THE CIRCULATION 293 



monary arteries sometimes, one valve may close a minute FlG - 157 

 fraction of a second before the others. The sound is inten- 

 sified and perhaps altered materially in tone by the vibra- 

 tion of the tense column of blood set in motion by the 

 ventricular systole and by the slapping shut of the valves 

 as the arterial walls passively (and hence almost instantly) 

 recoil. 



The Heart-beat as Muscular Action. The physiology of < 

 the heart as a muscle is studied in the laboratory (see 

 Expt. 69, etc., therefore, in the Appendix). Out of the , 

 large mass of facts learned about the complexities of 

 the cardiac muscle, one of the most important perhaps is 

 the rhythmicity apparently inherent in the heart. Whether 

 the rhythmic heart-beat is produced by the action of the 

 circulating salines (perhaps through the ions) on the heart 

 muscle directly or is brought about by the rhythmic stimu- 

 lation of nerve cells, as Dogiel has recently claimed so 

 vigorously, it is the most striking fact in relation to the 

 heart Another fact about the heart muscle recently come 

 into prominence is its duality of action; it has apparently 

 the characteristics of both smooth muscle and of cross- 

 striated muscle. The former kind of contractile tissue is 

 represented in the heart's action by its tonic variations in 

 size. These occur probably under the influence of the vaso- 

 motor apparatus and correlate its action with the caliber of 

 the arteries. The cross-striated aspect of the heart-muscle 

 is represented in its sharp, quick, and powerful beat. It is 

 hoped that the years soon to come will clear up these 

 matters concerning the heart, for they are of the utmost 

 practical importance to diseased humanity and in a theo- 

 retical way to physiology, striving to arrive at the principles 

 of organic things. 



The Influence of Nerves on the Heart. We do not yet 

 know the exact relations between the nerves of the heart 

 and its muscle-cells. One hypothesis, the neurogenic theory, 

 maintains that the heart's rhythmic beat depends on ' 

 rhythmic stimulation from the central nervous system or at 

 least from nerve cells in the heart. The other supposition, 

 the myogenic theory, maintains that, provided the heart- 

 muscle be supplied with its nutritive fluid (the blood and the 

 lymph) of the right temperature, etc., the heart tends to keep 

 up its rhythmicity without continual influence from the 

 nervous system. To the "myogenists," then, it seems that 



FIG. 157. This tracing shows in an unusual degree the tonus in the frog's 

 heart. Suspension-method. To be read from left to right. The dots are at 

 about ten-second intervals. 2 / 3 . 



