THE CAUSATION OF THE HEART BEAT 



985 



where the nerves and ganglia run in the basal portion of the auricles. This 

 can be excised, leaving the two auricular appendages in connection with the 

 sinus venosus and with the ventricles. 



(c) The heart in the developing chick can be seen beating at a time when 

 it is quite free from nerve cells, which only extend into it at a later date. 



(d) Remak's ganglia are situated at the point where the two vagus nerves 

 enter the heart, and under the microscope can be seen to be connected with 

 the fibres of these nerves. We have now, from the discovery of Langley and 

 Dickinson, a means of judging of the action of ganglion cells in the drug 

 nicotine, which first stimulates and then paralyses nerve cells themselves, 

 or the synapses between the cells 



and the nerve fibres in connection 

 with them. Direct application 

 of nicotine to the heart, after a 

 primary period of slowing, leaves 

 the heart beat practically un- 

 altered, the normal sequence of 

 beat in the various cavities being 

 unaffected. After the application 

 of the drug however, stimulation 

 of the trunk of the vagus is with- 

 out effect, though slowing or stop- 

 page of the heart may still be 

 produced by excitation of the 

 post-ganglionic nerve fibres of 

 the vagus, which arise from the 

 cells of Remak's ganglia. These 

 ganglia must therefore be re- 



garded not as a motor Centre for 

 ,11 ,1 T T . . 



the heart, but merely as a distri- 

 buting centre for the inhibitory 

 fibres of the vagus. Since tetaui- 



sation of the heart with weak currents also causes local inhibition, it would 

 seem that the finer nerve fibres ramifying throughout the muscular substance 

 are, to a large extent at all events, inhibitory in their function. This is 

 confirmed by the fatit that atropine, which paralyses the inhibitory fibres 

 of the vagus, also abolishes the direct inhibitory effect of tetanisation on the 

 heart muscle. Gaskell and Engelmann therefore came to the conclusion 

 that the source of the cardiac rhythm was to be found, not in the ganglia 

 scattered about its cavities, but in the muscular cells themselves. 



The normal sequence of events i. e. the subordination of the ventricle to 

 auricles and auricles to sinus so that the beat always follows in the order, 

 sinus, auricles, ventricle, bulbus can be ascribed to the difference between 

 the natural rhythms of these different cavities. It is possible to record the 

 contractions of each of these parts of the heart separately, after having divided 

 them either functionally by crushing the intervening tissue or by actual 



-n 



FIG. 432. Tortoise s heart from dorsal surface. 



(GASKELL.) 



S, sinus; J, sino-auricular junction; A, auri- 

 cles; C, coronary vein; V, ventricle. (The 



dotte d line shows how a strip may be cut from 

 the ventricle apex.) 



