144 ANIMAL PHYSIOLOGY. 
nerve on the heart, an action excited by the use of electricity. 
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 
