This is called the sinus node, or the sino-auricular node, and
consists of a small bundle of fibers resembling muscle tissue. Lewis
[Footnote: Lewis: Lecture in the Harvey Society, New York Academy of
Medicine, Oct. 31, 1914.] describes this bundle as from 2 to 3 cm.
in length, its upper end being continuous with the muscle fibers of
the wall of the superior vena cava. Its lower end is continuous with
the muscle fibers of the right auricle. From this node "the
excitation wave is conducted radially along the muscular strands at
a uniform rate of about a thousand millimeters per second to all
portions of the auricular musculature."

Though a wonderfully tireless mechanism, this region may fall out of
adjustment, and the stimuli proceeding from it may not be normal or
act normally. It has been shown recently not only that there must be
perfection of muscle, nerve and heart circulation but also that the
various elements in solution in the blood must be in perfect amounts
and relationship to each other for the heart stimulation to be
normal. It has also been shown that if for any reason this region of
the right auricle is disturbed, a stimulus or impulse might come
from some other part of the auricle, or even from the ventricle, or
from some point between them. Such stimulations may constitute
auricular, ventricular or auriculoventricular extra contractions or
extrasystoles, as they are termed. In the last few years it has been
discovered that the auriculoventricular handle, or "bundle of His,"
has a necessary function of conductivity of auricular impulse to
ventricular contraction. A temporary disturbance of this
conductivity will cause a heart block, an intermittent disturbance
will cause intermittent heart block (Stokes-Adams disease), and a
prolonged disturbance, death. It has also been shown that
extrasystoles, meaning irregular heart action, may be caused by