its impulse would indicate its new
position.

Sounds also accompany the heart's action. If the ear be applied over the
region of the heart, two distinct sounds will be heard following one
another with perfect regularity. Their character may be tolerably imitated
by pronouncing the syllables _lubb_, _dup_. One sound is heard immediately
after the other, then there is a pause, then come the two sounds again.
The first is a dull, muffled sound, known as the "first sound," followed
at once by a short and sharper sound, known as the "second sound" of the
heart.

The precise cause of the first sound is still doubtful, but it is made at
the moment the ventricles contract. The second sound is, without doubt,
caused by the sudden closure of the semilunar valves of the pulmonary
artery and the aorta, at the moment when the contraction of the ventricles
is completed.

[Illustration: Fig. 76.--Muscular Fibers of the Ventricles.

A, superficial fibers common to both ventricles;
B, fibers of the left ventricle;
C, deep fibers passing upwards toward the base of the heart;
D, fibers penetrating the left ventricle
]

The sounds of the heart are modified or masked by blowing "murmurs" when
the cardiac orifices or valves are roughened, dilated, or otherwise
affected as the result of disease. Hence these new sounds may often afford
indications of the greatest importance to physicians in the diagnosis of
heart-disease.

194. The Nervous Control of the Heart. The regular, rhythmic movement
of the heart is maintained by the action of certain nerves. In various
places in the substance of the heart are masses of nerve matter, called
ganglia. From these ganglia there proceed, at regular intervals,
discharges of nerve energy, some of which excite movement, while others
seem to restrain it. The heart would quickly become exhausted if the
exciting ganglia had it all their own way, while it would stand still if
the restraining ganglia had full sway. The influence of one, however,
modifies the other, and the result is a moderate and regular activity of
the heart.

The heart is also subject to other nerve influences, but from outside of
itself. Two nerves are connected with the heart, the pneumogastric
and the sympathetic (secs. 271 and 265). The former appears to be
connected with the restraining ganglia; the latter with the exciting
ganglia. Thus, if a person were the subject of some emotion which caused
fainting, the explanation