ix CAEDIAC MUSCLE AND NEEVES 307 



external stimuli ceases in the various segments, in the same order 

 in which the automatic excitability disappeared ; first, the ventricle 

 becomes inexcitable, next the auricles, and shortly after the sinus 

 venosus. On the basis of these facts it may be sustained that the - 

 two forms of excitability are in direct reciprocal relation, and that 

 the segments most excitable to internal stimuli are also the most . 

 excitable to external stimuli. 



V. Does the falling excitability (automatic and reflex) from 

 sinus venosus (or mouth of venae cavae) to auricles, from auricles 

 to ventricles, which determines cardiac rhythm, depend on the 

 varying number and arrangement of the ganglion cells in the, 

 different segments of the heart, since these have the property of 

 generating rhythmical impulses which are then transmitted to the 

 muscle cells, provoking contraction in the form of a peristaltic wave ; 

 or is it independent of the intracardiac nervous elements, and 

 inherent in the muscle cells of the myocardium ? For a long time 

 the first doctrine was very generally admitted. Two special 

 phenomena were adduced in its support which from their simplicity 

 appeared to be direct evidence : 



(a) The lower two-thirds of the apex of the frog's ventricle, 

 which shows no nerve-cells under the microscope, invariably ceases 

 to beat automatically when separated by an incision or ligature 

 from the other parts of the heart, which are provided with nerve 

 elements (Stannard, Eckhard and others). 



(&) Circumscribed excitation of any part of the frog's heart and , 

 its integuments always produces a contraction that commences in 

 the auricles, and not at the point directly stimulated (Kiirschner, 

 Budge, Pagliani). 



The first phenomenon seemed to be a direct proof of the 

 neurogenic origin of automatic excitability, and the second a direct 

 proof of the neurogenic origin of reflex excitability. Subsequently,, 

 however, the supposed constancy and affirmative character of both 

 was disputed. 



Eanvier and Engelmann observed rhythmical pulsations under 

 the microscope in fragments of the adult heart in which it was 

 impossible to detect any trace of nervous elements. The apex of 

 the ventricle, again, when excised and lightly attached to a 

 cannula will, after a pause (if conveniently distended and irrigated 

 with a nutrient fluid), begin to beat spontaneously, and continue to 

 do so for a long time, although the rhythm may be slower than 

 usual. This proves that even if this part has no ganglia, it also* 

 has an inherent automatic capacity, although to a much smaller * 

 extent than the rest of the heart. It should be added that this 

 masked excitability is a characteristic peculiar to the ventricle of 

 the adult frog. That of the tortoise, on the contrary, although it 

 has no nerve-cells, will beat for a considerable time after it has 

 been isolated (Gaskell). 



