. In this 

 form the theory assumes for the heart an intrinsic central nerv- 

 ous system, as it were, with a principal motor center in which 

 the property of automaticity is chiefly developed and subordinate 

 centers whose activity usually depends upon the principal center, 

 but which may show automatic properties of a lower order if the 

 connections between them and the main center are interrupted. 

 This intrinsic nervous system is responsible not only for the spon- 

 taneous origination and normal sequence of the beat, but also for 

 its co-ordination. The many muscular fibers of the ventricle 

 contract normally in a definite manner and sequence, so that their 

 effect is summated. Under abnormal conditions the fibers may 

 contract irregularly, giving the so-called fibrillary contractions of 

 the heart, which are incorordinaied. It may be said that this con-- 

 ception of the connections of the intrinsic nervous system rests 

 mainly upon deductions from physiological experiments. The 

 histological details regarding the connections of the nerve cells in the 

 heart are not yet sufficiently known, but it can not be said at present 

 that they give any positive support to such a view. In regard to 

 the neurogenic theory the following general statements may be made : 



1. Most of the very numerous facts known regarding the heart 



* For general presentations from different standpoints see Gaskell, article 

 on "The Contraction of Cardiac Muscle/' in Schafer's "Text-book of Physi- 



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in Hill's "Further Advances in Physiology," 1909, 53. 



