THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 141 



The Relation of the Heart to the Nervous System. A very simple 

 experiment is sufficient to prove that the beat of the heart does not 

 depend on its connection with the central nervous system, for an 

 excised frog's heart may, under favourable conditions, of which the 

 most important are a moderately low temperature, the presence of 

 oxygen, and the prevention of evaporation, continue to beat for days. 

 The mammalian heart also, after removal from the body, beats for 

 a time, and indeed, if defibrinated blood be artificially circulated 

 through the coronary vessels, for several or even many hours. But 

 although this proves that the heart can beat when separated from 

 the central nervous system, it does not prove that nervous influence 

 is not essential to its action, for in the cardiac substance nervous 

 elements, both cells and fibres, are to be found. 



The Intrinsic Nerves of the Heart. In the heart of the frog 

 numerous nerve-cells occur in the sinus venosus, especially near its 

 junction with the right auricle (Remak's ganglion). A branch 

 from each vagus, or rather from each vago-sympathetic nerve (for 

 in the frog the vagus is joined a little below its exit from the skull 

 by the sympathetic), enters the heart along the superior vena cava 

 (pp. 157, 196). 



Running through the sinus, with whose ganglion-cells the true vagus 

 fibres, or some of them, are believed to make physiological junction 

 (p. 163), the nerves pursue their course to the auricular septum. Here 

 they form an intricate plexus, studded with ganglion-cells. From the 

 plexus nerve-fibres issue in two main bundles, which pass down the 

 anterior and posterior borders of the septum to end in two clumps of 

 nerve-cells (Bidder's ganglia), situated at the auriculo-ventricular 

 groove. These ganglia in turn give off fine nerve-bundles to the ven- 

 tricle, which form three plexuses one under the pericardium, another 

 under the endocardium, and a third in the muscular wall itself, or myo- 

 cardium. From the last of these plexuses numerous non-medullated 

 fibres run in among the muscular fibres and end in close relation with 

 them. Similar plexuses of nerve-fibres exist in the mammalian ventricle. 

 But while scattered ganglion-cells are found in the upper part of the 

 ventricular wall, most observers have been unable to demonstrate any 

 either in the mammal or the frog in the apical half. In the rat's heart, 

 according to the careful observations of Schwartz, true ganglion-cells 

 are confined to an area on the posterior surface of the auricles, lying 

 always under the visceral pericardium. Other writers, however, have 

 stated that ganglion-cells do exist in the apex both of the cat's and of 

 the frog's heart. In connection with the whole question it must be 

 borne in mind that in other organs improved histological methods have 

 brought typical nerve -cells to light in situations where they were not 

 suspected or were denied to exist, and, further, that all investigators 

 are not agreed upon the histological criteria by which ganglion-cells are 

 to be distinguished. 



Cause of the Rhythmical Beat of the Heart. Scarcely any physio- 

 logical question has excited greater interest for many years than the 

 mechanism of the heart-beat. Several properties of the cardiac 

 tissue ought to be distinguished in discussing this question: (i) Its 



