THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
1015 
These facts alone make it probable that the vagus lowers the excitability of the 
muscle at one time and increases it at another. Now although this diminution of 
excitability occurs at that period of the vagus action when the stimulation of the 
nerve is known to cause a diminution of the force of the contractions, yet it is clear 
that the connexion between the two is relative and not absolute. As has been seen, 
when the contractions are synchronous with the impulses, stimulation of the nerve is 
able to reduce the force of the contractions to the vanishing point, and yet up to the 
last moment of visible contractions every impulse is followed by a contraction— i.e., the 
excitability does not in this case fall sufficiently, in relation to the strength of the 
impulses, to prevent the muscle from responding to every impulse, even although the 
contractions become so small as to be almost invisible. Since, therefore, we have 
reason to suppose that the vagus stimulation does diminish the excitability at the 
first period of its action— i.e., at the time when it diminishes the force of the con¬ 
tractions—and yet leaves the cardiac muscle still able to respond synchronously to 
the impulses, provided that the latter have not been diminished in strength, the 
inevitable conclusion is, that normally the excitability of the muscular tissue is greater 
than is necessary to enable it just to respond to every impulse. In other words, 
a range is allowed within which the excitability may fluctuate without thereby 
preventing the synchronous response of the muscle to the impulses, as long as the 
strength of the impulses does not diminish. As a rule the lower limit of this range 
is not passed, in consequence of vagus stimulation, when the strength of the impulses 
is not interfered with ; and therefore in these cases the diminution of excitability 
caused by the vagus is not rendered visible. This diminution can, however, be made 
manifest by first reducing the excitability of the muscle to some extent. Thus if 
muscarin be applied to the ventricle alone, when it is beating synchronously with 
every beat of the auricles, and the vagus be stimulated when the muscarin has, 
without altering the synchronism, greatly reduced the force of the ventricular contrac¬ 
tions, then the stimulation may cause the ventricle to beat synchronously with every 
second auricular beat for a short period, showing that the stimulation of the nerve, 
added to the effect of the muscarin, has reduced the excitability of the ventricle to a 
greater extent than the muscarin alone had done. 
Hitherto I have chiefly spoken about the diminution of excitability caused bv the 
vagus, and will therefore now proceed to give further proofs that the vagus increases 
the excitability of the muscle at the same time that it augments the force of its con¬ 
tractions. The most striking proof of this fact, in addition to what has been already 
said, is given by the effect upon the ventricle of stimulation of the nerve when the 
ventricle is rendered quiescent either by heating the sinus and auricles or by tightening 
the clamp in the auriculo-ventricular groove. 
In both cases the effect is the same, and is well exemplified in the series of Plate 69, 
fig. 17, curves A, B, C, D, which represent the effects of vagus stimulation in a single 
MDCCCLXXXTI. 6 O 
