PHYSIOLOGICAL RESEARCH AND REASONING. 151 



This does not, however, according to the principles of a rigid 

 logic, follow. The heart may have ceased beating from some 

 cause wholly unconnected with this experiment, or from the 

 electric current escaping along the nerve and affecting some 

 nervous mechanism within the heart, which is not a part of the 

 vagus nerve ; or it may have been due to the action of the cur- 

 rent on the muscular tissue of the heart directly, or in some other 

 way. But suppose that invariably, whenever this experiment 

 is repeated, the one result (arrest of the beat) follows, then it is 

 clear that the vagus nerve is in some way a factor in the causa- 

 tion. Now, if it could be ascertained that certain branches of 

 the nerve were distributed to the heart-muscle directly, and that 

 stimulation of these gave rise to arrest of the cardiac pulsation, 

 then would it be highly probable, though not certain, that there 

 was in the first instance no intermediate mechanism. ; while 

 this inference would become still more probable if in hearts 

 totally without any such nervous apparatus whatever, such a 

 result followed on stimulation of the vagus. Suppose, further, 

 that the application of some drug or poison to the heart pro- 

 vided with special nervous elements besides the vagus termi- 

 nals prevented the effect before noticed on stimulating the 

 vagus, while a like result followed under similar circumstances 

 in those forms of heart unprovided with such nervous struct- 

 ures, there would be additional evidence in favor of the view 

 that the result we are considering was due solely to some action 

 of the vagus nerve ; while, if arrest of the heart followed in the 

 first case but not in the second, and this result were invariable, . 

 there would be roused the suspicion that the action of the 

 vagus was not direct, but through the nervous structures with- 

 in the heart other than vagus endings. And if, again, there were 

 a portion of the rabbit's heart to which there were distributed 

 this intrinsic nervous supply, which on stimulation directly 

 was arrested in its pulsation, it would be still more probable 

 that the effect in the first instance we have considered was due 

 to these structures, and only indirectly to the vagus. But be it 

 observed, in all these cases there is only probability. The con- 

 clusions of physiology never rise above probability, though this 

 may be so strong as to be practically equal in value to absolute 

 certainty. Would it be correct, from any or all the experi- 

 ments we have supposed to have been made, to assert that the 

 vagus was the arresting (inhibitory) nerve of the heart ? All 

 hearts thus far examined have much in common in structure 



