IRRITABILITY OF THE AUTOMATIC MOTOR CENTERS. 115 



cedes that of the auricles). From this ganglion the vagus fibers pass as the 

 anterior and posterior septal nerves, each of which is provided with a ganglion at 

 the auriculo-ventricular junction, the ventricular ganglion, or Bidder's ganglion. 

 The nerve-fibers, which are for the most part non-medullated, can be traced 

 further in connection with the ganglia. 



The motor fibers terminate with slightly clubbed extremities in each muscle- 

 cell; the sensory, which are derived from medullated fibers, in flat, expanded 

 terminal plexuses, which are quite abundant in the endocardium and the peri- 

 cardium. 



All ganglion-cells are bipolar or multipolar. In the frog most of them are 

 surrounded by a network of fibers; in Bidder's ganglion spindle-shaped cells with 

 two processes, one at each extremity, predominate. In the rabbit and in the frog 

 the ganglion-cells belonging to the sympathetic system have two nuclei, while the 

 vagus ganglia have only one. After division of the vagus branches (in the frog) 

 the spiral process and the pericellular network from which it originates undergo 

 degeneration. The straight process gives off the muscle-nerves. The bulb of the 

 aorta contains numerous nerves for its muscle-fibers; but whether it contains 

 ganglia also is doubtful. 



IRRITABILITY OF THE AUTOMATIC MOTOR CENTERS IN THE 

 HEART AND IN THE HEART-MUSCLE. 



There are at the present time only two theories with regard to the 

 irritability of the heart and its spontaneous rhythmic action. 



1. The older theory teaches that the "automatic centers" that 

 excite the movements and maintain an orderly rhythm are situated 

 within the heart and that this function resides in the ganglia. 



2. It is assumed that not one but several such centers are present 

 in the heart and are connected with one another by conducting paths. 

 So long as the heart is intact the various centers are stimulated 

 to rhythmic activity in a definite order from the principal center, the 

 impulse being conveyed through the conducting paths from that center. 

 The forces that excite these regular continuous movements are not known. 

 If, however, diffuse stimuli, of which the simplest is a strong electrical 

 current, are applied to the heart, all of the centers are thrown into action 

 and a spasmodic contraction of the heart takes place without any 

 rhythm of movement. The dominating center is situated in the auri- 

 cles (in the frog), whence, therefore, the regular progressive movements 

 usually proceed. When its irritability is reduced, as by applying 

 opium to the septum with a cotton pledget, a different set of centers 

 appears to gain control, and the movement may then be propagated 

 from the ventricles to the auricles. 



3. The nerve-centers of the auricles are more irritable than those of 

 the ventricles; hence they continue to beat independently for a longer 

 time when the heart is left to itself. 



4. All stimuli of moderate strength acting directly on the heart cause 

 primarily an increase in the rhythmic heart-beats; stronger stimuli 

 cause, in a short while, diminution progressing to paralysis, often pre- 

 ceded by spasmodic tremulous "undulation or flickering." Increased 

 activity on the part of the heart exhausts its strength the more rapidly. 



5. Individual weak stimuli, such as are insufficient to exert any effect 

 on the heart, may be rendered efficient by repetition, as the heart is 

 capable of summation of the individual stimuli. 



6. Even the feeblest stimuli that are at all capable of exciting a 

 contraction always excite an active contraction, that is, "the minimal 

 stimulus has a maximal effect." 



