Il8 DIRECT STIMULATION OF THE HEART. 



fact that the embryonal heart, in which it has been impossible to dem- 

 onstrate the presence of ganglia, pulsates like the heart of certain inverte- 

 brates, some recent investigators assert that the automatism of the cardiac 

 action resides in the muscle itself. Similarly, His, Jr., and Romberg, 

 on developmental grounds, teach that the ganglia belong really to the 

 sensory nerves of the heart, and that, therefore, there are no automatic 

 nerve-centers at all. When Krehl and Romberg isolated portions of 

 the rabbit's heart devoid of ganglia by crushing, but in such a way that, 

 so long as the circulation was maintained, they represented anatomical 

 portions of the heart, they found that these pieces continued to pulsate 

 for hours. It is said that even excision of the entire septum of the 

 frog's heart, including Remak's ganglion, has no disturbing effect on 

 the heart-beat. 



The propagation of the contraction from the auricles to the ventricles 

 is said to take place through the muscle-fibers that pass from the former 

 to the latter. That the conduction of the stimuli from auricle to ven- 

 tricle, which does not take place continuously, but periodically in the 

 same rhythm as the heart-beats, is not transmitted through the nerve- 

 paths is proved by the slow rate at which it is effected, the conduction 

 being 300 times slower than in motor nerves. 



Engelmann expresses his views upon these questions as follows : 



The muscle-cells of the heart itself and not a system of nerve-ganglia constitute 

 the excito-motor central organ ; as such they generate the motor stimuli that cause 

 the heart to beat. As those muscle-cells that surround the large veins emptying 

 into the heart are most susceptible to the irritating influence of automatic move- 

 ment, the systolic contraction occurs first at this point, to spread then in a peris- 

 taltic manner successively to the auricles, the ventricles, and the bulb of the 

 aorta. The motor stimulus is propagated directly from muscle-cell to muscle-cell. 

 All of the muscle-cells of the entire heart form together a single physiologically 

 conducting contractile mass. Within each individual portion of the heart-^- venous 

 trunks, venous sinuses, auricles, ventricles, bulb of the aorta the motor stimulus 

 is propagated rapidly, in a manner comparable to the contraction of a striated 

 muscle. Those muscle-cells, on the other hand, that form the connecting bridges 

 between the individual portions of the heart conduct slowly, in a manner 

 comparable to unstriated or embryonal muscles. Consequently every individual 

 portion of the heart contracts practically at the same time as a whole; while, on 

 the other hand, the systole of each portion of the heart situated farther on in 

 the course of the blood-stream can take place only after an actual interval, long 

 enough for the blood to be carried from one part of the heart into the next. As 

 the fibers of the heart-muscle, in the act of contraction, temporarily lose their 

 contractility and conducting power, as a sort of fatigue-phenomenon, they contain 

 within themselves the periodicity of contraction and relaxation systole and dias- 

 tole. A cycle of the entire heart may be induced from any point in the large 

 veins. When the cardiac stimuli succeed one another slowly, each individual car- 

 diac cycle becomes shorter, but more powerful. The blood is then propelled in 

 larger quantities and with greater force; while if the succession is more rapid, 

 less blood is propelled with a lesser degree of force. 



Direct Stimulation of the Heart. All direct cardiac stimuli act much more 

 vigorously from the internal than from the external surface of the heart. When 

 the stimulation is severe or protracted, the ventricular portion is always paralyzed 

 first. 



(a) Thermic Stimuli. Descartes had already observed in 1644 that the eel's 

 heart could be made to pulsate more rapidly by the application of heat. Alex. v. 

 Humboldt explained the acceleration of the pulse that takes place in man in a 

 hot medium in the same way. As the temperature continues to rise, the heart- 

 beats at first often reach a considerable frequency. They then become more 

 infrequent again, and finally cease altogether, and the muscle is found to be con- 

 tracted. As a rule, the ventricular portion is arrested before the auricles, some- 

 times after a period of tetanic undulatory spasm. At a temperature of 25 C. 

 and above, the ligated frog's heart immersed in a 0.6 per cent, saline solution, 



