THE ACTION OF THE HEART 315 



opposing views are held. One of these is that the heart is sub- 

 ject to the influence of a constant stimulus, its property of " maxi- 

 mal" contractions with their accompanying refractory periods 

 sufficing to bring about rhythmic responses to such constant 

 stimulation. The other view is that the heart is a truly automatic 

 organ, the metabolic processes going on within the heart tissue 

 being of such a nature as to produce rhythmic activity quite 

 independently of "stimulation " as we ordinarily understand it. 



Those who believe the heart to be under the influence of a 

 constant stimulus .look to the blood as its source, and especially 

 to the inorganic blood-salts, it having been shown that the heart- 

 beat can be maintained for an astonishing length of time when 

 the heart is fed solutions containing only inorganic salts of sodium, 

 potassium, and calcium in proper proportion. Those who look 

 upon the heart as a truly automatic organ take the position that 

 their view is more in accordance with general physiological prin- 

 ciples than the other, and that no evidence yet brought forth 

 disproves their claim. They put the burden of proof upon the 

 supporters of the "constant stimulus" theory. It must be ad- 

 mitted that at present no conclusive evidence for either view is 

 available, nor are the supporters of either able to picture a satisfac- 

 tory mechanism of rhythmicity in terms of their particular theory. 



The Extrinsic Nerves of the Heart. The heart, as stated pre- 

 viously, is under the control of the sympathetic system. It 

 receives nerve-fibers over two pathways. One of these is by way 

 of the tenth cranial nerves, the vagi, the other by way of the sym- 

 pathetic system proper. The vagus nerves give off their cardiac 

 branches in the neck; the cardiac nerves from the sympathetic 

 system arise from the inferior cervical ganglion, a sympathetic 

 ganglion lying in the lower neck region. Both anatomically and 

 physiologically the two sets of nerve-fibers are distinct. Anatomic- 

 ally the vagus fibers arc prc-ganglionic ; they arise from cell-bodies 

 in the nucleus of the tenth nerve in the medulla and are myelin- 

 ated. They terminate about nerve-cells lying on or within the 

 heart itself. The fibers from the sympathetic system are post- 

 ganglionic ; they arise from cell-bodies in sympathetic ganglia, the 

 inferior cervical for the most part, and are non-myelinated. 

 They terminate in the tissues of the heart directly. Since nicotine 

 cuts the connection between pre- and post-ganglionic fibers, appli- 



