There is often a difference in effect
between the two vagi. Sometim'es one
vagus is found not to possess any
inhibitory fibers, in which case the
opposite vagus is usually found espec-
ially active. It is generally found
that the effect is not identical on the
two sides, one usually being more
powerful than the other.
The arrest, or period of inhibition, is manifest in the curve by
the lever recording merely a straight line. If the laryngeal muscles
contract, and thereby affect the position of the heart, divide the
laryngeal branches of the vagus.
There is an appreciable time or latent period, before the heart
reacts to the stim.ulus and likewise when the stimulus is removed
the heart do^ not at once regain its normal movement. Note that
when the heart begins to beat again the beats are at first small and
gradually rise to normal. In some instances, however, they are

109




Pig. 29. — ^Cardiac Nerves of
the Prog. (Poster).
more vigorous and quicker. Cut both vagus nerves and compare
the tracing with those just obtained.
332. Action op Drugs on Heart. The experiments may be
performed upon a heart which has been removed and placed in a
watch-glass or preferably upon the heart in its natural position.
Muscarine. Pith a frog, expose the heart and with a fine pipette
apply a drop of serum or saline solution containing a trace of
muscarine, which rapidly arrests the rhythmical action of the heart,
the ventricle being relaxeid, i. e., diastole, and distended with blood.
Get a tracing from the heart while under the influence of muscarine.
